The Universe of Disco

Daylight chart
My wife is always sad this time of the year because the days are so short and dark. I made a chart like this one a couple of years ago so that she could see the daily progress from dark to light as the winter wore on to spring and then summer.

This chart only works for Philadelphia and other places at the same latitude, such as Boulder, Colorado. For higher latitutdes, the y axis must be stretched. For lower latitudes, it must be squished. In the southern hemisphere, the whole chart must be flipped upside-down.

(The chart will work for places inside the Arctic or Antarctic circles, if you use the correct interpretations for y values greater than 1440 minutes and less than 0 minutes.)

This time of year, the length of the day is only increasing by about thirty seconds per day. (Decreasing, if you are in the southern hemishphere.) On the equinox, the change is fastest, about 165 seconds per day. (At latitudes 40°N and 40°S.) After that, the rate of increase slows, until it reaches zero on the summer solstice; then the days start getting shorter again.

As with all articles in the physics section of my blog, readers are cautioned that I do not know what I am talking about, although I can spin a plausible-sounding line of bullshit. For this article, readers are additionally cautioned that I failed observational astronomy twice in college.

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Notes on Neal Stephenson's Baroque novels
Earlier this year I was reading books by Robert Hooke, John Wilkins, Sir Thomas Browne, and other Baroque authors; people kept writing to me to advise me to read Neal Stephenson's "Baroque cycle", in which Hooke and Wilkins appear as characters.

I ignored this advice for a while, because those books are really fat, and because I hadn't really liked the other novels of Stephenson's that I'd read.

But I do like Stephenson's non-fiction. His long, long article about undersea telecommunications cables was one of my favorite reads of 1996, and I still remember it years later and reread it every once in a while. I find his interminable meandering pointless and annoying in his fiction, where I'm not sure why I should care about all the stuff he's describing. When the stuff is real, it's a lot easier to put up with it.

My problems with Stephenson's earlier novels, The Diamond Age and Snow Crash, will probably sound familiar: they're too long; they're disorganized; they don't have endings; too many cannons get rolled onstage and never fired.

Often "too long" is a pinheaded criticism, and when I see it I'm immediately wary. How long is "too long"? It calls to mind the asinine complaint from Joseph II that Mozart's music had "too many notes". A lot of people who complain that some book is "too long" just mean that they were too lazy to commit the required energy. When I say that Stephenson's earlier novels were "too long", I mean that he had more good ideas than he could use, and put a lot of them into the books even when they didn't serve the plot or the setting or the characters. A book is like a house. It requires a plan, and its logic dictates portions of the plan. You don't put in eleven bathtubs just because you happen to have them lying around, and you don't stick Ionic columns on the roof just because Home Depot had a sale on Ionic columns the week you were building it.

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The work totals about 2,700 pages. Considered as a trilogy, this is three very long books. Stephenson says in the introduction that it is actually eight novels, not three. He wants you to believe that he has actually written eight middle-sized books. But he hasn't; he is lying, perhaps in an attempt to shut up the pinheads who complain that his books are "too long". This is not eight middle-sized books. It is one extremely long book.

The narrative of the Baroque cycle is continuous, following the same characters from about 1650 up through about 1715. There is a framing story, introduced in the first chapters, which is followed by a flashback that lasts about 1,600 pages. Events don't catch up to the frame story until the third volume. If you consider Quicksilver to be a novel, the opening chapters are entirely irrelevant. If you consider it to be three novels, the opening chapters of the first novel are entirely irrelevant. It starts nowhere and ends nowhere, a vermiform appendix. But as a part of a single novel, it's not vestigial at all; it's a foreshadowing of later developments, which are delivered in volume III, or book 6, depending on how you count.

Another example: The middle volume, titled The Confusion, alternates chapters from two of the eight "novels" that make up the cycle. Events in these two intermingled ("con-fused") novels take place concurrently. Stephenson claims that they are independent, but they aren't.

So from now on I'm going to drop the pretense that this is a trilogy or a "cycle", and I'm just going to call this novel the "Baroque novel".

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This was quite a surprise to me. The world is full of incoherent ramblers, and most of them, if you really take the time to listen to them carefully, and at length, turn out to be completely full of shit. You get nothing but more incoherence.

Stephenson at 600 pages is a semi-coherent rambler; to really get what he is saying, you have to turn him up to 2,700 pages. Most people would have been 4.5 times as incoherent; Stephenson is at least 4.5 times as lucid. His ideas are great; he just didn't have enough space to explain them before! The Baroque novel has a single overarching theme, which is the invention of the modern world. One of the strands of this theme is the invention of science, and the modern conception of science; another is the invention of money, and the modern conception of money.

I've written before about what I find so interesting about the Baroque thinkers. Medieval, and even Renaissance thought seems very alien to me. In the baroque writers, I have the first sense of real understanding, of people grappling with the same sorts of problems that I do, in the same sorts of ways. For example, I've written before about John Wilkins' attempt to manufacture a universal language of thought. People are still working on this. Many of the particular features of Wilkins' attempt come off today as crackpottery, but to the extent that they do, it's only because we know now that these approaches won't work. And the reason we know that today is that Wilkins tried those approaches in 1668 and it didn't work.

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I find that almost all of Stephenson's annoying habits are much less annoying in the context of historical fiction. For example, many plot threads are left untied at the end. Daniel Waterhouse (fictional) becomes involved with Thomas Newcomen (real) and his Society for the Raising of Water by Fire. (That is, using steam engines to pump water out of mines.) This society figures in the plot of the last third of the novel, but what becomes of it? Stephenson drops it; we don't find out. In a novel, this would be annoying. But in a work of historical fiction, it's no problem, because we know what became of Newcomen and his steam engines: They worked well enough for pumping out coal mines, where a lot of coal was handy to fire them, and well enough to prove the concept, which really took off around 1775 when a Scot named James Watt made some major improvements. Sometime later, there were locomotives and nuclear generating plants. You can read all about it in the encyclopedia.

Another way in which Stephenson's style works better in historical fiction than in speculative fiction is in his long descriptions of technologies and processes. When they're fictitious technologies and imaginary processes, it's just wankery, a powerful exercise of imagination for no real purpose. Well, maybe the idea will work, and maybe it won't, and it is necessarily too vague to really give you a clear idea of what is going on. But when the technologies are real ones, the descriptions are illuminating and instructive. You know that the idea will work. The description isn't vague, because Stephnson had real source material to draw on, and even if you don't get a clear idea, you can go look up the details yourself, if you want. And Stephenson is a great explainer. As I said before, I love his nonfiction articles.

A lot of people complain that his novels don't have good endings. He's gotten better at wrapping things up, and to the extent that he hasn't, that's all right, because, again, the book is a historical novel, and history doesn't wrap up. The Baroque novel deals extensively with the Hanoverian succession to the English throne. Want to know what happened next? Well, you probably do know: a series of Georges, Queen Victoria, et cetera, and here we are. And again, if you want, the details are in the encyclopedia.

So I really enjoyed this novel, even though I hadn't liked Stephenson's earlier novels. As I was reading it, I kept thinking how glad I was that Stephenson had finally found a form that suits his talents and his interests.

[ Addendum 20170728: I revisited some of these thoughts in connection with Stephenson's 2016 novel Seveneves. ]

[ Addendum 20191216: I revived an article I wrote in 2002 about Stephenson's first novel The Big U. ]

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Linogram development: 20061227 Update
Defective constraints (see yesterday's article) are now handled. So the example from yesterday's article has now improved to:

        define regular_polygon[N] closed {
          param number radius, rotation=0;
          point vertex[N], center;
          line  edge[N];
          constraints {
            vertex[i] = center + radius * cis(rotation + 360*i/N);
            edge[i].start = vertex[i];
            edge[i].end   = vertex[i+1];
          }
        }

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Linogram development: 20061226 Update
I have made significant progress on the "arrays" feature. I recognized that it was actually three features; I have two of them implemented now. Taking the example from further down the page:

        define regular_polygon[N] closed {
          param number radius, rotation=0;
          point vertex[N], center;
          line  edge[N];
          constraints {
            vertex[i] = center + radius * cis(rotation + 360*i/N);
            edge[i].start = vertex[i];
            edge[i].end   =  vertex[i+1];
          }
        }

The following example works just fine:

        number p[3];
        number q[3];
        constraints {
          p[i] = q[i];
          q[i] = i*2;
        }

        number p[3];
        number q[3];
        constraints {
          p[i] = q[i+1];
        }

p₀ = q₁
p₁ = q₂
p₂ = q₃

The third of these is defective, because there is no q₃. If the figure is "closed" (which is the default) the subscripts should wrap around, turning the defective constraint into p₂ = q₀ instead. If the figure is declared "open", the defective constraint should simply be discarded.

The syntax for parameterized definitions (define regular_polygon[N] { ... }) is still a bit up in the air; I am now leaning toward a syntax that looks more like define regular_polygon { param index N; ... } instead.

The current work on the "arrays" feature is in the CVS repository on the branch arrays; get the version tagged tests-pass to get the most recent working version. Most of the interesting work has been on the files lib/Chunk.pm and lib/Expression.pm.

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Reader's disease
Reader's disease is an occupational hazard of editors and literary critics. Critics are always looking for the hidden meaning, the clever symbol, and, since they are always looking for it, they always find it, whether it was there or not. (Discordians will recognize this as a special case of the law of fives.) Sometimes the critics are brilliant, and find hidden meanings that are undoubtedly there even though the author was unaware of them; more often, they are less fortunate, and sometimes they even make fools of themselves.

The idea of reader's disease was introduced to me by professor David Porush, who illustrated it with the following anecdote. Nathaniel Hawthorne's The Scarlet Letter is prefaced by an introduction called The Custom-House, in which the narrator claims to have found documentation of Hester Prynne's story in the custom house where he works. The story itself is Hawthorne's fantasy, but the custom house is not; Hawthorne did indeed work in a custom house for many years.

Porush's anecdote concerned Mary Rudge, the daughter of Ezra Pound. Rudge, reading the preface of The Scarlet Letter, had a brilliant insight: the custom house, like so many other buildings of the era, was a frame house and was built in the shape of the letter "A". It therefore stands as a physical example of the eponymous letter.

Rudge was visiting Porush in the United States, and told him about her discovery.

"That's a great theory," said Porush, "But it doesn't look anything like a letter 'A'."

Rudge argued the point.

"Mary," said Porush, "I've seen it. It's a box."

Rudge would not be persuaded, so together they got in Porush's car and drove to Salem, Massachusetts, where the custom house itself still stands.

But Rudge, so enamored of her theory that she could not abandon it, concluded that some alternative explanation must be true: the old custom house had burnt down and been rebuilt, or the one in the book was not based on the real one that Hawthorne had worked in, or Porush had led her to the wrong building.

But anyway, all that is just to introduce my real point, which is to relate one of the most astounding examples of reader's disease I have ever encountered personally. The Mary Rudge story is secondhand; for all I know Porush made it up, or exaggerated, or I got the details wrong. But this example I am about to show you is in print, and is widely available.

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Each extract is accompanied by some introductory remarks by Fadiman and sometimes by one of the contributing editors. One long section in the book concerns early articles about human flight; in the Second Edition (published 1778-1783) there was an article on "Flying" by then editor James Tytler. Contributing editor Bruce L. Felknor's remarks include the following puzzled query:

What did Tytler mean by his interjection of "Fa" after Friar Bacon's famous and fanciful claim that man had already succeeded in flying? It hardly seems a credulous endorsement, an attitude sometimes attributed to Tytler.

Fadiman adds his own comment on this:

As for Tytler's "Fa": Could it have been an earlier version of our "Faugh!"? In any case we suddenly hear an unashamed human voice.

Gosh, what could Tytler have meant by this curious interjection? A credulous endorsement? An exclamation of disgust? An unedited utterance of the unashamed human voice? Let's have a look:

The secret consisted in a couple of large thin hollow copper-globes exhausted of air; which being much lighter than air, would sustain a chair, whereon a person might sit. Fa. Francisco Lana, in his Prodromo, proposes the same thing...

Felknor and Fadiman have mistaken "Fa" for a complete sentence. But it is apparently an abbreviation of Father Francisco Lana's ecclesiastical title.

Oops.

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ssh-agent
When you do a remote login with ssh, the program needs to use your secret key to respond to the remote system's challenge. The secret key is itself kept scrambled on the disk, to protect it from being stolen, and requires a passphrase to be unscrambled. So the remote login process prompts you for the passphrase, unscrambles the secret key from the disk, and then responds to the challenge.

Typing the passphrase every time you want to do a remote login or run a remote process is a nuisance, so the ssh suite comes with a utility program called "ssh-agent". ssh-agent unscrambles the secret key and remembers it; it then provides the key to any process that can contact it through a certain network socket.

The process for login is now:

        plover% eval `ssh-agent`
        Agent pid 23918
        plover% ssh-add
        Need passphrase for /home/mjd/.ssh/identity
        Enter passphrase for mjd@plover: (supply passphrase here)
        plover% ssh remote-system-1
        (no input required here)
        ...
        plover% ssh remote-system-2
        (no input required here either)
        ...

How does ssh know where to contact the agent process? ssh-agent prints an output something like this one:

        SSH_AUTH_SOCK=/tmp/ssh-GdT23917/agent.23917; export SSH_AUTH_SOCK;
        SSH_AGENT_PID=23918; export SSH_AGENT_PID;
        echo Agent pid 23918;

Now, suppose I log in to my home machine, say from work. The new login shell does not have any environment settings pertaining to ssh-agent. I can, of course, run a new agent for the new login shell. But there is probably an agent process running somewhere on the machine already. If I run another every time I log in, the agent processes will proliferate. Also, running a new agent requires that the new agent be supplied with the private key, which would require that I type the passphrase, which is long.

Clearly, it would be more convenient if I could tell the new shell to contact the existing agent, which is already running. I can write a command which finds an existing agent and manufactures the appropriate environment settings for contacting that agent. Or, if it fails to find an already-running agent, it can just run a new one, just as if ssh-agent had been invoked directly. The program is called ssh-findagent.

I put a surprising amount of work into this program. My first idea was that it can look through /tmp for the sockets. For each socket, it can try to connect to the socket; if it fails, the agent that used to be listening on the other end is dead, and ssh-findagent should try a different socket. If none of the sockets are live, ssh-findagent should run the real ssh-agent. But if there is an agent listening on the other end, ssh-findagent can print out the appropriate environment settings. The socket's filename has the form /tmp/ssh-XXXPID/agent.PID; this is the appropriate value for the SSH_AUTH_SOCK variable.

The SSH_AGENT_PID variable turned out to be harder. I thought I would just get it from the SSH_AUTH_SOCK value with a pattern match. Wrong. The PID in the SSH_AUTH_SOCK value and in the filename is not actually the pid of the agent process. It is the pid of the parent of the agent process, which is typically, but not always, 1 less than the true pid. There is no reliable way to calculate the agent's pid from the SSH_AUTH_SOCK filename. (The mismatch seems unavoidable to me, and the reasons why seem interesting, but I think I'll save them for another blog article.)

After that I had the idea of having ssh-findagent scan the process table looking for agents. But this has exactly the reverse problem: You can find the agent processes in the process table, but you can't find out which socket files they are attached to.

My next thought was that maybe linux supports the getpeerpid system call, which takes an open file descriptor and returns the pid of the peer process on other end of the descriptor. Maybe it does, but probably not, because support for this system call is very rare, considering that I just made it up. And anyway, even if it were supported, it would be highly nonportable.

I had a brief hope that from a pid P, I could have ssh-findagent look in /proc/P/fd/* and examine the process file descriptor table for useful information about which process held open which file. Nope. The information is there, but it is not useful. Here is a listing of /proc/31656/fd, where process 31656 is an agent process:

        lrwx------    1 mjd      users          64 Dec 12 23:34 3 -> socket:[711505562]

Not easily, anyway; there is a program called lsof which grovels over the kernel data structures and can associate a process with the names of the files it has open, or a filename with the ID numbers of the processes that have it open. My last attempt at getting ssh-findagent to work was to have it run lsof over the possible socket files, getting a listing of all processes that had any of the socket files open, and, for each process, which file. When it found a process-file pair in the output of lsof, it could emit the appropriate environment settings.

The final code was quite simple, since lsof did all the heavy lifting. Here it is:

        #!/usr/bin/perl

        open LSOF, "lsof /tmp/ssh-*/agent.* |"
            or die "couldn't run lsof: $!\n";
        <LSOF>;  # discard header
        my $line = <LSOF>;
        if ($line) {
          my ($pid, $file) = (split /\s+/, $line)[1,-1];
          print "SSH_AUTH_SOCK=$file; export SSH_AUTH_SOCK;
        SSH_AGENT_PID=$pid; export SSH_AGENT_PID;
        echo Agent pid $pid;
        ";
          exit 0;
        } else {
          print "echo starting new agent\n";
          exec "ssh-agent";
          die "Couldn't run new ssh-agent: $!\n";
        }

It's hard to figure out what the settings should be. So instead of trying to figure them out, one should save them to a file when they're first generated, then load them from the file as required.

Oh, yeah. A file. Duh.

It looks like this: Instead of loading the environment settings from ssh-agent directly into the shell, with eval `ssh-agent`, you should save the settings into a file:

        plover% ssh-agent > ~/.ssh/agent-env

        plover% . ~/.ssh/agent-env
        Agent pid 23918
        plover% ssh remote-system-1
        (no input required here)

Oh well, I'm not yet a perfect sage.

[ Addendum 20070105: I have revisited this subject and the reader's responses. ]

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Serendipitous web searches
In yesterday's article about a paper of Robert French, I mentioned that I had run across it while looking for something entirely unrelated. The unrelated thing is pretty interesting itself.

Back in the 1950's and 1960's, James V. McConnell at the University of Michigan was doing some really interesting work on learning and memory in planaria flatworms, shown at right. They used to publish their papers in their own private journal of flatworm science, The Journal of Biological Psychology. They didn't have enough material for a full journal, so when you were done reading the Journal, you could flip it over and read the back half, which was a planaria-themed humor magazine called The Worm-Runner's Digest. I swear I'm not making this up.

(I think it's time to revive the planaria-themed humor magazine. Planaria are funny even when they aren't doing anything in particular. Look at those googly eyes!)

(For some reason I've always found planaria fascinating, and I've known about them from an early age. We would occasionally visit my cousin in Oradell, who had a stuffed toy which was probably intended to be a snake, but which I invariably identified as a flatworm. "We're going to visit your Uncle Ronnie," my parents would say, and I would reply. "Can I play with Susan's flatworm?")

Anyway, to get on with the point of this article, McConnell made the astonishing discovery that memory has an identifiable chemical basis. He trained flatworms to run mazes, and noted how long it took to do so. (The mazes were extremely simple T shapes. The planarian goes in the bottom foot of the T. Food goes in one of the top arms, always the same one. Untrained planaria swim up the T and then turn one way or the other at random; trained planaria know to head toward the arm where the food always is. Pretty impressive, for a worm.)

Then McConnell took the trained worms and ground them up and fed them to untrained worms. The untrained worms learned to run the maze a lot faster than the original worms had, apparently demonstrating that there was some sort of information in the trained worms that survived being ground up and ingested. The hypothesis was that the information was somehow encoded in RNA molecules, and could be physically transferred from one individual to another. Isn't that a wonderful dream?

You can still see echoes of this in the science fiction of the era. For example, a recurring theme in Larry Niven's early work is "memory RNA", people getting learning injections, and pills that impart knowledge when you swallow them. See World Out of Time and The Fourth Profession, for example. And I once had a dream that I taught a giant planarian to speak Chinese, then fried it in cornmeal and ate it, after which I was able to speak Chinese. So when I say it's a wonderful dream, I'm speaking both figuratively and literally.

Unfortunately, later scientists were not able to reproduce McConnell's findings, and the "memory RNA" theory has been discredited. How to explain the cannibal flatworms' improved learning times, then? It seems to have been sloppy experimental technique. The original flatworms left some sort of chemical trail in the mazes, that remained after they had been ground up. McConnell's team didn't wash the mazes in between tests, and the cannibal flatworms were able to follow the trails later; it had nothing to do with their diet. Bummer.

So a couple of years ago I was poking around, looking for more information about this, and in particular for a copy of McConnell's famous paper Memory transfer through cannibalism in planarium, which I didn't find. But I did find the totally unrelated Robert French paper. It mentions McConnell, as an example of another cool-sounding and widely-reported theory that took a long time to dislodge, because it's hard to produce clear evidence that cannibal flatworms aren't in fact learning from their lunch meat, and because the theory that they aren't learning is so much less interesting-sounding than the theory that they are. News outlets reported a lot about the memory RNA breakthrough, and much less about the later discrediting of the theory.

French's paper, you will recall, refutes the interesting-sounding hypothesis that infants resemble their fathers more strongly then they do their mothers, and has many of the same difficulties.

There are counterexamples. Everyone seems to have heard that the Fleischmann and Pons tabletop cold fusion experiment was an error. And the Hwang Woo-Suk stem cell fraud is all over the news these days.

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Do infants resemble their fathers more than their mothers?
Back in 1995, Christenfeld and Hill published a paper claimed to have found evidence that infants tended to resemble their fathers more than they resembled their mothers. The evolutionary explanation for this, it was claimed, is that children who resemble their fathers are less likely to be abandoned by them, because their paternity would be less likely to be doubted. The pop science press got hold of it—several years later, as they often do—and it was widely reported for a while. Perhaps you heard about it.

A couple years ago, while looking for something entirely unrelated, I ran across the paper of French et al. titled The Resemblance of One-year-old Infants to Their Fathers: Refuting Christenfeld & Hill. French and his colleagues had tried to reproduce Christenfeld and Hill's results, with little success; they suggested that the conclusion was false, and offered a number of arguments as to why the purported resemblance should not exist. Of course, the pop science press was totally uninterested.

At the time, I thought, "Wow, I wish I had a way to get a lot of people to read this paper." Then last month I realized that my widely-read blog is just the place to do this.

Before I go on, here is the paper. I recommend it; it's good reading, and only six pages long. Here's the abstract:

In 1995 Christenfeld and Hill published a paper that purported to show at one year of age, infants resemble their fathers more than their mothers. Evolution, they argued, would have produced this result since it would ensure male parental resources, since the paternity of the infant would no longer be in doubt. We believe this result is false. We present the results of two experiments (and mention a third) which are very far from replicating Christenfeld and Hill's data. In addition, we provide an evolutionary explanation as to why evolution would not have favored the result reported by Christenfeld and Hill.

In the first study done by French, participants were presented with a 1-, 3-, or 5-year-old child's face, and the faces of either the father and two unrelated men, or the mother and two unrelated women. The participants were invited to identify the child's parent. They did indeed succeed in identifying the children's parents somewhat more often than would have been obtained by chance alone. But the participants did not identify fathers more reliably than they identified mothers.

The second study was similar, but used only 1-year-old infants. (The Christenfeld and Hill claim is that one year is the age at which children most resemble their fathers.)

French points out that although the argument from evolutionary considerations is initially attractive, it starts to disintegrate when looked at more closely. The idea is that if a child resembles its father, the father is less likely to doubt his paternity, and so is less likely to withhold resources from the child. So there might be a selection pressure in favor of resembling one's father.

But now turn this around: if a father can be sure of paternity because the children look like him, then he can also be sure when the children aren't his because they don't resemble him. This will create a very strong selection pressure in favor of children resembling their fathers. And the tendency to resemble one's father will create a positive feedback loop: the more likely kids are to look like their fathers, the more likely that children who don't resemble their fathers will be abandoned, neglected, abused, or killed. So if there is a tendency for infants to resemble their fathers more than their mothers, one would expect it to be magnified over time, and to be fairly large by now. But none of the studies (including the original Christenfeld and Hill one) found a strong tendency for children to resemble their fathers.

But, as French notes, it's hard to get people to pay attention to a negative result, to a paper that says that something interesting isn't happening.

[ Addendum 20061206: Here's the original Christenfeld and Hill paper. ]

[ Addendum 20171101: A recent web search refutes my claim that “the pop science press was totally uninterested”. Results include this article from Scientific American and a mention in the book Bumpology: The Myth-Busting Pregnancy Book for Curious Parents-To-Be. ]

[ Addendum 20171101: A more extensive study in 2004 confirmed French's results. ]

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Clockwise
A couple of weeks ago I was over at a friend's house, and was trying to explain to her two-year-old daughter which way to turn the knob on her Etch-a-Sketch. But I couldn't tell her to turn it clockwise, because she can't tell time yet, and has no idea which way is clockwise.

It occurs to me now that I may not be giving her enough credit; she may know very well which way the clock hands go, even though she can't tell time yet. Two-year-olds are a lot smarter than most people give them credit for.

Anyway, I then began wonder what "clockwise" and "counterclockwise" were called before there were clocks with hands that went around clockwise. But I knew the answer to that one: "widdershins" is counterclockwise; "deasil" is clockwise.

Or so I thought. This turns out not to be the answer. "Deasil" is only cited by the big dictionary back to 1771, which postdates clocks by several centuries. "Widdershins" is cited back to 1545. "Clockwise" and "counter-clockwise" are only cited back to 1888! And a full-text search for "clockwise" in the big dictionary turns up nothing else. So the question of what word people used in 1500 is still a mystery to me.

That got me thinking about how asymmetric the two words "deasil" and "widdershins" are; they have nothing to do with each other. You'd expect a matched set, like "clockwise" and "counterclockwise", or maybe something based on "left" and "right" or some other pair like that. But no. "Widdershins" means "the away direction". I thought "deasil" had something to do with the sun, or the day, but apparently not; the "dea" part is akin to dexter, the right hand, and the "sil" part is obscure. Whereas the "shins" part of "widdershins" does have something to do with the sun, at least by association. That is, it is not related historically to the sun, except that some of the people using the word "widdershins" were apparently thinking it was actually "widdersun". What a mess. And the words have nothing to do with each other anyway, as you can see from the histories above; "widdershins" is 250 years older than "deasil".

The OED also lists "sunways", but the earliest citation is the same as the one for deasil.

Anyway, I did not know any of this at the time, and imagined that "deasil" meant "in the direction of the sun's motion". Which it is; the sun goes clockwise through the sky, coming up on the left, rising to its twelve-o'-clock apex, and then descending on the right, the way the hands of a clock do. (Perhaps that's why the early clockmakers decided to make the hands of the clock go that way in the first place. Or perhaps it's because of the (closely related) reason that that's the direction that the shadow on a sundial moves.)

And then it hit me that in the southern hemisphere, the sun goes the other way: instead of coming up on the left, and going down on the right, the way clock hands do, it comes up on the right and goes down on the left. Wowzers! How bizarre.

I'm a bit sad that I figured this out before actually visiting the southern hemisphere and seeing it for myself, because I think I would have been totally freaked out on that first morning in New Zealand (or wherever). But now I'm forewarned that the sun goes the wrong way down there and it won't seem so bizarre when I do see it for the first time.

[Other articles in category /lang] permanent link

Trivial calculations
Back in September, I wrote about how I tend to plunge ahead with straightforward calculations whenever possible, grinding through the algebra, ignoring the clever shortcut. I'll go back and look for the shortcut, but only if the hog-slaughtering approach doesn't get me what I want. This is often an advantage in computer programming, and often a disadvantage in mathematics.

This occasionally puts me in the position of feeling like a complete ass, because I will grind through some big calculation to reach a simple answer which, in hindsight, is completely obvious.

One early instance of this that I remember occurred more than twenty years ago when a friend of mine asked me how many spins of a slot machine would be required before you could expect to hit the jackpot; assume that the machine has three wheels, each of which displays one of twenty symbols, so the chance of hitting the jackpot on any particular spin is 1/8,000. The easy argument goes like this: since the expected number of jackpots per spin is 1/8,000, and expectations are additive, 8,000 spins are required to get the expected number of jackpots to 1, and this is in fact the answer.

But as a young teenager, I did the calculation the long way. The chance of getting a jackpot in one spin is 1/8000. The chance of getting one in exactly two spins is (1 - 1/8000)·(1/8000). The chance of getting one in exactly three spins is (1 - 1/8000)²·(1/8000). And so on; so you sum the infinite series:

$$\sum_{i=1}^\infty {i{\left({k-1\over k}\right)^{i-1}} {1\over k}}$$

I'd like to be able say I never forgot this. I wish it were true. I did remember it a lot of the time. But I can think of a couple of times when I forgot, and then felt like an ass when I did the problem the long way.

One time was in 1996. There is a statistic in baseball called the on-base percentage or "OBP"—please don't all go to sleep at once. This statistic measures, for each player, the fraction of his plate appearances in which he is safe on base, typically by getting a hit, or by being walked. It is typically around 1/3; exceptional players have an OBP as high as 2/5 or even higher. You can also talk about the OBP of a team as a whole.

A high OBP is a very important determiner of the number of runs a baseball team will score, and therefore of how many games it will win. Players with a higher OBP are more likely to reach base, and when a batter reaches base, another batter comes to the plate. Teams with a high overall OBP therefore tend to bring more batters to the plate and so have more chances to score runs, and so do tend to score runs, than teams with a low overall OBP.

I wanted to calculate the relationship between team OBP and the expected number of batters coming to the plate each inning. I made the simplifying assumption that every batter on the team had an OBP of p, and calculated the expected number of batters per inning. After a lot of algebra, I had the answer: 3/(1-p). Which makes perfect sense: There are only two possible outcomes for a batter in each plate appearance: he can reach base, or he can be put out; these are exclusive. A batter who reaches base p of the time is put out 1-p of the time, consuming, on average, 1-p of an out. A team gets three outs in an inning; the three outs are therefore consumed after 3/(1-p) batters. Duh.

This isn't the only baseball-related mistake I've made. I once took a whole season's statistics and wrote a program to calculate the average number of innings each pitcher pitched. Okay, no problem yet. But then I realized that the average was being depressed by short relievers; I really wanted the average only for starting pitchers. But how to distinguish starting pitchers from relievers? Simple: the statistics record the number of starts for each pitcher, and relievers never start games. But then I got too clever. I decided to weight each pitcher's contribution to the average by his number of starts. Since relievers never start, this ignores relievers; it allows pitchers who do start a lot of games to influence the result more than those who only pitched a few times. I reworked the program to calculate the average number of innings pitched per start.

The answer was 9. (Not exactly, but very close.)

It was obviously 9. There was no other possible answer. There is exactly one start per game, and there are 9 innings per game,¹ and some pitcher pitches in every inning, so the number of innings pitched per start is 9. Duh.

Anyway, the real point of this article is to describe a more sophisticated mistake of the same general sort that I made a couple of weeks ago. I was tinkering around with a problem in pharmacology. Suppose Joe has some snakeoleum pills, and is supposed to take one capsule every day. He would like to increase the dosage to 1.5 pills every day, but he cannot divide the capsules. On what schedule should he take the pills to get an effect as close as possible to the effect of 1.5 pills daily?

I assumed that we could model the amount of snakeoleum in Joe's body as a function f(t), which normally decayed exponentially, following f(t) = ae^-kt for some constant k that expresses the rate at which Joe's body metabolizes and excretes the snakeoleum. Every so often, Joe takes a pill, and at these times d₀, d₁, etc., the function f is discontinuous, jumping up by 1. The value a here is the amount of snakeoleum in Joe's body at time t=0. If Joe takes one pill every day, the (maximum) amount of snakeoleum in his body will tend toward 1/(1-e^-k) over time, as in the graph below:

I wanted to compare this with what happens when Joe takes the pill on various other schedules. For some reason I decided it would be a good idea to add up the total amount of pill-minutes for a particular dosage schedule, by integrating f. That is, I was calculating !!\int_a^b f(t) dt!! for various a and b; for want of better values, I calculated the total amount !!\int_0^\infty f(t) dt!!.

Doing this for the case in which Joe takes a single pill at time 0 is simple; it's just !!\int_0^\infty e^{-kt} dt!!, which is simply 1/k.

But then I wanted to calculate what happens when Joe takes a second pill, say at time M. At time M, the amount of snakeoleum left in Joe's body from the first pill is e^-kM, so the function f has f(t) = e^-kt for 0 ≤ t ≤ M and f(t) = (e^-kM+1)e^-k(t-M) for M ≤ t < ∞. The graph looks like this:

$$\halign{ \hfil$#$ & $= # \hfil $ & $+ #\hfil$ \cr \int_0^\infty f(t) dt & \int_0^M f(t) dt & \int_M^\infty f(t) dt \cr & \int_0^M e^{-kt} dt & \int_M^\infty (e^{-kM} + 1)e^{-k(t-M)} dt \cr & {1 \over k }(1-e^{-kM}) & {1\over k}(e^{-kM} + 1)\cr & \ldots & \omit \cr }$$.

In retrospect, this is completely obvious.

This is because of the way I modeled the pills. When the decay of f(t) is exponential, as it is here, that means that the rate at which the snakeoleum is metabolized is proportional to the amount: twice as much snakeoleum means that the decay is twice as fast. or, looked at another way, each pill decays independently of the rest. Put two pills in, and an hour later you'll find that you have twice as much left as if you had only put one pill in.

Since the two pills are acting independently, you can calculate their effect independently. That is, f(t) can be decomposed into f₁(t) + f₂(t), where f₁ is the contribution from the first pill:

Anyway, I think what I really want is to find !!\int_0^\infty {\left(f_1(t) - f_{1.5}(t, d)\right)}^2 dt!!, where f₁ is the function that describes the amount of snakeoleum when Joe takes one pill a day, and f_1.5 is the function that describes the amount when Joe takes 1.5 pills every d days. But if there's one thing I think you should learn from a dumbass mistake like this, it's that it's time to step back and try to consider the larger picture for a while, so I've decided to do that before I go on.

¹[ Addendum 20070124: There is a brief explanation of why the average baseball game has almost exactly 9 innings. ]

[Other articles in category /oops] permanent link

Legal status of corpses in 1911 England
As you might expect from someone who browses at random in the library stacks, I own several encyclopedias, which I also browse in from time to time. You never know what you are going to find when you do this.

I got rid of one recently. It was a 1962 Grolier's. Obviously, it was out of date, but I was using it for general reference anyway, conscious of its shortcomings. But one day I picked it up to read its article on Thurgood Marshall. It said that Marshall was an up-and-coming young lawyer, definitely someone to watch in the future. That was too much, and I gave it away.

But anyway, my main point is to talk about the legal status of corpses. One of the encyclopedias I have is a Twelfth Edition Encyclopaedia Britannica. This contains the complete text of the famous 1911 Eleventh Edition, plus three fat supplementary volumes that were released in 1920. The Britannica folks had originally planned the Twelfth Edition for around 1930, but so much big stuff happened between 1911 and 1920 that they had to do a new edition much earlier.

The Britannica is not as much fun as I hoped it would be. But there are still happy finds. Here is one such:

CORPSE (Lat. corpus, the body), a dead human body. By the common law of England a corpse is not the subject of property nor capable of holding property. It is not therefore larceny to steal a corpse, but any removal of the coffin or grave-cloths is otherwise, such remaining the property of the persons who buried the body. It is a misdemeanour to expose a naked corpse to public view. . .

(The complete article is available online.)

[ Addendum 20191213: it is a felony in California to sexually penetrate or to have sexual contact with human remains, except as authorized by law. ]

[Other articles in category /law] permanent link

favicon.ico Results
A couple of days ago I asked for suggestions for a favicon icon to represent the blog in people's shortcut menus. There were only two responses, but they were both very helpful, and solved the problem.

My first thought was, of course, to use an octopus, but I immediately rejected this idea since I didn't think I would be able to draw a recognizable octopus in 16×16 pixels. Neil Kandalgaonkar was braver than I was: .

However, the concept I decided to go with was suggested by David Eppstein, who provided this attractive interpretation: . To explain this, I need to explain my domain name, which I haven't done here before.

For nine years I was an independent consultant, working under the name Plover Systems. Why Plover? Many people assume that it is an abbreviation for "Perl lover"; this is not the case. The plover domain predates my involvement with Perl. (Some people have also interpreted it as "piss lover", a veiled statement of a fetishistic attraction to urination. It is not.)

A plover is a small bird. Typically, they make their nests on the seashore. The Egyptian plover is the little bird that is reputed to snatch food scraps from between the teeth of the crocodile. The American golden plover migrates all the way from Alaska to South America, and sometimes across the ocean to Europe.

Immediately prior to my becoming an independent consultant and setting up Plover Systems and the plover.com domain, I was employed as the senior systems engineer for Pathfinder, the Time Warner web site. I did not like this job very much. After I quit, I joked that my company had grown too big, so I downsized most of the management and employees, divested the magazine business, and cut the organization's mission back to core competences. Time Warner, the subsidiary I spun off to publish the magazines, was very large. I named my new, lean, trim company "Plover" because the plover is small and agile.

There is another reason for "Plover". For about thirty years, I have been a devoted fan of the old computer game Adventure. When time came to choose a domain name, I wanted to choose something with an Adventure connection. Here the obvious choices are xyzzy, which was already taken, and plugh, which is ugly. Both of these are magic words which, uttered at the correct spot, will teleport the player to another location. The game has a third such magic word, which is "plover"; from the right place, it transports the player to the "Plover room":

You're in a small chamber lit by an eerie green light. An extremely narrow tunnel exits to the west. A dark corridor leads NE.

The Plover room is so-called because it contains "an emerald the size of a plover's egg". A plover's egg is not very big, as eggs go, because the plover is not a very large bird, as birds go. But an emerald the size of a plover's egg is enormous, as emeralds go. The description is a reference to an off-color joke that was current in the early 1970's when Adventure was written: a teenage girl, upon hearing that the human testicle is the size of a plover's egg, remarks "Oh, so that's how big a plover's egg is." I think this was somewhat more risqué in 1974 than it is today.

For his contribution, M. Eppstein has won a free two-year subscription to The Universe of Discourse. Neil Kandalgaonkar gets the runner-up prize of two free six-month subscriptions, to run concurrently. Thank you both!

[Other articles in category /meta] permanent link

Baseball team nicknames, again
Some addenda to my recent article about baseball team nicknames.

Several people wrote to complain that I mismatched the cities and the nicknames in this sentence:

The American League [has] the Boston Royals, the Kansas City Tigers, the Detroit Indians, the Oakland Orioles...

My apologies for the error. It should have been the Boston Tigers, the Kansas City Indians, the Detroit Orioles, and the Oakland Royals.

Phil Varner reminded me that the Chicago Bulls are in fact a "local color" name; they are named in honor of the Chicago stockyards.

This raises a larger point, brought up by Dave Vasilevsky: My classification of names into two categories conflates some issues. Some names are purely generic, like the Boston Red Sox, and can be transplanted anywhere. Other names are immovable, like the Philadelphia Phillies. In between, we have a category of names, like the Bulls, which, although easily transportable, are in fact local references.

The Milwaukee Brewers are a good baseball example. The Brewers were named in honor of the local German culture and after Milwaukee's renown as a world center of brewing. Nobody would deny that this is a "local color" type name. But the fact remains that many cities have breweries, and the name "Brewers" would work well in many places. The Philadelphia Brewers wouldn't be a silly name, for example. The only place in the U.S. that I can think of offhand that fails as a home for the Brewers is Utah; the Utah Brewers would be a bad joke. (This brings us full circle to the observation about the Utah Jazz that inspired the original article.)

The Baltimore Orioles are another example. I cited them as an example of a generic and easily transportable name. But the Baltimore Oriole is in fact a "local color" type name; the Baltimore Oriole is named after Lord Baltimore, and is the state bird of Maryland. (Thanks again to Dave Vasilevsky and to Phil Gregory for pointing this out.)

Or consider the Seattle Mariners. The name is supposed to suggest the great port of Seattle, and was apparently chosen for that reason. (I have confirmed that the earlier Seattle team, the Seattle Pilots, was so-called for the same reason.) But the name is transportable to many other places: it's easy to imagine alternate universes with the New York Mariners, the Brooklyn Mariners, the San Francisco Mariners, or the Boston Mariners. Or even all five.

And similarly, although in the previous article I classed the New York Yankees with the "local color" names, based on the absurdity of the Selma or the Charleston Yankees, the truth is that the Boston Yankees only sounds strange because it didn't actually happen that way.

I thought about getting into a tremendous cross-check of all 870 name-city combinations, but decided it was too much work. Then I thought about just classing the names into three groups, and decided that the issue is too complex to do that. For example, consider the Florida Marlins. Local color, certainly. But immovable? Well, almost. The Toronto Marlins or the Kansas City Marlins would be jokes, but the Tampa Bay Marlins certainly wouldn't be. And how far afield should I look? I want to class the Braves as completely generic, but consideration of the well-known class AA Bavarian League Munich Braves makes it clear that "Braves" is not completely generic.

So in ranking by genericity, I think I'd separate the names into the following tiers:

1. Pirates, Cubs, Reds, Cardinals, Giants, Red Sox, Blue Jays, White Sox, Tigers, Royals, Athletics
2. Braves, Mets, Dodgers, Orioles, Yankees, Indians, Angels, Mariners, Nationals, Brewers
3. Marlins, Astros, Diamondbacks, Rockies, Padres, Devil Rays, Twins
4. Phillies, Rangers

Readers shouldn't take this classification as an endorsement of the Phillies' nickname, which I think is silly. I would have preferred the Philadelphia Brewers. Or even the Philadelphia Cheese Steaks. Maybe they didn't need the extra fat, but wouldn't it have been great if the 1993 Phillies had been the 1993 Cheese Steaks instead? Doesn't John Kruk belong on a team called the Cheese Steaks?

Another oddity, although not from baseball: In a certain sense, the Montreal Canadiens have an extremely generic name. And yet it's clearly not generic at all!

[Other articles in category /lang] permanent link

Etymological oddity
Sometimes you find words that seem like they must be related, and then it turns out to be a complete coincidence.

Consider pen and pencil.

Pen is from French penne, a long feather or quill pen, akin to Italian penne (the hollow, ribbed pasta), and ultimately to the word feather itself.

Pencil is from French pincel, a paintbrush, from Latin peniculus, also a brush, from penis, a tail, which is also the source of the English word penis.

A couple of weeks ago someone edited the Wikipedia article on "false cognates" to point out that day and diary are not cognate. "No way," I said, "it's some dumbass putting dumbassery into Wikipedia again." But when I checked the big dictionary, I found that it was true. They are totally unrelated. Diary is akin to Spanish dia, Latin dies, and other similar words, as one would expect. Day, however, is "In no way related to L. dies..." and is akin to Sanskrit dah = "to burn", Lithuania sagas = "hot season", and so forth.

[Other articles in category /lang/etym] permanent link

Linogram: The EaS as a component
In an earlier article, I discussed the definition of an Etch-a-Sketch picture as a component in the linogram drawing system. I then digressed for a day to rant about the differences between specification-based and WYSIWYG systems. I now return to the main topic.

Having defined the EAS component, I can use it in several diagrams. The typical diagram looks like this:

        require "eas";

        number WIDTH = 2;

        EAS the_eas(w = WIDTH);

        gear3 gears(width=WIDTH, r1=1/4, r3=1/12);

        constraints { 
            the_eas.left = gears.g1.c;
            the_eas.right = gears.g3.c; 
        }

This sort of thing was exactly what I was hoping for when I started designing linogram. I wanted it to be easy to define a new component type, like EAS or gear3, and to use it as if it were a built-in type. (Indeed, as noted before, even linogram's "built-in" types are not actually built in! Everything from point on up is defined by a .lino file just like the one above, and if the user wants to redefine point or line, they are free to do so.)

(It may not be clear redefining point or line is actually a useful thing to do. But it could be. Consider, for example, what would happen if you were to change the definition of point to contain a z coordinate, in addition to the x and y coordinates it normally has. The line and box definitions would inherit this change, and become three-dimensional objects. If provided with a suitably enhanced rendering component, linogram would then become a three-dimensional diagram-drawing program. Eventually I will add this enhancement to linogram.)

I am a long-time user of the AT&T Unix tool pic, which wants to allow you to define and use compound objects in this way, but gets it badly wrong, so that it's painful and impractical. Every time I suffered through pic's ineptitude, I would think about how it might be done properly. I think linogram gets it right; pic was a major inspiration.

Slanty lines

  line L(slope=..., start=..., ...);

In general, though, this is too hard to handle. The resulting equations are quadratic, or worse, trigonometric, and linogram does not know how to solve those kinds of equations.

But if the slope is required to be constant, then the quadratic equations become linear, and there is no problem. And in this case, I only needed constant slopes. Once I realized this, it was easy to define a constant-slope line type:

        require "line";

        define sline extends line {
          param number slope;
          constraints {
            end.y - start.y = slope * (end.x - start.x);
          }
        }

The diagram above was generated from this specification:

        require "eas";
        require "sline";

        number WIDTH = 2;

        EAS the_eas(w = WIDTH);

        gear3 gears(width=WIDTH, r1=1/4, r3=1/12);
        sline L1(slope=1/3, start=the_eas.screen.ne);
        sline L2(slope=3, start=the_eas.screen.ne);

        constraints { 
            the_eas.left = gears.g1.c;
            the_eas.right = gears.g3.c; 
            L1.end.x = the_eas.screen.w.x;
            L2.end.y = the_eas.screen.s.y;
        }

The additions here are the sline items, named L1 and L2. Both lines start at the northeast corner of the screen. Line L1 has slope 1/3, and its other endpoint is constrained to lie somewhere on the west edge of the screen. The y-coordinate of that endpoint is not specified, but is implicitly determined by the other constraints. To locate it, linogram must solve some linear equations. The complete set of constraints no the line is:

L1.start.x = the_eas.screen.ne.x

L1.start.y = the_eas.screen.ne.y

L1.end.x = the_eas.screen.w.x

L1.end.y - L1.start.y = L1.slope × (L1.end.x - L1.start.x);

The L1.slope is required to be specified before the equations are solved, and in the example above, it is 1/3, so the last of these equations becomes:

L1.end.y - L1.start.y = 1/3 × (L1.end.x - L1.start.x);

So the resulting system is entirely linear, and is easily solved.

The other line, L2, with slope 3, is handled similarly; its endpoint is constrained to lie somewhere on the south side of the screen.

Surprise features

One of the signs that you have designed a system well is that it comes out to be more powerful than you expected when you designed it, and lends itself to unexpected uses. The slines are an example of that. When it occurred to me to try doing them, my first thought was "but that won't work, will it?" But it does work.

Here's another technique I hadn't specifically planned for, that is already be supported by linogram. Suppose Fred Flooney wants to use the eas library, but doesn't like the names of the reference points in the EAS component. Fred is quite free to define his own replacement, with whatever names for whatever reference points he likes

        require "eas";

        define freds_EAS {
          EAS it;
          point middle = it.screen.c;
          point bernard = (it.body.e + 2*it.body.se)/3;
          line diag(start=it.body.nw, end=it.body.se);
          draw { it; }
        }

The draw section tells linogram that only the component named it—that is, the Etch-a-Sketch itself—should be drawn; this suppresses the diag line. which would otherwise have been rendered also.

If Fred inherits some other diagram or component that includes an Etch-a-Sketch, he can still use his own aliases to refer to the parts of the Etch-a-Sketch, without modifying the diagram he inherited. For example, suppose Fred gets handed my specification for the diagram above, and wants to augment it or incorporate it in a larger diagram. The diagram above is defined by a file called "eas4-3-12.lino", which in turn requires EAS.lino. Fred does not need to modify eas4-3-12.lino; he can do:

        require "eas4-3-12.lino";
        require "freds_eas";

        freds_EAS freds_eas(it = the_eas);

        constraints { 
          freds_eas.middle = ...;
          freds_eas.bernard = ...;
        }
        ...

Why Fred wants these names for these components, I don't know; it's just a contrived example. But the important point is that if he does want them, he can have them, with no trouble.

What next?

  g1.e = g2.w;
  g2.e = g3.w;

  define gears[n] open {
    gear[n] g;
    constraints {
      g[i].e = g[i+1].w;
    }
  }

For gear[3], two constraints are generated: g[0].e = g[1].w, and g[1].e = g[2].w. Normally, arrays are cyclic, and a third constraint, g[2].e = g[0].w, would be generated as well. The open keyword suppresses this additional constraint.

This feature is under development now. I originally planned it to support splines, which can have any number of control points. But once again, I found that the array feature was going to be useful for many other purposes.

When the array feature is finished, the next step will be to create a spline type: spline[4] will be a spline with 4 control points; spline[7] will be a spline with 7 control points, and so on. PostScript will take care of drawing the splines for me, so that will be easy. I will also define a regular polygon type at that time:

        define polygon[n] closed {
          param number rotation = 0;
          number radius;
          point v[n], center;
          line  e[n];
          constraints {
            v[i] = center + radius * cis(rotation + i * 360/n);
            e[i].start = v[i];
            e[i].end   = v[i+1];
          }
        } 

Buy Higher-Order Perl from Bookshop.org (with kickback) (without kickback)

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Damning with faint praise
If you have used evaporated milk, you may have noticed that the label says something like:

By adding one part water to one part of the contents of this can, a resulting milk product will be obtained which will not be below the legal standard for whole milk.

This sounds ghastly, doesn't it? "Will not be below the legal standard...". Shudder.

The purpose of this note is to let you know that:

1. I have done this,
2. more than once, and
3. it not only wasn't as ghastly as it sounds,
4. it wasn't ghastly at all.

From the warning on the label, you would expect maybe a 30% resemblance to milk; in truth, the resemblance is more like 85%. That's close enough to drink plain, if you're not too fussy, and it's certainly close enough to pour over your cereal without noticing the difference.

The wording of the label scares people off, but it works quite well, well enough that it is probably worth keeping a couple of cans in the closet for emergencies, like when you run out of milk for your cereal at 2 AM after the store is closed.

This has been a public service announcement of the Universe of Discourse. Happy Thanksgiving, everyone!

[Other articles in category /food] permanent link

Baseball team nicknames
Lorrie and I were in the car, and she noticed another car with a Detroit Pistons sticker. She remarked that "Pistons" was a good name for a basketball team, and particularly for one from Detroit. I agreed. But then she mentioned the Utah Jazz, a terrible mismatch, and asked me how that happened to be. Even if you don't know, you can probably guess: They used to be the New Orleans Jazz, and the team moved to Utah. They should have changed the name to the Teetotalers or the Salt Flats or something, but they didn't, so now we have the Utah Jazz. I hear that next month they're playing the Miami Fightin' Irish.

That got us thinking about how some sports team names travel, and others don't. Jazz didn't. The Miami Heat could trade cities or names with the Phoenix Suns and nobody would notice. But consider the Chicago Bulls. They could pick up and move anywhere, anywhere at all, and the name would still be fine, just fine. Kansas City Bulls? Fine. Honolulu Bulls? Fine. Marsaxlokk Bulls? Fine.

We can distinguish two categories of names: the "generic" names, like "Bulls", and the "local color" names, like "Pistons". But I know more about baseball, so I spent more time thinking about baseball team names.

In the National League, we have the generic Braves, Cardinals, Cubs, Giants, Pirates, and Reds, who could be based anywhere, and in some cases were. The Braves moved from Boston to Milwaukee to Atlanta, although to escape from Boston they first had to change their name from the Beaneaters. The New York Giants didn't need to change their name when they moved to San Francisco, and they won't need to change their name when they move to Jyväskylä next year. (I hear that the Jyväskylä city council offered them a domed stadium and they couldn't bear to say no.)

On the other hand, the Florida Marlins, Arizona Diamondbacks, and Colorado Rockies are clearly named after features of local importance. If the Marlins were to move to Wyoming, or the Rockies to Nebraska, they would have to change their names, or turn into bad jokes. Then again, the Jazz didn't change their name when they moved to Utah.

The New York Mets are actually the "Metropolitans", so that has at least an attempt at a local connection. The Washington Nationals ditto, although the old name of the Washington Senators was better. At least in that one way. Who could root for a team called the Washington Senators? (From what I gather, not many people could.)

The Nationals replaced the hapless Montreal Expos, whose name wasn't very good, but was locally related: they were named for the 1967 Montreal World's Fair. Advice: If you're naming a baseball team, don't choose an event that will close after a year, and especially don't choose one that has already closed.

The Houston Astros, and their Astrodome filled with Astroturf, are named to recall the NASA manned space center, which opened there in 1961. The Philadelphia club is called the Phillies, which is not very clever, but is completely immovable. Boston Phillies, anyone? Pittsburgh Phillies? New York Phillies? No? I didn't think so.

I don't know why the San Diego Padres are named that, but there is plenty of Spanish religious history in the San Diego area, so I am confident in putting them in the "local color" column. Milwaukee is indeed full of Brewers; there are a lot of Germans up there, brewing up lager. (Are they back in the National League again? They seem to switch leagues every thirty years.)

That leaves just the Los Angeles Dodgers, who are a bit of an odd case. The team, as you know, was originally the Brooklyn Dodgers. The "Dodgers" nickname, as you probably didn't know, is short for "Trolley Dodgers". The Los Angeles Trolley Dodgers is almost as bad a joke as the Nebraska Rockies. Fortunately, the "Trolley" part was lost a long time ago, and we can now imagine that the team is the Los Angeles Traffic Dodgers. So much for the National League; we have six generic names out of 16, counting the Traffic Dodgers in the "local color" group, and ignoring the defunct Expos.

The American League does not do so well. They have the Boston Royals, the Kansas City Tigers, the Detroit Indians, the Oakland Orioles, and three teams that are named after sox: the Red, the White, and the Athletics.

Then there are the Blue Jays. They were originally owned by Labatt, a Canadian brewer of beer, and were so-named to remind visitors to the park of their flagship brand, Labatt's Blue. I might have a harder time deciding which group to put them in, if it weren't for the (1944-1945) Philadelphia Blue Jays. If the name is generic enough to be transplanted from Toronto to Philadelphia, it is generic. I have no idea what name the Toronto club could choose if they wanted to avail themselves of the "local color" option rather than the "generic" option; it's tempting to make a cruel joke and suggest that the name most evocative of Toronto would be the Toronto Generics. But no, that's unfair. They could always call their baseball club the Toronto Hockey Fans.

Anyway, moving on, we have the New York Yankees, which is not the least generic possible name, but clearly qualifies as "local color" once you pause to think about the Charleston Yankees, the Shreveport Yankees, and the Selma Yankees. The Tampa Bay Devil Rays are clearly "local color". The Minnesota Twins play in the Twin Cities of Minneapolis and St. Paul. The California, Anaheim, or Los Angeles Angels, whatever they're called this week, are evidently named for the city of Los Angeles. I would ridicule the Los Angeles Angels for having a redundant name, but as an adherent of the Philadelphia Phillies, I am living in a glass house.

The Texas Rangers are named for the famous Texas Rangers. I don't know exactly why the Seattle club is named Mariners; I wouldn't have considered Seattle to be an unusually maritime city, but their previous team was the Seattle Pilots, so the folks in Seattle must think of themselves so, and I'm willing to go along with it.

The tally for the American League is therefore eight generic, six local color. The total for Major League Baseball as a whole is 14 generic names out of 30.

This is a lot better than the Japanese Baseball League, which has a bunch of teams with names like the Lions, Tigers, Dragons, Giants, and Fighters. They make up for this somewhat in the names of the teams' corporate sponsors, so, for example, the Nippon Ham Fighters. They are sponsored by Nippon Ham, which does not make it any less funny. And the Yakult Swallows, which, if you interpret it as a noun phrase, sounds just a little bit like a gay porn flick set in Uzbekistan.

Incidentally, my favorite team name is the Wilmington Blue Rocks. The Blue Rocks' mascot is, alas, not a rock but a moose. Sometimes I dream of a team from Lansing, Michigan, called the Lansing Boils, but I know it will remain an unfulfilled fantasy.

[ Warning for non-Americans: Almost, but not quite everything in this article is the truth. Marsaxlokk does not actually have a Major League baseball club yet; however, they do have a class-A affiliate in the Mediterranean league, called the Marsaxlokk Moghzaskops. Also, the Giants are not scheduled to move to Jyväskylä until after the 2008 season. ]

[ Addendum 20061127: There is a followup article to this one. ]

[ Addendum 20230425: I can't believe it took me this long to realize it, but the Los Angeles Lakers is just as strange a mismatch as the Utah Jazz. There are no lakes near Los Angeles. That name itself tells you what happened: the team was originally located in Minneapolis. ]

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Linogram: Declarative drawing
As we saw in yesterday's article, The definition of the EAS component is twenty lines of strange, mostly mathematical notation. I could have drawn the Etch-a-Sketch in a WYSIWYG diagram-drawing system like xfig. It might have been less trouble. Many people will prefer this. Why invent linogram?

Some of the arguments should be familiar to you. The world is full of operating systems with GUIs. Why use the Unix command line? The world is full of WYSIWYG word processors. Why use TeX?

Text descriptions of processes can be automatically generated, copied, and automatically modified. Common parts can be abstracted out. This is a powerful paradigm.

Collectively, the diagrams contained 19 "gears". Partway through, I decided that the black dot that represented the gear axle was too small, and made it bigger. Had I been using a WYSIWYG system, I would have had the pleasure of editing 19 black dots in 10 separate files. Then, if I didn't like the result, I would have had the pleasure of putting them back the way they were. With linogram, all that was required was to change the 0.02 to an 0.05 in eas.lino:

        define axle {
          param number r = 0.05;
          circle a(fill=1, r=r);
        }

The Etch-a-Sketch article contained seven similar diagrams with slight differences. Each one contained a require "eas"; directive to obtain the same definition of the EAS component. Partway through the process, I decided to alter the aspect ratio of the Etch-a-Sketch body. Had I been drawing these with a WYSIWYG system, that would have meant editing each of the seven diagrams in the same way. With linogram, it meant making a single trivial change to eas.lino.

A linogram diagram has a structure: it is made up of component parts with well-defined relationships. A line in a WYSIWYG diagram might be 4.6 inches long. A line in a linogram diagram might also be 4.6 inches long, but that is probably not all there is to it. The south edge of the body box in my diagrams is 4.6 inches long, but only because it has been inferred (from other relationships) to be 1.15 w, and because w was specified to be 4 inches. Change w, and everything else changes automatically to match. Each part moves appropriately, to maintain the specified relationships. The distance from the knob centers to the edge remains 3/40 of the distance between the knobs. The screen remains 70% as tall as the body. A WYSIWYG system might be able to scale everything down by 50%, but all it can do is to scale down everything by 50%; it doesn't know enough about the relationships between the elements to do any better. What will happen if I reduce the width but not the height by 50%? The gears are circles; will the WYSIWYG system keep them as circles? Will they shrink appropriately? Will their widths be adjusted to fit between the two knobs? Maybe, or maybe not. In linogram, the required relationships are all explicit. For example, I specified the size of the black axle dots in absolute numbers, so they do not grow or shrink when the rest of the diagram is scaled.

Finally, because the diagrams are mathematically specified, I can leave the definitions of some of the components implicit in the mathematics, and let linogram figure them out for me. For example, consider this diagram:

        gear3 gears(width=WIDTH, r1=1/4, r3=1/12);

I specified r1, the radius of the left gear, and r3, the radius of the right gear. Where is the middle gear? It's implicit in the definition of the gear3 type. The definition knows that the three gears must all touch, so it calculates the radius of the middle gear accordingly:

define gear3 {
  ...
  number r2 = (1 - r1 - r3) / 2;
  ...
}

linogram gives me the option of omitting r2 and having it be calculated for me from this formula, or of specifying r2 anyway, in which case linogram will check it against this formula and raise an error if the values don't match.

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More complete information about linogram is available in Chapter 9 of Higher-Order Perl; complete source code is available from the linogram web site.

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Linogram: The EaS component
In yesterday's article I talked about the basic facilities provided by linogram. What about the Etch-a-Sketch diagrams?

The core of these diagrams was a specification I wrote for an Etch-a-Sketch itself, in a file called eas.lino. The specification is complicated, because an Etch-a-Sketch has many parts, but it is conceptually just like the definitions above: it defines a diagram component called an EAS that looks like an Etch-a-Sketch:

        define EAS {
          param number w;
          number knobrad = w * 1/16;
          circle hknob(r=knobrad, fill=0.25), vknob(r=knobrad, fill=0.25);
          point left = hknob.c, right = vknob.c;
          number margin = 3/40 * w;
          box body(sw = left + (-margin, -margin), se = right + (margin,  -margin),
                   ht = w * 1);
          box screen(wd = body.wd * 0.9, 
                     n = body.n + (0, -margin),
                     ht = body.ht * 0.7);

          number nudge = body.ht * 0.025;
          label Brand(text="Etch A Sketch") = (body.n + screen.n)/2 + (0, -nudge);

          constraints { left + (w, 0) = right; 
                        left.y = right.y = 0;
                        left.x = 0;
                      }
        }

I didn't, of course, write this all in one fell swoop. I built it up a bit at a time. Each time I changed the definition in the eas.lino file, the changes were inherited by all the files that contained require "eas".

The two main parts of the Etch-a-Sketch are the body (large outer rectangle) and screen (smaller inner rectangle), which are defined to be boxes:

        box body(...);
        box screen(...);

But most of the positions are ultimately referred to the centers of the two knobs. The knobs themselves are hknob and vknob, and their centers, hknob.c and vknob.c, are given the convenience names left and right:

          point left = hknob.c, right = vknob.c;

Down in the constraints section is a crucial constraint:

        left + (w, 0) = right; 

This constrains the right point (and, by extension, vknob.c and the circle vknob of which it is the center, and, by further extension, anything else that depends on vknob) to lie exactly w units east and 0 units south of the left point. The number w ("width") is declared as a "param", and is special: it must be specified by the calling context, to tell the Etch-a-Sketch component how wide it is to be; if it is omitted, compilation of the diagram fails. By varying w, we can have linogram draw Etch-a-Sketch components in different sizes. The diagram above had w=4; this one has w=2:

          number knobrad = w * 1/16;

So if you make w twice as big, the knobs get twice as big also.

The quantity margin is the amount of space between knobs and the edge of the body box, defined to be 3/40 of w:

          number margin = 3/40 * w;

Since the margin is 0.075 w, and the knobs have size 0.0625 w, there is a bit of extra space between the knobs and the edge of the body. Had I wanted to state this explicitly, I could have defined margin = knobrad * 1.15 or something of the sort.

The southwest and southeast corners of the body box are defined as offsets from the left and right reference points, which are at the centers of the knobs:

          box body(sw = left + (-margin, -margin), 
                   se = right + (margin, -margin),
                   ht = w * 1);

This implicitly specifies the width of the body box, since linogram can deduce it from the positions of the two bottom corners. The height of the body box is defined as being equal to w, making the height of the body equal to the distance between the to knobs. This is not realistic, since a real Etch-a-Sketch is not so nearly a square, but I liked the way it looked. Earlier drafts of the diagram had ht = w * 2/3, to make the Etch-a-Sketch more rectangular. Changing this one number causes linogram to adjust everything else in the entire diagram correspondingly; everything else moves around to match.

The smaller box, the screen, is defined in terms of the larger box, the body:

          box screen(wd = body.wd * 0.9, 
                     n = body.n + (0, -margin),
                     ht = body.ht * 0.7);

The only other features are the label and the fill property of the knobs. A label is defined by linogram's standard library. It is like a point, but with an associated string:

        require "point";

        define label extends point {
                param string text;
                draw { &put_string; }
        }

          number nudge = body.ht * 0.025;
          label Brand(text="Etch A Sketch") = 
                (body.n + screen.n)/2 + (0, -nudge);

The text="..." supplies the text parameter, which is handed off directly to put_string(). The rest of the constraint says that the text should be positioned halfway between the north points of the body and the screen boxes, but nudged southwards a bit. The nudge value is a fudge factor that I put in because I haven't yet gotten the PostScript drawing component to position the text at the exact right location. Indeed, I'm not entirely sure about the best way to specify text positioning, so I left that part of the program to do later, when I have more experience with text.

The fill parameter of the knobs is handled similarly to the text parameter of a label: it's an opaque piece of information that is passed to the drawing component for handling:

          circle hknob(r=knobrad, fill=0.25), vknob(r=knobrad, fill=0.25);

The PostScript drawing component then uses this when drawing the circles, eventually generating something like gsave 0.75 setgray fill grestore. The default value is 0, indicating white fill; here we use 0.25, which is light gray. (The PostScript drawing component turns this into 0.75, which is PostScript for light gray. PostScript has white=1 and black=0, but linogram has white=0 and black=1. I may reverse this before the final release, or make it a per-diagram configuration option.)

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More complete information about linogram is available in Chapter 9 of Higher-Order Perl; complete source code is available from the linogram web site.

[Other articles in category /linogram] permanent link

favicon.ico
I'm getting hundreds of failed requests for favicon.ico. I need to come up with a good-looking icon for the blog. Since the blog isn't about Higher-Order Perl, I don't want to use the HOP cover or the HOP quilt block icon .

I have not had any good ideas. So I am asking LazyNet.

The best suggestion will win a free one-year subscription to The Universe of Discourse blog. If the winner supplies an icon that I can actually use, the prize will be increased to a free two-year subscription.

Any suggestions?

[ Addendum 20061120: David Eppstein responded immediately with , which I will explain in a future article that summarizes the suggestions. M. Eppstein's suggestion is good enough that I am comfortable installing it right away. But don't let that stop you from coming up with your own suggestion. ]

[ Addendum 20061120: Neil Kandalgaonkar has contributed . Thank you, Neil! ]

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A series of articles about Linogram
Every six months or so I do a little work on linogram, which for about 18 months now has been about one week of hard work short of version 1.0. Every time I try to use it, I am surprised by how pleased I am with it, how easy I find it to use, and how flexible the (rather spare) basic feature set is—all properties of a well-designed system.

I was planning to write a short note about this, but, as always happens when I write about linogram, it turned into a very long note that included a tutorial about how linogram works, a rant about why it is a good idea, and a lot of technical examples. So I decided to break the 4,000-word article into smaller pieces, which I will serialize.

Most recently I used linogram to do schematic diagrams of an Etch-a-Sketch for some recent blog articles ([1] [2]). In case you have forgotten, here is an example:

Basic ideas of linogram

        B.start.x = A.end.x;
        B.start.y = A.end.y;

which one can (and normally would) abbreviate to:

        B.start = A.end;

The computer is very good at solving systems of linear equations, and can figure out all the things the equations imply. For example, consider two boxes, X and Y:

        X.ht = 2;     # height of box X
        Y.ht = 3;     # height of box Y
        X.s = Y.n;    # 's' is 'south'; 'n' is 'north'

These simple examples are not very impressive, but please bear with me.

The other fundamental idea in linogram is the idea of elements of a diagram as components that can be composed into more complicated elements. The linogram libraries have a definition for a box, but it is not a primitive. In fact, linogram has one, and only one primitive type of element: the number. A point is composed of two numbers, called x and y, and is defined by the point.lino file in the library:

        define point {
          number x, y;
        }

        require "point";

        define line {
          point start, end;
        }

        require "point";

        define line {
          point start, end, center;
          constraints { 
            center = (start + end) / 2;
          }
        }

center.x = (start.x + end.x) / 2

center.y = (start.y + end.y) / 2

From the specification above, the program can deduce the location of the center point given the two endpoints. But it can also deduce the location of either endpoint given the locations of the center and the other endpoint. The treatment of equations is completely symmetrical. In fact, the line is really, at the bottom, an agglomeration of six numbers (start.x, center.y, and so forth) and from any specification of two x coordinates and two y coordinates, the program can deduce the missing values.

The linogram standard library defines hline and vline as being like ordinary lines, but constrained to be horizontal and vertical, and the then box.lino defines a box as being two hlines and two vlines, constrained so that they line up in a box shape:

        require "hline";
        require "vline";

        define box {
          vline left, right;
          hline top, bottom;
          constraints {
            left.start  = top.start;
            right.start = top.end;
            left.end    = bottom.start;
            right.end   = bottom.end;
          }
        }

I have abridged this definition for easy reading; the actual definition in the file has more to it. Here it is, complete:

        require "hline";
        require "vline";
        require "point";

        define box {
          vline left, right;
          hline top, bottom;
          point nw, n, ne, e, se, s, sw, w, c;
          number ht, wd;
          constraints {
            nw = left.start  = top.start;
            ne = right.start = top.end;
            sw = left.end    = bottom.start;
            se = right.end   = bottom.end;

            n = (nw + ne)/2;
            s = (sw + se)/2;
            w = (nw + sw)/2;
            e = (ne + se)/2;

            c = (n + s)/2;

            ht = left.length;
            wd = top.length;
          }
        }

The additional components, like sw, make it easy to refer to the corners and edges of the box; you can refer to the southwest corner of a box B as B.sw. Even without this convenience, it would not have been too hard: B.bottom.start and B.left.end are names for the same place, as you can see in the constraints section.

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More complete information about linogram is available in Chapter 9 of Higher-Order Perl; complete source code is available from the linogram web site.

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Etch-a-Sketch blue-skying, corrected
In my last article I discussed a scheme for improving the Etch-a-Sketch which contained a serious mechanical error. I was discussing attaching gears to the two knobs of the Etch-a-Sketch to force them to turn at the exact same rate. Supposing that the distance between the knobs is 1 unit, I said, then we can gear the two knobs together by attaching a gear of radius 1/2 to each knob; the two gears will mesh, and the knobs will then turn at the same rate, in opposite directions. This was fine.

Then I went astray, and suggested adding an axle peg midway between the two knobs, and putting gears of radius 1/3 on the peg and on the two knobs. This won't work.

The one person who wrote to me to ask about the problem is a very bright person, but been seriously confused about how I was planning to set up the gears, so I evidently I didn't explain it very well. It needed a picture. So this time I'm going to try to get it right, with pictures. Here is an Etch-a-Sketch:

Here are some gears, which happen to have radii 1/3, 1/4, and 1/6:

Here's a picture of an Etch-a-Sketch with a radius-1/2 gear mounted on each knob:

Here the knobs have been fitted with different-sized gears, one with radius 1/3 and the other with radius 2/3:

Then I suggested that you could drill a little hole in between the two knobs, and use it to mount a third axle and a third gear. If all three gears are the same size, the two knobs are forced to turn at the same rate, this time in the same direction, and you get a line with slope 1, from southeast to northwest:

This wrecks the rest of the details of my other article. Since we were already including gears of size 1/2 and 1/3, I reasoned, we can throw in a gear of size 1/6 and get some new behaviors from the 1/2 + 1/3 + 1/6 combination. The corresponding combination for 1/2 and 1/4 is 1/8:

So what next? The calculations are a bit less obvious than they were back in the happy days when I thought that installing two gears of size p and q left space for one of size 1-(p+q). It's tempting to consider a radius-1/3 gear next, since it's the simplest size I haven't yet installed. But to mount it on the knobs along with a size-1/2 gear, we need to include a size-1/12 gear to go in between:

Once we have the size-1/12 gear, we can mount it with the size-1/4 and size-1/3 that we already had:

[Other articles in category /games] permanent link

Etch-a-Sketch
I've always felt that the Etch-a-Sketch is a superb example of a toy that doesn't do as much as it could.

An Etch-a-Sketch is a drawing toy invented in 1959 by Arthur Granjean and marketed by the Ohio Art company since shortly afterward. It looks superficially like a flat-screen television with two knobs.

It is very easy to draw horizontal and vertical lines, but very difficult to draw diagonal lines. (Wikipedia says "Creating a straight diagonal line or smoothly curved line with an Etch A Sketch is notoriously difficult and a true test of coordination.") So although extremely complex drawings can be made with an Etch-a-Sketch:

(Etch-a-Sketch drawing of Albert Einstein by Nicole Falzone)

But it needn't be so. The most frustrating thing about the Etch-a-Sketch, I think, is that its potential has not yet begun to be unlocked.

Consider a forty-five degree line. To draw such a line, one must turn both the horizontal and the vertical knobs at the same time, at exactly the same rate. Suppose, for concreteness, that we're drawing a line from the upper left to the lower right. If you turn the horizontal knob a little too quickly, the diagonal line will bend rightward; if you turn it a little too slowly the diagonal line will bend downward. So in contrast to the mathematically exact vertical and horizontal lines that are easy to draw, it's next to impossible to draw a diagonal line that doesn't wiggle. And when you screw up, you can't fix the mistake without erasing the whole thing and starting over.

But the solution is obvious: If you can link the two knobs somehow, so that they can only turn simultaneously, you can easily draw a diagonal line. As a child, I experimented with rubber bands, trying to get one knob to drive the other. This wasn't successful. Clearly, a better solution is to use gears.

There are plenty of examples of toys that have good-quality cast-plastic gears. (Spirograph is one such.) The knobs on the Etch-a-Sketch could be geared together. If the gears are the same size, the knobs will rotate at the same rate, and the result will be a perfect 45° line.

If you gear the two knobs together directly, they will rotate in opposite directions, so that you can only draw lines with slope -1 (northwest to southeast), not with slope 1 (northeast to southwest). To fix this problem, we need to introduce more gears. There can be an axle peg sticking up from the case of the Etch-a-Sketch, in between the two knobs. Mounting three equal-sized gears on the two knobs and the axle peg gears will force the knobs to rotate in the same direction, at the same rate.

[ The remainder of this article contains a number of very dumb arithmetic errors. For example, you cannot fit three gears of size 1/3 on the knobs and pegs; you need to use three gears of size 1/4 instead. I will correct this on Monday, and provide an illustration to make it clearer what I mean. —MJD ]

[ Addendum 20061116: I have posted the correction, with illustrations. ]

Let's say that the distance between the centers of the two knobs is 1. We can get a line of slope -1 by mounting two gears, each with radius 1/2, on the two knobs; we can get a line of slope +1 by mounting three gears, each with radius 1/3, on the two knobs and on the axle peg. If we want to do both, we had better make the axle peg removable, or else it will interfere with the size-1/2 gears. This is no problem. It can mount into a socket on the front of the Etch-a-Sketch, and be pulled out when not needed.

But why have only one socket? We're including five gears already (two of size 1/2 and three of size 1/3) so we may as well put them to some more use. Throw in a size 1/6 gear, and add another socket for the axle peg, this time 1/3 of the way between the two knobs. Now you can mount a size 1/2 gear on the left knob, a size 1/6 gear on the axle peg, and a size 1/3 gear on the right knob. If the left knob turns at rate r, the middle gear turns at rate -3r and the right knob turns at rate 3r/2. This produces a line with slope 3/2, which is about a 56-degree angle.

Or put in another socket for the axle peg, 1/6 of the way between the knobs, and then mount size 1/2, size 1/3, and size 1/6 gears, in that order. The knobs are now producing a line with slope 3, a 72-degree angle. If you want a line with slope 1/3 (18°) instead, just reverse the order of the gears. (That is, exchange the large and the small ones.)

At this point adding a few more gears expands the repertoire significantly. Add a radius-2/3 gear and another radius-1/6 gear and you can mount [2/3, 1/3] to get lines with slope -1/2, [1/3, 2/3] to get slope -2, [2/3, 1/6, 1/6] to get slope 4, [1/6, 1/6, /23] to get slope 1/4.

Clearly, you can carry this onwards, limited only by the space for the axle holes and the expense of adding in more gears. Spirograph used to deliver fifteen or twenty plastic gears for a reasonable price, so it's clearly not implausible that Ohio Art could have done something like this.

Sometimes I even dare to think that they might have provided cams or elliptical gears. Properly designed cams could gear together the knobs to produce mathematically exact curved lines, squiggles, maybe even circles.

But no, as far as I can tell, it's never been done. Why not?

[Other articles in category /games] permanent link

Boring answers to Powell's questions
A while back I answered some questions for Powell's City of Books web site. I didn't know they had posted the answers until it was brought to my attention by John Gabriele. Thank you, John.

They sent fifteen questions and asked me to pick at least five. I had a lot of trouble finding five of their questions that I wanted to answer. Most of the questions were not productive of interesting answers; I had to work hard to keep my answers from being super-dull.

The non-super-dull answers are on Powell's site. Here are the questions I didn't answer, with their super-dull answers:

1. Have you ever taken the Geek Test? How did you rate?

   Hardly anyone seems to answer this question, and really, who cares? Except that Sir Roger Penrose said something like "There's a Geek Test?".

   I did take it once, but I forget how I scored. But if you read this blog, you can probably extrapolate: high on math, science, and programming. But really, who cares? Telling someone else about your geek test score is even more boring than telling them about your dreams.

2. What do you do for relaxation?

   I didn't answer this one because my answer seemed so uninteresting. I program. I read a lot; unlike most people who read a lot, I read a lot of different things. Sometimes I watch TV. I go for walks and drive the car.

   One thing I used to do when I was younger was the "coffee trick". I'd go to an all-night diner with pens and a pad of paper and sit there drinking coffee all night and writing down whatever came out of my caffeine-addled brain. I'm too old for that now; it would make me sick.

3. What's your favorite blog right now?

   I answered this one for Powell's, and cited my own blog and Maciej Ceglowski's. But if I were answering the question today I would probably mention What Jeff Killed. Whenever a new What Jeff Killed post shows up in the aggregator, I get really excited. "Oh, boy!" I say. "I can't wait to see What Jeff Killed today!".

   It occurs to me that just that one paragraph could probably give plenty of people a very clear idea of what I'm like, at least to the point that they would be able to decide they didn't want to know me.

4. Douglas Adams or Scott Adams?

   I think they're both boring. But I wasn't going to say so in my Powell's interview.

5. What was your favorite book as a kid?

   This should have been easy to answer, but none of the books I thought of seemed particularly revealing. When I was in sixth grade my favorite book was "The Hero from Otherwhere," by Jay Williams. (He also wrote the Danny Dunn books.) A few years back Andrew Plotkin posted on rec.arts.sf.written that he had recently read this, and that it occurred to him that it might have been his favorite book, had he read it in sixth grade, and had anyone had that experience. I wasn't the only one who had.

   I reread it a few years ago and it wasn't that good anymore.

   Robertson Davies writes about the awful juvenile-fiction magazines that he loved when he was a juvenile. Yes, they were terrible, but they fed something in him that needed to be fed. I think a lot of the books we love as children are like that.

6. What new technology do you think may actually have the potential for making people's lives better?

   I couldn't think of any way to answer this question that wouldn't be really boring. That probably says a lot more about me than about the question. I thought about gene therapy, land mine detection, water purification. But I don't personally have anything to do with those things, so it would just be a rehash of what I read in some magazine. And what's the point of reading an interview with an author who says, "Well, I read in Newsweek..."?

7. If you could be reincarnated for one day to live the life of any scientist or writer, who would you choose and why?

   This seems like it could have been interesting, but I couldn't figure out what to do with it. I might like to be Galileo, or to know what it's like to be Einstein, but that's not what the question says; it says that I'm me, living the life of Galileo or Einstein. But why would I want to do that? If I'm living the life of Einstein, that means I get to get up in the morning, go to an office in Zurich or Princeton, and sit behind a desk for eight hours, wishing I was smart enough to do Einstein's job.

   Some writers and scientists had exciting lives. I could be reincarnated as Evariste Galois, who was shot to death in a duel. That's not my idea of a good time.

   I once knew a guy who said he'd like to be David Lee Roth for one day, so that he could have sex with a groupie. Even if I wanted to have sex with a groupie, the question ("scientist or writer") pretty much rules out that form of entertainment. I suppose there's someone in the world who would want to be Pierre Curie, so that he know what it was like to fuck Marie Curie. That person isn't me.

8. What are some of the things you'd like your computer to do that it cannot now do?

   I came really close to answering this question. I had an answer all written. I wrote that I wanted the computer to be able to manufacture pornography on demand to the user's specification: if they asked for a kneecap fetish movie featuring Celine Dion and an overalls-wearing midget, it should be able to do that.

   Then I came to my senses and I realized I didn't want that answer to appear on my interview on the Powell's web site.

   But it'll happen, you wait and see.

   I also said I'd settle for having the computer discard spam messages before I saw them. I think the porn thing is a lot more likely.

9. By the end of your life, where do you think humankind will be in terms of new science and technological advancement?

   First I was stumped on this one because I don't know when the end of my life will be. I could be crushed in a revolving door next week, right?

   And assuming that I'll live another thirty years seems risky too. I'm hoping for a medical breakthrough that will prolong my life indefinitely. I expect it'll be along sooner or later. So my goal is to stay alive and healthy long enough to be able to take advantage of it when it arrives.

   Some people tell me they don't want to be immortal, that they think they would get bored. I believe them. People are bored because they're boring. Let them die; I won't miss them. I know exactly what I would do with immortality: I would read every book in the library.

   A few months ago I was visiting my mother, and she said that as a child I had always wanted to learn everything, and that it took me a long time to realize that you couldn't learn everything.

   I got really angry, and I shouted "I'm not done yet!"

   Well, even assuming that I live another thirty years, I don't think I can answer the question. When I was a kid my parents would go to the bank to cash a check. We got seven channels on the TV, and that was more than anyone else; we lived in New York. Nobody owned a computer; few people even owned typewriters. Big companies stored records on microfiche. The only way to find out what the law was was to go to the library and pore over some giant dusty book for hours until you found what you wanted.

   And sixty years ago presidential campains weren't yet advertising on television. Harry Truman campaigned by going from town to town on the back of a train (a train!) making speeches and shaking hands with people.

   Thirty years from now the world will be at least that different from the way things are now. How could I know what it'll be like?

10. Which country do you believe currently leads the world in science and technology? In ten years?

    In case you hadn't noticed, I hate trying to predict the future; I don't think I'm good at it and I don't think anyone else is. Most people who try don't seem to revisit their old predictions to see if they were correct, or to learn from their past errors, and the people who listen to them never do this.

    Technology prognosticators remind me of the psychics in the National Enquirer who make a hundred predictions for 2007: Jennifer Aniston will get pregnant with twins; space aliens will visit George Bush in the White House. Everyone can flap their mouth about what will happen next year, but it's not clear that anyone has any useful source of information about it, or is any better than anyone else at predicting.

    I read a book a few years back called The Year 2000: A Framework for Speculation on the Next Thirty Years, by Kahn and Weiner. It has a bunch of very carefully-done predictions about the year 2000, and was written in 1967. The predictions about computers are surprisingly accurate, if you ignore the fact that they completely failed to predict the PC. The geopolitical predictions are also surprisingly accurate, if you ignore the fact that they completely failed to predict the fall of the Soviet Union.

    But hardly anyone predicted the PC or the fall of the Soviet Union. And even now it's not clear whether the people who did predict those things did so because they were good at predicting or if it was just lucky guesses, like a stopped clock getting the time right twice a day.

    Sometimes I have to have dinner with predictors. It never goes well. Two years ago at OSCON I was invited to dinner with Google. I ended up sitting at a whole table of those people. Last year I was invited again. I said no thanks.

[ Addendum 20180423: Andrew Plotkin's rec.arts.sf.written post. ]

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A statistical puzzle
I heard a nice story a few years back. I don't know if it's true, but it's fun anyway. The story was that the University of North Carolina surveyed their graduates, asking them how much money they made, and calculated the average salary for each major. Can you guess which major had the highest average salary?

Can you explain this?

[Other articles in category /math] permanent link

Why it was the wrong pi
It my last article, I pointed out that the value of π carved into the wall of the Washington Park Portland MAX station is wrong:

  3.1415926535   8979323846   2643383279   5028841971   6939937510  5820974944   5923078164   0628620899   8628034825   3421170679
    8214808651   3282306647   0938446095   5058223172   5359408128  4811174502   8410270193   8521105559   6446229489   5493038196
    4428810975   6659334461   2847564823   3786783165   2712019091  4564856692   3460348610   4543266482   1339360726   0249141273
    7245870066   0631558817   4881520920   9628292540   9171536436  7892590360   0113305305   4882046652   1384146951   9415116094
    3305727036   5759591953   0921861173   8193261179   3105118548  0744623799   6274956735   1885752724   8912279381   8301194912
    9833673362   4406566430   8602139494   6395224737   1907021798  6094370277   0539217176   2931767523   8467481846   7669405132
    0005681271   4526356082   7785771342   7577896091   7363717872  1468440901   2249534301   4654958537   1050792279   6892589235
    4201995611   2129021960   8640344181   5981362977   4771309960  5187072113   4999999837   2978049951   0597317328   1609631859
    5024459455   3469083026   4252230825   3344685035   2619311881  7101000313   7838752886   5875332083   8142061717   7669147303
    5982534904   2875546873   1159562863   8823537875   9375195778  1857780532   1712268066   1300192787   6611195909   2164201989
    897932     

This explanation was apparently discovered by Oregonians for Rationality. I found it by doing Google search for "portland 'washington park' max station pi wrong value". Thank you, Google.

One thing that struck me about the digits as written is that there seemed to be too many repeats; this is what made me wonder if the digits were invented by the artist out of laziness or an attempt to communicate a secret message. We now know that they weren't. But I wondered if my sense that there were an unusually large number of duplicates was accurate, so I counted. If the digits are normal, we would expect exactly 1/10 of the digits to be the same as the previous digit.

In fact, of 97 digit pairs, 14 are repeats; we would expect 9.7. So this does seem to be on the high side. Calculating the likelihood of 14 repeats appearing entirely by chance seems like a tedious chore, without using somewhat clever methods. I'm in the middle of reading some books by Feller and Gnedenko about probability theory, and they do explain the clever methods, but they're at home now, so perhaps I'll post about this further tomorrow.

[Other articles in category /math] permanent link

It's not pi
In August I was in Portland for OSCON. One afternoon I went out to Washington Park to visit the museums there. The light rail station is underground, inside a hill, and the walls are decorated with all sorts of interesting things. For example, there's an illuminated panel with pictures of a sea urchin, a cactus, and a guy with a mohawk; another one compares an arm bone and a trombone. They bored a long lava core out of the hill, and display the lava core on the wall:

The inbound platform walls have a bunch of mathematics displays, including a display of Pascal's triangle. Here's a picture of one of them that I found extremely puzzling:

So what's the deal? Did they just screw up? Did they think nobody would notice? Is it a coded message? Or is there something else going on that I didn't get?

[ Addendum 20061017: The answer! ]

[Other articles in category /math] permanent link

Why two ears?
Aaron Swartz, remembering my earlier article about interesting science questions, sent me a reference to an interesting article about odd questions asked of students applying for admission to Cambridge and Oxford universities.

The example given in the article that I found most interesting was "Why don't we just have one ear in the middle of our face?". As I said earlier, I think the mark of a good question is that it's quick to ask and long to answer. I've been thinking about this one for several days now, and seems pretty long to answer.

Any reasonable answer to this question is going to be based on evolutionary and adaptive considerations, I think. When you answer from evolutionary considerations, there are only a few kinds of answers you can give:

1. It's that way because it confers a survival or reproductive advantage.
2. It's that way because that's the only way it can be made to work.
3. It's that way because it doesn't really matter, and that's just the way it happened to come out.

(Why only one heart? There's no benefit to having two; if you lose 50% of your cardiac capacity, you'll die anyway. Why one mouth? It needs to be big enough to eat with, and anyway, you can't lose it. Why one liver? No reason; that's just the way it's made; two livers would work just as well as one. Why two lungs? I'm not sure; I suppose it's a combination between "no reason, that's the way it's made" (#3 above) and "because that way you can still breathe even if one lung gets clogged up" (#1).)

The positioning of your ears is important. Having two ears far apart on the sides of your head allows you to locate sounds by triangulation. Triangulation requires at least two ears, and requires that they be as far apart as possible. This also explains why the ears are on the sides rather than the front.

Consider what would go wrong if the positions of the eyes and ears were switched. The ears would be pointed in the same direction, which would impede the triangulation-by-sound process. The eyes would be pointed in opposite directions, which would completely ruin the triangulation-by-sight process; you would completely lose your depth perception. So the differing position of the eyes and ears can be seen a response to the differing physical properties of light and sound: light travels in straight lines; sound does not.

The countervailing benefit to losing your depth perception would be that you would be able to see almost 180 degrees around you. Many animals do have their eyes on the side of their heads: antelopes, rabbits, and so forth. Prey, in other words. Predators have eyes on the fronts of their heads so that they can see the prey they are sneaking up on. Prey have eyes on the sides of their heads so that predators can't sneak up on their flanks. Congratulations: you're predator, not prey.

Animals do have exactly one nose in the middle of their face. Why not two? Here, triangulation is not an issue at all. Having one nose on each side of your head would not help you at all to locate the source of an odor. So the nose is stuck in the middle of the head, I suppose for mostly mechanical reasons: animals with noses evolved from animals with a long breathing tube down the middle of their bodies. The nose arises as sensors stuck in the end of the tube. This is another explanation for the one mouth.

Another consideration is symmetry. The body is symmetric, so if you want two ears, you have to put one on each side. Why is this? I used to argue that it was to save information space in the genome: there is only so much room in your chromosomes for instructions about how to build your body, so the information must be compressed. One excellent way to compress it is to make some parts like other parts and then express the differences as diffs. This, I used to say, is why the body is symmetric, why your feet look like your hands, and why men's and women's bodies are approximately the same.

I now think this is wrong. Well, wrong and right, essentially right, but mostly wrong. The fact is, there is plenty of space in the chromosomes for instructions about all sorts of stuff. Chromosomes are really big, and full of redundancy and junk. And if it's so important to save space in the chromosome, why is the inside of your body so very asymmetric?

I now think the reason for symmetries and homologies between body parts is less to do with data compression and storage space in the chromosome, and more to do with the shortness of the distance between points in information space. Suppose you are an animal with two limbs, each of which has a hand on the end. Then a freak mutation occurs so that your descendants now have four limbs. The four limbs will all have similar hands, because mutation cannot invent an entirely new kind of hand out of thin air. Your genome contains only one set of instructions for appendages that go on the ends of limbs, so these are the instructions that are available to your descendants. These instructions can be duplicated and modified, but again, there is no natural process by which a new set of instructions for a new kind of appendage can be invented from whole cloth. So your descendants' hands will look something like their feet for quite a long time.

Similarly, there is a certain probability, say p, of an earless species evolving something that functions as an ear. The number p is small, and ears arise only because of natural selection in favor of having ears. The chance that the species will simultaneously and independently evolve two completely different kinds of ear structures is no more than p², which is vanishingly small. And once the species has something earlike, the selection pressure in favor of the second sort of ear is absent. So a species gets one kind of ear. If having two ears is beneficial, it is extremely unlikely to arise through independent evolution, and much more likely to arise through a much smaller mutation that directs the same structure, the one for which complete instructions already exist in the genome, to appear on each side of the head.

So this is the reason for bodily symmetry. Think of (A) an earless organism, (B) an organism with two completely different ears, and (C) an organism with two identical ears. Think of these as three points in the space of all possible organisms. The path from point A to C is both much shorter than the path from A to B, and also much more likely to be supported by selection processes.

Now, why is the outside of the body symmetric while the inside is not? I haven't finished thinking this through yet. But I think it's because the outside interacts with the gross physical world to a much greater extent than the inside, and symmetry confers an advantage in large-scale physical interactions. Consider your legs, for example. They are approximately the same length. This is important for walking. If you had a choice between having both legs shortened six inches each, and having one leg shortened by six inches, you would certainly choose the former. (Unless you were a sidehill winder.) Similarly, having two different ears would mess up your hearing, particularly your ability to locate sounds. On the other hand, suppose one of your kidneys were much larger than the other. Big deal. Or suppose you had one giant liver on your right side and none on the left. So what? As long as your body is generally balanced, it is not going to matter, because the liver's interactions with the world are mostly on a chemical level.

So I think that's why you have an ear on each side, instead of one ear in the middle of your head: first, it wouldn't work as well to have one. Second, symmetry is favored by natural selection for information-conserving reasons.

[Other articles in category /bio] permanent link

Bone names
Names of bones are usually Latin. They come in two types. One type is descriptive. The auditory ossicles (that's Latin for "little bones for hearing") are named in English the hammer, anvil, and stirrup, and their formal, Latin names are the malleus ("hammer"), incus ("anvil"), and stapes ("stirrup")

The fibula is the small bone in the lower leg; it's named for the Latin fibula, which is a kind of Roman safety pin. The other leg bone, the tibia, is much bigger; that's the frame of the pin, and the fibula makes the thin sharp part.

The kneecap is the patella, which is a "little pan". The big, flat parietal bone in the skull is from paries, which is a wall or partition. The clavicle, or collarbone, is a little key.

"Pelvis" is Latin for "basin". The pelvis is made of four bones: the sacrum, the coccyx, and the left and right os innominata. Sacrum is short for os sacrum, "the sacred bone", but I don't know why it was called that. Coccyx is a cuckoo bird, because it looks like a cuckoo's beak. Os innominatum means "nameless bone": they gave up on the name because it doesn't look like anything. (See illustration to right.)

On the other hand, some names are not descriptive: they're just the Latin words for the part of the body that they are. For example, the thighbone is called the femur, which is Latin for "thigh". The big lower arm bone is the ulna, Latin for "elbow". The upper arm bone is the humerus, which is Latin for "shoulder". (Actually, Latin is umerus, but classical words beginning in "u" often acquire an initial "h" when they come into English.) The leg bone corresponding to the ulna is the tibia, which is Latin for "tibia". It also means "flute", but I think the flute meaning is secondary—they made flutes out of hollowed-out tibias.

Some of the nondescriptive names are descriptive in Latin, but not in English. The vertebra in English are so called after Latin vertebra, which means the vertebra. But the Latin word is ultimately from the verb vertere, which means to turn. (Like in "avert" ("turn away") and "revert" ("turn back").) The jawbone, or "mandible", is so-called after mandibula, which means "mandible". But the Latin word is ultimately from mandere, which means to chew.

The cranium is Greek, not Latin; kranion (or κρανιον, I suppose) is Greek for "skull". Sternum, the breastbone, is Greek for "chest"; carpus, the wrist, is Greek for "wrist"; tarsus, the ankle, is Greek for "instep". The zygomatic bone of the face is yoke-shaped; ζυγος ("zugos") is Greek for "yoke".

The hyoid bone is the only bone that is not attached to any other bone. (It's located in the throat, and supports the base of the tongue.) It's called the "hyoid" bone because it's shaped like the letter "U". This used to puzzle me, but the way to understand this is to think of it as the "U-oid" bone, which makes sense, and then to remember two things. First, that classical words beginning in "u" often acquire an initial "h" when they come into English, as "humerus". And second, classical Greek "u" always turns into "y" in Latin. You can see this if you look at the shape of the Greek letter capital upsilon, which looks like this: Υ. Greek αβυσσος ("abussos" = "without a bottom") becomes English "abyss"; Greek ανωνυμος ("anonumos") becomes English "anonymous"; Greek υπος ("hupos"; there's supposed to be a diacritical mark on the υ indicating the "h-" sound, but I don't know how to type it) becomes "hypo-" in words like "hypothermia" and "hypodermic". So "U-oid" becomes "hy-oid".

(Other parts of the body named for letters of the alphabet are the sigmoid ("S-shaped") flexure of the colon and the deltoid ("Δ-shaped") muscle in the arm. The optic chiasm is the place in the head where the optic nerves cross; "chiasm" is Greek for a crossing-place, and is so-called after the Greek letter Χ.)

The German word for "auditory ossicles" is Gehörknöchelchen. Gehör is "for hearing". Knöchen is "bones"; Knöchelchen is "little bones". So the German word, like the Latin phrase "auditory ossicles", means "little bones for hearing".

[Other articles in category /lang/etym] permanent link

MadHatterDay 2006

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Really real examples of HOP techniques in action
I recently stopped working for the University of Pennsylvannia's Informations Systems and Computing group, which is the organization that provides computer services to everyone on campus that doesn't provide it for themselves.

I used HOP stuff less than I might have if I hadn't written the HOP book myself. There's always a tradeoff with the use of any advanced techniques: it might provide some technical benefit, like making the source code smaller, but the drawback is that the other people you work with might not be able to maintain it. Since I'm the author of the book, I can be expected to be biased in favor of the techniques. So I tried to compensate the other way, and to use them only when I was absolutely sure it was the best thing to do.

There were two interesting uses of HOP techniques. One was in the username generator for new accounts. The other was in a generic server module I wrote.

Name generation

        george    bauer     georgef   fgeorge   fbauer    bauerf
        gf        georgeb   fg        fb        bauerg    bf
        georgefb  georgebf  fgeorgeb  fbauerg   bauergf   bauerfg
        ge        ba        gef       gbauer    fge       fba
        bgeorge   baf       gfbauer   gbauerf   fgbauer   fbgeorge
        bgeorgef  bfgeorge  geo       bau       geof      georgeba
        fgeo      fbau      bauerge   bauf      fbauerge  bauergef
        bauerfge  geor      baue      georf     gb        fgeor
        fbaue     bg        bauef     gfb       gbf       fgb
        fbg       bgf       bfg       georg     georgf    gebauer
        fgeorg    bageorge  gefbauer  gebauerf  fgebauer

It was convoluted because people kept asking that the generation algorithm be tweaked in various ways. Each tweak was accompanied by someone hacking on the code to get it to do things a little differently.

I threw it all away and replaced it with a lazy generator based on the lazy stream stuff of Chapter 6. The underlying stream library was basically the same as the one in Chapter 6. Atop this, I built some functions that generated streams of names. For example, one requirement was that if the name generator ran out of names like the examples above, it should proceed by generating names that ended with digits. So:

        sub suffix {
          my ($s, $suffix) = @_;
          smap { "$_$suffix" } $s;          
        }       

        # Given (a, b, c), produce a1, b1, c1, a2, b2, c2, a3...
        sub enumerate {
          my $s = shift;
          lazyappend(smap { suffix($s, $_) } iota());
        }

        # Given (a, b, c), produce a, b, c, a1, b1, c1, a2, b2, c2, a3...
        sub and_enumerate {
          my $s = shift;
          append($s, enumerate($s));
        }

        # Throw away names that are already used
        sub available_filter {
          my ($s, $pn) = @_;
          $pn ||= PennNames::Generate::InUse->new;
          sgrep { $pn->available($_) } $s;
        }

Second, the frequent changes and tinkerings to the name generation algorithm in the past suggested that an extremely modular approach would be a benefit. In fact, the requirements for the generation algorithm chanced several times as I was writing the code, and the stream approach made it really easy to tinker with the order in which names were generated, by plugging together the prefabricated stream modules.

Generic server

The callback was responsible for communicating with the client. It was passed the client socket:

        sub child_callback {
          my $socket = shift;
          # ... read and write the socket ...
          return;   # child process exits
        }

        # typical client callback:
        sub child_callback {
          my $socket = shift; 
          while (my $request = <$socket>) {
            # generate response to request
            print $socket $response;
          }
        }

        sub child_behavior {
          my $request = shift;
          if ($request =~ /^LOOKUP (\w+)/) {
            my $input = $1;
            if (my $result = lookup($input)) {
              return "OK $input $result";
            } else {
              return "NOK $input";
            }
          } elsif ($request =~ /^QUIT/) {
            return;
          } elsif ($request =~ /^LIST/) {
            my $N = my @N = all_names();
            return join "\n", "OK $N", @N, ".";
          } else {
            return "HUH?";
          }
        }

        $server->run(CALLBACK => make_callback(\&child_behavior));

        sub make_callback {
          my $behavior = shift;
          return sub {
            my $socket = shift;
            while (my $request = <$socket>) {
              chomp $request;
              my $response = $behavior->($request);
              return unless defined $response;
              print $socket $response;
            }
          };
        }

[Other articles in category /prog] permanent link

Ralph Johnson on design patterns
Last month I wrote an article about design patterns which attracted a lot of favorable attention in blog world. I started by paraphrasing Peter Norvig's observation that:

"Patterns" that are used recurringly in one language may be invisible or trivial in a different language.

and ended by concluding:

Patterns are signs of weakness in programming languages.

When we identify and document one, that should not be the end of the story. Rather, we should have the long-term goal of trying to understand how to improve the language so that the pattern becomes invisible or unnecessary.

Johnson raises several points. First there is a meta-issue to deal with. Johnson says:

He clearly thinks that what he says is surprising. And other people think it is surprising, too. That is surprising to me.

One thing that I did find surprising is the uniformity of other people's surprise and interest. There were dozens of blog posts and comments in the following two weeks, all pretty much saying what a great article I had written and how right I was. I tracked the responses as carefully as I could, and I did not see any articles that called me a dumbass; I did not see any except for Johnson's that suggested that what I was saying was unsurprising.

We can't conclude from this that I am right, of course; people agree with all sorts of stupid crap. But we can conclude that that what I said was surprising and interesting, since people were surprised and interested by it, even people who already have some knowledge of this topic. Johnson is right to be surprised by this, because he thought this was obvious and well-known, and that it was clearly laid out in his book, and he was mistaken. Many or most of the readers of his book have completely missed this point. I didn't miss it, but I didn't get it from the book, either.

Johnson and his three co-authors wrote this book, Design Patterns, which has had a huge influence on the way that programming is practiced. I think a lot of that influence has been malign. Any practice can be corrupted, of course, by being reduced to its formal aspects and applied in a rote fashion. (There's a really superb discussion of this in A. Ya. Khinchin's essay On the Teaching of Mathematics, and a shorter discussion in Polya's How to Solve It, in the section on "Pedantry and Mastery".) That will happen to any successful movement, and the Gang of Four can't take all the blame for that.

But if they really intended that everyone should understand that each design pattern is a demonstration of a weakness in its target language, then they blew it, because it appears that hardly anyone understood that.

Let's pause for a moment to imagine an alternate universe in which the subtitle of the Design Patterns book was not "Elements of Reusable Object-Oriented Software" but "Solutions for Recurring Problems in Object-Oriented Languages". And let's imagine that in each section, after "Pattern name", "Intent", "Motivation", "Applicability", and so forth, there was another subsection titled "Prophylaxis" that went something like this: "The need for the Iterator pattern in C++ appears to be due partly to its inflexible type system and partly to its lack of abstract iteration structures. The iterator pattern is unnecessary in the Python language, which avoids these defects as follows: ... at the expense of ... . In Common Lisp, on the other hand, ... (etc.)".

I would have liked to have seen that universe, but I suppose it's too late now. Oh well.

Anyway, moving on from meta-issues to the issues themselves, Johnson continues:

At the very end, he says that patterns are signs of weakness in programming languages. This is wrong.

At the very end of his response, Johnson says:

No matter how complicated your language will be, there will always be things that are not in the language. These things will have to be patterns. So, we can eliminate one set of patterns by moving them into the language, but then we'll just have to focus on other patterns. We don't know what patterns will be important 50 years from now, but it is a safe bet that programmers will still be using patterns of some sort.

I think I know where it is. When I said "[Design] Patterns are signs of weakness in programming languages," what I meant was something like "Each design pattern is a sign of a weakness in the programming language to which it applies." But it seems that Johnson thinks that I meant that the very existence of design patterns, at all, is a sign of weakness in all programming languages everywhere.

If I thought that the existence of design patterns, at all, was a sign that current programming languages are defective, as a group, I would see an endpoint to programming language development: someday, we would have a perfect überlanguage in which it would be unnecessary to use patterns because all possible patterns would have been built in already.

I think Johnson thinks this was my point. In the passage quoted above, I think he is addressing the idea of the überlanguage that incorporates all patterns everywhere at all levels of abstraction. And similarly:

Some people like languages with a lot of features. . . . I prefer simple languages.

No matter how complicated your language will be, there will always be things that are not in the language.

What I imagine is that when pattern P applies to language L, then, to the extent that some programmer on some project finds themselves needing to use P in their project, the use of P indicates a deficiency in language L for that project.

The absence of a convenient and simple way to do P in language L is not always a problem. You might do a project in language L that does not require the use of pattern P. Then the problem does not manifest, and, whatever L's deficiencies might be for other projects, it is not deficient in that way for your project.

This should not be difficult for anyone to understand. Perl might be a very nice language for writing a program to compile a bioinformatic data file into a more reasonable form; it might be a terrible language for writing a real-time missile guidance system. Its deficiencies operate in the missile guidance project in a way that they may not in the data munging project.

But to the extent that some deficiency does come up in your project, it is a problem, because you are implementing the same design over and over, the same arrangement of objects and classes, to accomplish the same purpose. If the language provided more support for solving this recurring design problem, you wouldn't need to use a "pattern". Consider again the example of the "subroutine" pattern in assembly language: don't you have anything better to do than redesign and re-implement the process of saving the register values in a stack frame, over and over? Well, yes, you do. And that is why you use a language that has that built in. Consider again the example of the "object-oriented class" pattern in C: don't you have anything better to do than redesign and re-implement object-oriented method dispatch with inheritance, over and over? Yes, you do. And that is why you use a language that has that built in, if that is what you need.

By Gamma, Helm, Johnson, and Vlissides' own definition, the problems solved by patterns are recurring problems, and programmers must address them recurringly.

If these problems recurred in every language, we might conclude that they were endemic to programming itself. We might not, but it's hard to say, since if there are any such problems, they have not yet been brought to my attention. Every pattern discovered so far seems to be specific to only a small subset of the world's languages.

So it seems a small step to conclude that these recurring, language-specific problems are actually problems with the languages themselves. No problem is a problem in every language, but rather each problem is a red arrow, pointing at a design flaw in the language in which it appears.

Johnson continues:

Patterns might be a sign of weakness, but they might be a sign of simplicity. . . .

The alternative, remember, is to require the programmer to use a "pattern": to make them consult a manual of "patterns" to implement a "general arrangement of objects and classes" to solve the subroutine-call problem every time it comes up.

I guess you could interpret that as a sign of "simplicity", but it's the wrong kind of simplicity. Language designers have a hard problem to solve. If they don't put enough stuff into the language, it'll be too hard to use. But if they put in too much stuff, it'll be confusing and hard to program, like C++. One reason it's hard to be a language designer is that it's hard to know what to put in and what to leave out. There is an extremely complex tradeoff between simplicity and functionality.

But in the case of "patterns", it's much easier to understand the tradeoff. A pattern, remember, is a general method for solving "a recurring design problem". Patterns might be a sign of "simplicity", but if so, they are a sign of simplicity in the wrong place, a place where the language needs to be less simple and more featureful. Because patterns are solutions to recurring design problems.

If you're a language designer, and a "pattern" comes to your attention, then you have a great opportunity. The programmers using your language have a recurring problem. They have to implement the same solution to it, over and over. Clearly, this is a good place to try to expend some design effort; perhaps you can trade off a little simplicity for some functionality and fix the language so that the problem is a problem no longer.

Getting rid of one recurring design problem might create new ones. But if the new problems are operating at a higher level of abstraction, you may have a win. Getting rid of the need for the "subroutine call" pattern in assembly language opened up all sorts of new problems: when and how do I do recursion? When and how do I do coroutines?

Getting rid of the "object-oriented class" pattern in C created a need for higher-level patterns, including the ones described in the Design Patterns book. When people didn't have to worry about implementing inheritance themselves, a lot of their attention was freed up, and they could notice patterns like Façade.

As Alfred North Whitehead says, civilization advances by extending the number of important operations which we can perform without thinking about them. The Design Patterns approach seems to be to identify the important operations and then to think about them over and over and over and over and over.

Or so it seems to me. Johnson's next paragraph makes me wonder if I've completely missed his point, because it seems completely senseless to me:

There is a trade-off between putting something in your programming language and making it be a convention, or perhaps putting it in the library. Smalltalk makes "constructor" be a convention. Arithmetic is in the library, not in the language. Control structures and exception handling are from the library, not in the language.

Or consider Perl's dbmopen function. Prior to version 5.000, it was part of the "language", in some sense; in 5.000 and later, it became part of the "library". But what's the difference, really? I can't find any.

Is Johnson talking about some syntactic or semantic difference here? Maybe if I knew more about Smalltalk, I would understand his point. As it is, it seems completely daft, which I interpret to mean that there's something that went completely over my head.

Well, the whole article leaves me wondering if maybe I missed his point, because Johnson is presumably a smart guy, but his argument about the built-in features vs. libraries makes no sense to me, his argument about simplicity seems so clearly and obviously dismantled by his own definition of patterns, and his apparent attack on a straw man seems so obviously erroneous.

But I can take some consolation in the thought that if I did miss his point, I'm not the only one, because the one thing I can be sure of in all of this is that a lot of other people have been missing his point for years.

Johnson says at the beginning that he "wasn't sure whether to be happy or unhappy". If I had written a book as successful and widely read as Design Patterns and then I found out that everyone had completely misunderstood it, I think I would be unhappy. But perhaps that's just my own grumpy personality.

[ Addendum 20080303: Miles Gould wrote a pleasant and insightful article on Johnson's point about libraries vs. language features. As I surmised, there was indeed a valuable point that went over my head. I said I couldn't find any difference between "language" and "library", but, as M. Gould explains, there is an important difference that I did not appreciate in this context. ]

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The world's worst macro preprocessor
Last week I added another plugin to my Blosxom installation. As I wrote before, the sole benefit of Blosxom is that it's incredibly simple and lightweight. So when I write plugins for it, I try to keep them incredibly simple and lightweight, lest I spoil the single major benefit of Blosxom. Sometimes I'm more successful, sometimes less so. This time I think I did a good job.

The goal last time was a macro processor. I write a lot of math articles. I get tired of writing ² every time I want a superscript 2. Even if I bind a function key to that sequence of characters, it's hard to read. But now, with my new Blosxom macro processor, I just insert a line into my article that says:

  #define ^2 <sup>2</sup>

and for the rest of the article, ^2 is expanded to ².

This has turned out really well, and I'm using it for all sorts of stuff. I use it for math notations, such as for making -> an abbreviation for &rarr; (→), and for making ~ an abbreviation for &not; (¬).

But I've also used it to #define Godel G&ouml;del. I've used it to #define KK <b>K</b> and #define SS <b>S</b>, which makes an article I'm writing about combinatory logic readable, where it wasn't readable before. In my recent article about job hunting, I used it to #define CV r&eacute;sum&eacute;, which saved me from having to interrupt my train of thought several times in the article.

There are some important points about the design that I think I got right on the first try. Whenever you write a macro system, you have to ask about escape sequences: what do you do if you don't want a macro expanded? For example, in the combinatory logic article I defined a macro SS. This meant that if I had written MOUSSE in the article somewhere, it would have turned into MOUSE. How should I prevent that kind of error?

Answer: I don't. I'm unlikely to do that. But if I do, I'll pick it up during the article proofreading phase. If I can't avoid writing MOUSSE, I have two choices: I can change the name of the SS macro to something easier to avoid—like S*, say, or I can define a second macro: #define !MOUSSE MOUSSE. But so far, it hasn't come up.

One alternative solution is to say that macros are expanded only in certain contexts. For example, SS might only be expanded when it is a complete word, not when it is in the middle of a word, as MOUSSE. I resisted this solution. It is much simpler to remember that every macro is expanded everywhere. And it it is much easier to fix the problem of a macro being expanded when I don't want it than it is to fix the problem of a macro not being expanded when I do want it. So every macro is expanded no matter where it appears.

Related to the unintentional-expansion issue is that each article has its own private macro set. I don't have to worry that by defining a macro named -> in one article that I might be sabotaging my opportunity to actually write -> in some unknown future article. Each set of macros can be totally ad hoc. I don't have to worry about global tradeoffs. Do I #define --- &mdash;, knowing that that will foreclose my opportunity to use --- in any other way? I can make the decision based on simple, local information.

It would have been tempting to over-engineer the system and add all sorts of complex escape facilities. I think I made the right choice here by not doing any of that.

Another escaping issue: What if I want to write something that looks like a definition but isn't? Here I avoided the problem by choosing a definition syntax that I was unlikely to write in any other context: #define in the leftmost column indicates a definition. In this article, I had to write some similar text. It was no trouble to indent it a couple of spaces, disabling the special meaning. But HTML is already full of escape mechanisms, and it would have been no trouble to write &#35;define instead of #define if for some reason I had really needed it to appear in the leftmost column. (Unlikely anyway, since HTML has no column semantics.)

Another right choice I think I made was not to parametrize the macros. An article on algebra might well have:

  #define ^2 <sup>2</sup>
  #define ^3 <sup>3</sup>

  #define ^(\w+) <sup>$1</sup>

Also tempting is to extend the macro system to support something like this:

  #define BF(.*) <b>$1</b>

I did run into one trouble with the macro system. Originally, it was invoked before some of my other plugins and after others. The earlier plugins automatically inserted certain text into the article that sometimes accidentally triggered my macros. I have not had any trouble with this since I changed the plugin order to invoke the macro processor before any of the other plugins.

The macro-processing code is about 19 lines long, of which three are diagnostic. It is the world's worst macro system. It has exactly one feature. It is, I think the simplest thing that could possibly work, and so a good companion to Blosxom. For this application, the world's worst macro system is the world's best.

[ Addendum 20071004: There's now a one-year retrospective analysis. ]

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Job hunting stories
A guy named Jonathan Rentzsch bookmarked my article about creeping featurism and the ratchet effect, saying "I'd like to hire this guy just so I could fire him." Since I was looking for a new job last month, I sent him my résumé, inquiring about his company's severance package. He didn't reply.

I talked it over with my wife, and we decided that for $750,000 we would be willing for me to spend the year working in Milford, Iowa. $500,000, we decided, would not be sufficient, but $750,000 would. I forget by now how we arrived at this figure, but we took some care in coming up with it.

The headhunter called back. "Have you thought it over?" Yes, I had, I said. I had decided that $750,000 would be required to get me to Milford, Iowa.

He was really angry that I had wasted his time.

One day I was on a five-week business trip to Asia. (I hate commuting, but I like travelling.) I got email from a headhunter who was offering me a long-term contract in Elkton, Maryland. I had told this headhunter's company repeatedly that I was only interested in working in the Philadelphia area. (Elkton, Maryland is about as close to Philadelphia as you can get and still be in Maryland, which means that the only thing between Elkton and Philadelphia is the state of Delaware.)

I wrote back, from Tokyo, and said that his company should stop contacting me, because I had told them over and over that I would only work in Philadelphia, and they kept sending me offers of employment in places like Elkton. I said it was a shame that we should waste each others' time like this.

He suggested that the problem could be solved if I could just give him a "courtesy call". From Tokyo.

Instead, I solved the problem by putting his company in my spam filter file.

He didn't like my advice. I still think it was good advice. No nineteen-year-old needs a three-page résumé. I wonder if he eventually figured this out. You'd think so, but now that I have to read the résumés of people who are applying to me for jobs, it seems that hardly any of the applicants have figured out that you should leave out the job where you washed dishes for your fraternity.

I've talked to a couple of people lately who tell me that it's very rare to get a cover letter from an applicant that actually helps their chances of getting the job. At best, it's neutral. At worst, you get to see all their spelling and grammar errors.

I think most people don't know how to write a letter, or that they write one letter and send it with every application. Maybe they think it would be dishonest to send a different letter with every application.

I became a conference speaker and teacher of Perl classes in a similar way. I wanted to go to the second Perl conference, but I couldn't afford it. Someone mentioned to me that there was a $1,000 prize for the best user paper. I thought I could write a good paper, but I also thought that the best paper often doesn't win the prize. But I also found out that conference tutorial speakers were paid $1,500 and given free conference admission and airfare and hotel fees.

When you're submitting a proposal for a tutorial, it's perfectly honorable to go talk to the program committee behind the scenes and lobby them to accept your proposal instead of someone else's. If you do that with the contest judges, it's cheating. So I ignored the user paper contest and submitted a proposal for a tutorial, which was accepted.

I got back a peevish letter telling me that I wasn't qualified and to stop wasting the search committee's time. It was signed, in pen, "The Search Committee".

I accepted a job with the University of Pennsylvania instead.

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More about automorphisms
In a recent article, I asserted that "there aren't even any reasonable [automorphisms of R] that preserve addition.". This is patently untrue.

My proof started by referring to a previous result that any such automorphism f must have f(1) = 1. But actually, I had only proved this for automorphisms that must preserve multiplication. For automorphisms that preserve addition only, f(1) need not be 1; it can be anything. In fact, x → kx is an automorphism of R for all k except zero. It is not hard to show, following the technique in the earlier article, that every continuous automorphism has this form.

In hopes of salvaging something from my embarrassing error, I thought I'd spend a little time talking about the other automorphisms of R, the ones that aren't "reasonable". They are unreasonable in at least two ways: they are everywhere discontinuous, and they cannot be exhibited explicitly.

To manufacture the function, we first need a mathematical horror called a Hamel basis. A Hamel basis is a set of real numbers H_α such that every real number r has a unique representation in the form:

$$r = \sum_{i=1}^n q_i H_{\alpha_i}$$

The sum here is finite, so only a finite number of the uncountably many H_α are involved for any particular r; this is what characterizes it as a Hamel basis.

Leaving aside the proof that the Hamel basis exists at all, if we suppose we have one, we can easily construct an automorphism of R. Just pick some rational numbers m_α, one for each H_α. Then if as above, we have:

$$f(r) = \sum_{i=1}^n q_i H_{\alpha_i}m_{\alpha_i}$$

By a simple argument involving commutativity and associativity of addition, c_H(r+s) = c_H(r) + c_H(s) for all r, s, and H.

Also, c_H(f(r)) = m·c_H(r), for all r and H, where m is the multiplier we chose for H back when we were making up the automorphism, because that's how we defined f.

Then c_H(f(r+s)) = m·c_H(r+s) = m·(c_H(r) + c_H(s)) = m·c_H(r) + m·c_H(s) = c_H(f(r)) + c_H(f(s)) = c_H(f(r) + f(s)), for all H.

This means that f(r+s) and f(r) + f(s) have the same Hamel basis representation. They are therefore the same number. This is what we wanted to show.

If anyone actually found that in the least enlightening, I would be really interested to hear about it.

The problem with this construction is that it is completely abstract. Nobody can exhibit an example of a Hamel basis, being, as it is, an uncountably infinite set of mainly irrational numbers. So the discontinuous automorphisms constructed here are among the most utterly useless of all mathematical examples.

I think that is the full story about additive automorphisms of R. I hope I got everything right this time.

I should add, by the way, that there seems to be some disagreement about what is called a Hamel basis. Some people say it is what I said: a basis for the reals over the rationals, with the properties I outlined above. However, some people, when you say this, will sniff, adjust their pocket protectors, and and correct you, saying that that a Hamel basis is any basis for any vector space, as long as it has the analogous property that each vector is representable as a combination of a finite subset of the basis elements. Some say one, some the other. I have taken the definition that was convenient for the exposition of this article.

[ Thanks to James Wetterau for pointing out the error in the earlier article. ]

[ Previous articles in this series: Part 1 Part 2 Part 3 Part 4 ]

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Automorphisms of the complex numbers
In an earlier article, I wrote a proof that the only automorphisms of the complex numbers are the identity function and the function a + bi → a - bi.

Robert C. Helling points out that there is a much simpler proof that this is the case. Suppose that f is an automorphism, and that x² = y. Then f(x²) = (f(x))² = f(y), so that if x is a square root of y, then f(x) is a square root of f(y).

As I pointed out, f(1) = 1. Since -1 is a square root of 1, f(-1) must be a square root of 1, and so it must be -1. (It can't be 1, since automorphisms may not map two different arguments to the same value.) Since i is a square root of -1, f(i) must also be a square root of -1. So f(i) must be either ±i, and the theorem is proved.

This is a nice example of why I am not a mathematician. When I want to find the automorphisms of C, my first idea is to explicitly write down the general automorphism and then start bashing away on the algebra. This sort of mathematical pig-slaughtering gets the pig cut up all right, but mathematicians are not interested in slaughtering pigs. By which I mean that the approach gets the result I want, usually, but not new or mathematically interesting results.

In computer programming, the pig-slaughtering approach often works really well. Most programs are oversubtle, and can be easily improved by doing the necessary tasks in the simplest and most straightforward possible way, rather than in whatever baroque way the original programmer dreamed up.

[ Previous articles in this series: Part 1 Part 2 Part 3 Followup article: Part 5 ]

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Russell and Whitehead or Whitehead and Russell?
In an earlier article, I asked:

Everyone always says "Russell and Whitehead". Google results for "Russell and Whitehead" outnumber those for "Whitehead and Russell" by two to one, for example. Why? The cover and the title page [of Principia Mathematica] say "Alfred North Whitehead and Bertrand Russell, F.R.S.". How and when did Whitehead lose out on top billing?

I was going to write that I thought the answer was that when Whitehead died, he left instructions to his family that they destroy his papers; this they did. So Whitehead's work was condemned to a degree of self-imposed obscurity that Russell's was not.

I was planning to end this article there. But now, on further reflection, I think that this theory is oversubtle. Russell was a well-known political and social figure, a candidate for political office, a prolific writer, a celebrity, a famous pacifist. Whitehead was none of these things; he was a professor of philosophy, about as famous as other professors of philosophy.

The obvious answer to my question above would be "Whitehead lost out on top billing on 10 December, 1950, when Russell was awarded the Nobel Prize."

Oh, yeah. That.

I'm reminded of the advertising for the movie Space Jam. The posters announced that it starred Bugs Bunny and Michael Jordan, in that order. I reflected for a while on the meaning of this. Was Michael Jordan incensed at being given second billing to a fictitious rabbit? (Probably not, I think; I imagine that Michael Jordan is entirely unthreatened by the appurtenances of any else's fame, and least of all by the fame of a fictitious rabbit.) Why does Bugs Bunny get top billing over Michael Jordan? I eventually decided that while Michael Jordan is a hero, Bugs Bunny is a god, and gods outrank heroes.

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Imaginary units, again
In my earlier discussion of i and -i I said " The point about the square roots of -1 is that there is no corresponding criterion for distinguishing the two roots. This is a theorem."

The proof of the theorem is not too hard. What we're looking for is what's called an automorphism of the complex numbers. This is a function, f, which "relabels" the complex numbers, so that arithmetic on the new labels is the same as the arithmetic on the old labels. For example, if 3×4 = 12, then f(3) × f(4) should be f(12).

Let's look at a simpler example, and consider just the integers, and just addition. The set of even integers, under addition, behaves just like the set of all integers: it has a zero; there's a smallest positive number (2, whereas it's usually 1) and every number is a multiple of this smallest positive number, and so on. The function f in this case is simply f(n) = 2n, and it does indeed have the property that if a + b = c, then f(a) + f(b) = f(c) for all integers a, b, and c.

Another automorphism on the set of integers has g(n) = -n. This just exchanges negative and positive. As far as addition is concerned, these are interchangeable. And again, for all a, b, and c, g(a) + g(b) = g(c).

What we don't get with either of these examples is multiplication. 1 × 1 = 1, but f(1) × f(1) = 2 × 2 = 4 ≠ f(1) = 2. And similarly g(1) × g(1) = -1 × -1 = 1 ≠ g(1) = -1.

In fact, there are no interesting automorphisms on the integers that preserve both addition and multiplication. To see this, consider an automorphism f. Since f is an automorphism that preserves multiplication, f(n) = f(1 × n) = f(1) × f(n) for all integers n. The only way this can happen is if f(1) = 1 or if f(n) = 0 for all n.

The latter is clearly uninteresting, and anyway, I neglected to mention that the definition of automorphism rules out functions that throw away information, as this one does. Automorphisms must be reversible. So that leaves only the first possibility, which is that f(1) = 1. But now consider some positive integer n. f(n) = f(1 + 1 + ... + 1) = f(1) + f(1) + ... + f(1) = 1 + 1 + ... + 1 = n. And similarly for 0 and negative integers. So f is the identity function.

One can go a little further: there are no interesting automorphisms of the real numbers that preserve both addition and multiplication. In fact, there aren't even any reasonable ones that preserve addition. The proof is similar. First, one shows that f(1) = 1, as before. Then this extends to a proof that f(n) = n for all integers n, as before. Then suppose that a and b are integers. b·f(a/b) = f(b)f(a/b) = f(b·a/b) = f(a) = a, so f(a/b) = a/b for all rational numbers a/b. Then if you assume that f is continuous, you can fill in f(x) = x for the irrational numbers also.

(Actually this is enough to show that the only continuous addition-preserving automorphism of the reals is the identity function. There are discontinuous addition-preserving functions, but they are very weird. I shouldn't need to drag in the continuity issue to show that the only addition-and-multiplication-preserving automorphism is the identity, but it's been a long day and I'm really fried.)

[ Addendum 20060913: This previous paragraph is entirely wrong; any function x → kx is an addition-preserving automorphism, except of course when k=0. For more complete details, see this later article. ]

But there is an interesting automorphism of the complex numbers; it has f(a + bi) = a - bi for all real a and b. (Note that it leaves the real numbers fixed, as we just showed that it must.) That this function f is an automorphism is precisely the content of the statement that i and -i are numerically indistinguishable.

The proof that f is an automorphism is very simple. We need to show that if f(a + bi) + f(c + di) = f((a + bi) + (c + di)) for all complex numbers a+bi and c+di, and similarly f(a + bi) × f(c + di) = f((a + bi) × (c + di)). This is really easy; you can grind out the algebra in about two steps.

What's more interesting is that this is the only nontrivial automorphism of the complex numbers. The proof of this is also straightforward, but a little more involved. The purpose of this article is to present the proof.

Let's suppose that f is an automorphism of the complex numbers that preserves both addition and multiplication. Let's say that f(i) = p + qi. Then f(a + bi) = f(a) + f(b)f(i) = a + bf(i) (because f must leave the real numbers fixed) = a + b(p + qi) = (a + bp) + bqi.

Now we want f(a + bi) + f(c + di) = f((a + bi) + (c + di)) for all real numbers a, b, c, and d. That is, we want (a + bp + bqi) + (c + dp + dqi) = (a + c) + (b + d)(p + qi). It is, so that part is just fine.

We also want f(a + bi) × f(c + di) = f((a + bi) × (c + di)) for all real numbers a, b, c, and d. That means we need:

(a + b(p + qi)) × (c + d(p + qi))	=	f((ac-bd) + (ad+bc)i)
(a + bp + bqi) × (c + dp + dqi)	=	ac - bd + (ad + bc)(p + qi)
ac + adp + adqi + bcp + bdp2 + 2bdpqi + bcqi - bdq2	=	ac - bd + adp + bcp + adqi + bcqi
bdp2 + 2bdpqi - bdq2	=	- bd
p2 + 2pqi - q2	=	-1

Equating the real and imaginary parts gives us two equations:

1. p² - q² = -1
2. 2pq = 0

Equation 2 implies that either p or q is 0. If they're both zero, then f(a + bi) = a, which is not reversible and so not an automorphism.

Trying q=0 renders equation 1 insoluble because there is no real number p with p² = -1. But p=0 gives two solutions. One has p=0 and q=1, so f(a+bi) = a+bi, which is the identity function, and not interesting. The other has p=0 and q=-1, so f(a+bi) = a-bi, which is the one we already knew about. But we now know that there are no others, which is what I wanted to show.

[ Previous articles in this series: Part 1 Part 2 Followup articles: Part 4 Part 5 ]

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Design patterns of 1972
"Patterns" that are used recurringly in one language may be invisible or trivial in a different language.

Extended Example: "object-oriented class"

        struct st_employee_object *emp;

        EMPLOYEE emp;

        emp->method(emp, args...);

There are a bunch of variations on this. For example, you can get opaque implementation by defining two header files for each class. One defines the implementation:

        struct st_employee_object {
           unsigned salary;
           struct st_manager_object *boss;
           METHOD fire, transfer, competence;
        };

        struct st_employee_object {
           char __SECRET_MEMBER_DATA_DO_NOT_TOUCH[4];
           struct st_manager_object *boss;
           METHOD fire, transfer, competence;
        };

You get abstract classes by defining a constructor function that sets all the methods to NULL or to:

        void _abstract() { abort(); }

        struct st_manager_object;   /* forward declaration */

        #define EMPLOYEE_FIELDS \
           unsigned salary; \
           struct st_manager_object *boss; \
           METHOD fire, transfer, competence;
                
        struct st_employee_object {
           EMPLOYEE_FIELDS
        };

        struct st_manager_object {
           EMPLOYEE_FIELDS
           unsigned num_subordinates;
           struct st_employee_object **subordinate;
           METHOD delegate_task, send_to_conference;
        };

This may seem weird or contrived, but the technique is widely used. The C standard contains guarantees that the common fields of struct st_manager_object and struct st_employee_object will be laid out identically in memory, specifically so that this object-oriented class technique can work. The code of the X window system has this structure. The code of the Athena widget toolkit has this structure. The code of the Linux kernel filesystem has this structure.

Rob Pike, one of the primary architects of the Plan 9 operating system (the Bell Labs successor to Unix) and co-author (with Brian Kernighan) of The Unix Programming Environment, recommends this technique in his article "Notes on Programming in C".

This is a pattern

A design pattern systematically names, motivates, and explains a general design that addresses a recurring design problem in object-oriented systems. It describes the problem, the solution, when to apply the solution, and its consequences. It also gives implementation hints and examples. The solution is a general arrangement of objects and classes that solve the problem. The solution is customized and implemented to solve the problem in a particular context.

The definition in Wikipedia is:

In software engineering, a design pattern is a general solution to a common problem in software design. A design pattern isn't a finished design that can be transformed directly into code; it is a description or template for how to solve a problem that can be used in many different situations.

Codification of patterns

Invisible

So much a part of language that you don't notice

Formal

Implement pattern itself within the language
Instantiate/call it for each use
Usually implemented with macros

Informal

Design pattern in prose; refer to by name, but Must be reimplemented from scratch for each use

The single major driver for the invention of C++ was to codify this pattern into the language so that it was "invisible". In C++, you don't have to think about the structs and you don't have to worry about keeping data and methods private. You just declare a "class" (using syntax that looks almost exactly like a struct declaration) and annotate the items with "public" and "private" as appropriate.

But underneath, it's doing the same thing. The earliest C++ compilers simply translated the C++ code into the equivalent C code and invoked the C compiler on it. There's a reason why the C++ method call syntax is object->method(args...): it's almost exactly the same as the equivalent code when the pattern is implemented in plain C. The only difference is that the object is passed implicitly, rather than explicitly.

In C, you have to make a conscious decision to use OO style and to implement each feature of your OOP system as you go. If a program has fifty modules, you need to decide, fifty times, whether you will make the next module an OO-style module. In C++, you don't have to make a decision about whether or not you want OO programming and you don't have to implement it; it's built into the language.

Sherman, set the wayback machine for 1957

Recurring problem: Two or more parts of a machine language program need to perform the same complex operation. Duplicating the code to perform the operation wherever it is needed creates maintenance problems when one copy is updated and another is not.

Solution: Put the code for the operation at the end of the program. Reserve some extra memory (a "frame") for its exclusive use. When other code (the "caller") wants to perform the operation, it should store the current values of the machine registers, including the program counter, into the frame, and transfer control to the operation. The last thing the operation does is to restore the register values from the values saved in the frame and jump back to the instruction just after the saved PC value.

For machine language programmers of the 1950s and early 1960's, this was a pattern, reimplemented from scratch for each use. As assemblers improved, the pattern became formal, implemented by assembly-language macros. Shortly thereafter, the pattern was absorbed into Fortran and Lisp and their successors, and is now invisible. You don't have to think about the implementation any more; you just call the functions.

Iterators and model-view-controller

One of the things I pointed out was essentially what Norvig does: that many patterns aren't really addressing recurring design problems in object-oriented programs; they are actually addressing deficiencies in object-oriented programming languages, and that in better languages, these problems simply don't come up, or are solved so trivially and so easily that the solution doesn't require a "pattern". In assembly language, "subroutine call" may be a pattern; in C, the solution is to write result = function(args...), which is too simple to qualify as a pattern. In a language like Lisp or Haskell or even Perl, with a good list type and powerful primitives for operating on list values, the Iterator pattern is to a great degree obviated or rendered invisible. Henry G. Baker took up this same point in his paper "Iterators: Signs of Weakness in Object-Oriented Languages".

I received many messages about this, and curiously, some made the same point in the same way: they said that although I was right about Iterator, it was a poor example because it was a very simple pattern, but that it was impossible to imagine a more complex pattern like Model-View-Controller being absorbed and made invisible in this way.

This remark is striking for several reasons. It is an example of what is perhaps the most common philosophical fallacy: the writer cannot imagine something, so it must therefore be impossible. Well, perhaps it is impossible—or perhaps the writer just doesn't have enough imagination. It is worth remembering that when Edgar Allan Poe was motivated to investigate and expose Johann Maelzel's fraudulent chess-playing automaton, it was because he "knew" it had to be fraudulent because it was inconceivable that a machine could actually exist that could play chess. Not merely impossible, but inconceivable! Poe was mistaken, and the people who asserted that MVC could not be absorbed into a programming language were mistaken too. Since I gave my talk in 2002, several programming systems, such as Ruby on Rails and Subway have come forward that attempt to codify and integrate MVC in exactly the way that I suggested.

Progress in programming languages

Identification of patterns is an important driver of progress in programming languages. As in all programming, the idea is to notice when the same solution is appearing repeatedly in different contexts and to understand the commonalities. This is admirable and valuable. The problem with the "Design Patterns" movement is the use to which the patterns are put afterward: programmers are trained to identify and apply the patterns when possible. Instead, the patterns should be used as signposts to the failures of the programming language. As in all programming, the identification of commonalities should be followed by an abstraction step in which the common parts are merged into a single solution.

Multiple implementations of the same idea are almost always a mistake in programming. The correct place to implement a common solution to a recurring design problem is in the programming language, if that is possible.

The stance of the "Design Patterns" movement seems to be that it is somehow inevitable that programmers will need to implement Visitors, Abstract Factories, Decorators, and Façades. But these are no more inevitable than the need to implement Subroutine Calls or Object-Oriented Classes in the source language. These patterns should be seen as defects or missing features in Java and C++. The best response to identification of these patterns is to ask what defects in those languages cause the patterns to be necessary, and how the languages might provide better support for solving these kinds of problems.

With Design Patterns as usually understood, you never stop thinking about the patterns after you find them. Every time you write a Subroutine Call, you must think about the way the registers are saved and the return value is communicated. Every time you build an Object-Oriented Class, you must think about the implementation of inheritance.

People say that it's all right that Design Patterns teaches people to do this, because the world is full of programmers who are forced to use C++ and Java, and they need all the help they can get to work around the defects of those languages. If those people need help, that's fine. The problem is with the philosophical stance of the movement. Helping hapless C++ and Java programmers is admirable, but it shouldn't be the end goal. Instead of seeing the use of design patterns as valuable in itself, it should be widely recognized that each design pattern is an expression of the failure of the source language.

If the Design Patterns movement had been popular in the 1980's, we wouldn't even have C++ or Java; we would still be implementing Object-Oriented Classes in C with structs, and the argument would go that since programmers were forced to use C anyway, we should at least help them as much as possible. But the way to provide as much help as possible was not to train people to habitually implement Object-Oriented Classes when necessary; it was to develop languages like C++ and Java that had this pattern built in, so that programmers could concentrate on using OOP style instead of on implementing it.

Summary

When we identify and document one, that should not be the end of the story. Rather, we should have the long-term goal of trying to understand how to improve the language so that the pattern becomes invisible or unnecessary.

[ Thanks to Garrett Rooney for pointing out some minor errors that I have since corrected. - MJD ]

[ Addendum 20061003: There is a followup article to this one, replying to a response by Ralph Johnson, one of the authors of the "Design Patterns" book. This link URL is correct, but Johnson's website will refuse it if you come from here. ]

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Imaginary units, revisited
Last night, shortly after posting my article about the fact that i and -i are mathematically indistinguishable, I thought of what I should have said about it—true to form, forty-eight hours too late. Here's what I should have said.

The two square roots of -1 are indistinguishable in the same way that the top and bottom faces of a cube are. Sure, one is the top, and one is the bottom, but it doesn't matter, and it could just as easily be the other way around.

Sure, you could say something like this: "If you embed the cube in R³, then the top face is the set of points that have z-coordinate +1, and the bottom face is the set of points that have z-coordinate -1." And indeed, once you arbitrarily designate that one face is on the top and the other is on the bottom, then one is on the top, and one is on the bottom—but that doesn't mean that the two faces had any a priori difference, that one of them was intrinsically the top, or that the designation wasn't completely arbitrary; trying to argue that the faces are distinguishable, after having made an arbitrary designation to distinguish them, is begging the question.

Now can you imagine anyone seriously arguing that the top and bottom faces of a cube are mathematically distinguishable?

[ Previous article in this series: Part 1 Followup articles: Part 3 Part 4 Part 5 ]

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I get a new job
Where did my blog go for the past six weeks? Well, I was busy with another project. Usually, when I am busy with a project, it shows up here, because I am thinking about it, and I want to write about what I am thinking. As Hans Arp said, it grows out of me and I keep cutting it off, like toenails. But in this case I could not write about the project here, because it was a secret. I was looking for a new job, and I did not want my old job to find out before I was ready.

(Many people have been surprised to learn that I have a job; they remember that for many years I was intermittently a software consultant and itinerant programming trainer. But since January 2004 I have been regularly employed to do maintenance programming for the University of Pennsylvania's Networking and Telecommunications group.)

Anyway, the job hunt has come to a close. I accepted a new job, put in my resignation letters at the old one, and can stop thinking about it for a while. The new work will be head software engineer at the Penn Genomics Institute. I will try to develop software for genetic biologists to use in their research. I expect that the new job will suit me somewhat better than the old one. I like that it is connected to science, and that I will be working with scientists. The work itself is important; genomics is going to change everything in the world. Also, it pays rather more than the old one, although that was not the principal concern.

So with any luck blog posts will resume here, and eventually some genomics-related articles may start appearing.

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Imaginary units
Yesterday I had a phenomenally annoying discussion with the pedants on the IRC #math channel. Someone was talking about square roots, and for some reason I needed to point out that when you are considering square roots of negative numbers, it is important not to forget that there are two square roots.

I should back up and discuss square roots in more detail. The square root of x, written √x, is defined to be the number y such that y² = x. Well, no, that actually contains a subtle error. The error is in the use of the word "the". When we say "the number y such that...", we imply that there is only one. But every number (except zero) has two square roots. For example, the square roots of 16 are 4 and -4. Both of these are numbers y with the property that y² = 16.

In many contexts, we can forget about one of the square roots. For example, in geometry problems, all quantities are positive. (I'm using "positive" here to mean "≥ 0".) When we consider a right triangle whose legs have lengths a and b, we say simply that the hypotenuse has length √(a² + b²), and we don't have to think about the fact that there are actually two square roots, because one of them is negative, and is nonsensical when discussing hypotenuses. In such cases we can talk about the square root function, sqrt(x), which is defined to be the positive number y such that y² = x. There the use of "the" is justified, because there is only one such number. But pinning down which square root we mean has a price: the square root function applies only to positive arguments. We cannot ask for sqrt(-1), because there is no positive number y such that y² = -1. For negative arguments, this simplification is not available, and we must fall back to using √ in its full generality.

In high school algebra, we all learn about a number called i, which is defined to be the square root of -1. But again, the use of the word "the" here is misleading, because "the" square root is not unique; -1, like every other number (except 0) has two square roots. We cannot avail ourselves of the trick of taking the positive one, because neither root is positive. And in fact there is no other trick we can use to distinguish the two roots; they are mathematically indistinguishable.

The annoying discussion was whether it was correct to say that the two roots are mathematically indistinguishable. It was annoying because it's so obviously true. The number i is, by definition, a number such that i² = -1. This is its one and only defining property. Since there is another number which shares this single defining property, it stands to reason that this other root is completely interchangeable with i—mathematically indistinguishable from it, in other words.

This other square root is usually written "-i", which suggests that it's somehow secondary to i. But this is not the case. Every numerical property possessed by i is possessed by -i as well. For example, i³ = -i. But we can replace i with -i and get (-i)³ = -(-i), which is just as true. Euler's famous formula says that e^ix = cos x + i sin x. But replacing i with -i here we get e^-ix = cos x + -i sin x, which is also true.

Well, one of them is i, and the other is -i, so can't you distinguish them that way? No; those are only expressions that denote the numbers, not the numbers themselves. There is no way to know which of the numbers is denoted by which expression, and, in fact, it does not even make much sense to ask which number is denoted by which expression, since the two numbers are entirely interchangeable. One is i, and one is -i, sure, but this is just saying that one is the negative of the other. But so too is the other the negative of the one.

One of the #math people pointed out that there is a well-known Im() function, the "imaginary part" function, such that Im(i) = 1, but Im(-i) = -1, and suggested, rather forcefully, that they could be distinguished that way. This, of course, is hopeless. Because in order to define the "imaginary part" function in the first place, you must start by making an entirely arbitrary choice of which square root of -1 you are using as the unit, and then define Im() in terms of this choice. For example, one often defines Im(z) as !!z - \bar{z} \over 2i!!. But in order to make this definition, you have to select one of the imaginary units and designate it as i and use it in the denominator, thus begging the question. Had you defined Im() with -i in place of i, then Im(i) would have been -1, and vice versa.

Similarly, one #math inhabitant suggested that if one were to define the complex numbers as pairs of reals (a, b), such that (a, b) + (c, d) = (a + c, b + d), (a, b) × (c, d) = (ac - bd, ad + bc), then i is defined as (0,1), not (0,-1). This is even more clearly begging the question, since the definition of i here is solely a traditional and conventional one; defining i as (0, -1) instead of (0,1) works exactly as well; we still have i² = -1 and all the other important properties.

As IRC discussions do, this one then started to move downwards into straw man attacks. The #math folks then argued that i ≠ -i, and so the two numbers are indeed distinguishable. This would have been a fine counterargument to the assertion that i = -i, but since I was not suggesting anything so silly, it was just stupid. When I said that the numbers were indistinguishable, I did not mean to say that they were numerically equal. If they were, then -1 would have only one square root. Of course, it does not; it has two unequal, but entirely interchangeable, square roots.

The that the square roots of -1 are indistinguishable has real content. 1 has two square roots that are not interchangeable in this way. Suppose someone tells you that a and b are different square roots of 1, and you have to figure out which is which. You can do that, because among the two equations a² = a, b² = b, only one will be true. If it's the former, then a=1 and b=-1; if the latter, then it's the other way around. The point about the square roots of -1 is that there is no corresponding criterion for distinguishing the two roots. This is a theorem. But the result is completely obvious if you just recall that i is merely defined to be a square root of -1, no more and no less, and that -1 has two square roots.

Oh well, it's IRC. There's no solution other than to just leave. [ Addenda: Part 2 Part 3 Part 4 Part 5 ]

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Oyster jokes
Last week I heard a pathetically bad joke about oysters. Here it is:

What did the girl oyster say to the boy oyster?

"You never open up to me."

But it seems to me that there is a lot of unexploited material to be gotten from oysters.

For example, oysters, considered as food, are famous for their aphrodisiac properties. It ought to be possible to do something with that. What do the boy and the girl oyster use as aphrodisiacs? Does it involve oyster cannibalism? So much the better. Can the aphrodisiac cannibalism be tied to oral sex somehow? Better still. How could a joke about oyster cunnilingus fail to be hilarious?

Moreover, oysters are hermaphrodites. Surely there is some farcical oyster humor available from the fact that the boy and the girl oysters might in fact be the same individual. Now we have oyster autofellatial autocannibalism. It's both dirty and disgusting!

I was not able to come up with any oyster jokes, however, and a quick web search turned up nothing of value. Really nothing. Don't waste your time. I found one joke that was introduced with "Jennifer sent in this great oyster joke..." and then the joke wasn't even about oysters; it was about the ingestion of testicles. And I had heard it before.

I think there's a small gap in the world just the size and shape of a good oyster-themed joke. Don't you? Here is your big chance to make up a joke that nobody has ever heard before. Please send me your oyster jokes.

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Flipping coins, corrected
In a recent article about coin flipping, I said:

After a million tosses of a fair coin, you can expect that the numbers of heads and tails will differ by about 1,000.

...

In general, if you flip the coin n times, the expected difference between the numbers of heads and tails will be about √n.

I correctly remembered that the expected difference is on the order of √n, but forgot that the proportionality constant was not 1.

The main point of my article, however, is still correct. I said that the following assertion is not quite right (although not quite wrong either):

Over a million tosses you'll have almost the same amount of heads as tails

I pointed out that although the relative difference tends to get small, the absolute difference tends to infinity. This is still true.

Thanks to James Wetterau for pointing out my error.

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Flipping coins
A gentleman on IRC recently said:

Over a million tosses you'll have almost the same amount of heads as tails

Well, yes, and no. It depends on how you look at it.

After a million tosses of a fair coin, you can expect that the numbers of heads and tails will differ by about 1,000. This is a pretty big number.

On the other hand, after a million tosses of a fair coin, you can expect that the numbers of heads and tails will differ by about 0.1%. This is a pretty small number.

In general, if you flip the coin n times, the expected difference between the numbers of heads and tails will be about √n. As n gets larger, so does √n. So the more times you flip the coin, the larger the expected difference in the two totals.

But the relative difference is the quotient of the difference and the total number of flips; that is, √n/n = 1/√n. As n gets larger, 1/√n goes to zero. So the more times you flip the coin, the smaller the expected difference in the two totals.

It's not quite right to say that you will have "almost the same amount of heads as tails". But it's not quite wrong either. As you flip the coin more and more, you can expect the totals to get farther and farther apart—but the difference between them will be less and less significant, compared with the totals themselves.

[ Addendum 20060720: Although the main point of this article is correct, I made some specific technical errors. A correction is available. ]

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Someone else's mistake
I don't always screw up, and here's a story about mistake I could have made, but didn't.

Long ago, I was fresh out of college, looking for work. I got a lead on what sounded like a really excellent programming job, working for a team at University of Maryland that was preparing a scientific experiment that would be sent up into space in the Space Shuttle. That's the kind of work you dream about at twenty-one.

I went down for an interview, and met the folks working on the project. All except for the principal investigator, who was out of town.

The folks there were very friendly and helpful, and answered all my questions, except for two. I asked each person I met what the experiment was about, and what my programs would need to do. And none of them could tell me. They kept saying what a shame it was that the principal investigator was out of town, since he would have been able to explain it all to my satisfaction. After I got that answer a few times, I started to get nervous.

I'm sure it wasn't quite that simple at the time. I imagine, for example, that there was someone there who didn't really understand what was going on, but was willing to explain it to me anyway. I think they probably showed me some other stuff they were working on, impressed me with a tour of the lab, and so on. But at the end of the day, I knew that I still didn't know what they were doing or what they wanted me to do for them.

If that happened to me today, I would probably just say to them at 5PM that their project was doomed and that I wouldn't touch it with a ten-foot pole. At the time, I didn't have enough experience to be sure. Maybe what they were saying was true, maybe it really wasn't a problem that nobody seemed to have a clue as to what was going on? Maybe it was just me that was clueless? I had never seen anything like the situation there, and I wasn't sure what to think.

I didn't have the nerve to turn the job down. At twenty-one, just having gotten out of school, with no other prospects at the time, I didn't have enough confidence to be quite sure to throw away this opportunity. But I felt in my guts that something was wrong, and I never got back to them about the job.

A few years afterward I ran into one of those guys again, and asked him what had become of the project. And sure enough, it had been a failure. They had missed the deadline for the shuttle launch, and the shuttle had gone up with a box of lead in it where their experiment was supposed to have been.

Looking back on it with a lot more experience, I know that my instincts were right on, even though I wasn't sure at the time. Even at twenty-one, I could recognize the smell of disaster.

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Phrasal verbs
My mom teaches English to visiting foreign students, and last time I met her she was talling me about phrasal verbs. A phrasal verb is a verb that incorporates a preposition. Examples include "speed up", "try out", "come across", "go off", "turn down". The prepositional part is uninflected, so "turns down", "turned down", "turning down", not *"turn downs", *"turn downed", *"turn downing". My mom says she uses a book that has a list of all of them; there are several hundred. She was complaining specifically about "go off", which has an unusually peculiar meaning: when the alarm clock goes off in the morning, it actually goes on.

This reminded me that "slow up" and "slow down" are synonymous. And there is "speed up", but no "speed down". And you cannot understand "stand down" by analogy with "stand up", "sit up", and "sit down". And you also cannot understand "nose job" by analogy with "hand job". But I digress.

One of the things about the phrasal verbs that gives the foreign students so much trouble is that the verbs don't all obey the same rules. For example, some are separable and some not. Consider "turned down". I can turn down the thermostat, but I can also turn the thermostat down. And I can try out my new game, and I can also try my new game out. And I can stand up my blind date, and I can stand my blind date up. But while I can come across a fountain in the park, I can't *come a fountain across in the park. And while I can go off to Chicago, I can't *go to Chicago off. There's no way to know which of these work and which not, except just by memorizing which are allowed and which not.

And sometimes the separable ones can't be unseparated. I can give back the map, and I can give the map back, and I can give it back, but I can't *give back it. I can hold up the line, and I can hold the line up, and I can hold us up, but I can't *hold up us. I don't know what the rule is exactly, and I don't want to go to the library again to get the Cambridge Grammar, because last time I did that I dropped it on my toe.

I hadn't realized any of this until I read this article about them, but when I did, I had a sudden flash of insight. I had not realized before what was going on when someone set up us the bomb. "Set up" is separable: I can set up the bomb, or set the bomb up, or someone can set us up. But "us", as noted above, is not deseperable, so you cannot have *set up us. But I think I understand the mistake better now than I did before; it seems less like a complete freak and more like a member of a common type of error.

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A programmer had a problem...
A while back, I wrote an article in which I mentioned a programmer who had a problem, tried to solve it with weak references, and, as a result, had two problems. I said that weak references work unusually well in that little formula.

Yesterday I was about to make the same mistake. I had a problem, and weak references seemed like the solution. Fortunately, it was time to go home, which is a two-mile walk. Taking a two-mile walk is a great way to fix mistakes, especially the ones you haven't made yet. On this particular walk, I came to my senses and avoided the weak references.

The problem concerns the following classes and methods. You have a database object $db. You can call @rec = $db->lookup, which may return some record objects that represent records. You then call methods on the records, say $rec[3]->get_color, to extract data from them, or $rec[3]->set_color("purple"), to modify the data in the records. The updating is done in-memory only, and a later call to $db->flush writes all the updates back to the database.

The database object needs to store the changes that have been made but not yet written out. The easy way to do this is to have it store a change log of the modified record objects. So set_color first makes its change to the target record object, and then calls an internal _update method on the original database object to attach the record to the change log. Later on, flush will process this array, writing out the indicated changes.

In order for set_color to know which database to direct the _update call to, each record object must have a pointer back to the database that created it. This is convenient for other purposes too. Fine. But then if the record object is stored in the change log inside the database object, we now have a reference loop: the database contains a change log with a pointer to the record, which contains a pointer back to the database itself. This means that neither the database nor the record will ever be garbage collected. (This problem is common in complex Perl programs, and would simply vanish if Perl had even a slightly less awful garbage collector. Improvement is unlikely to occur before the release of Perl 6, now scheduled for October 28, 2073.)

My first reaction when faced with a problem like this one is to gurgle contentedly in my sleep, turn over, and pull the blankets over my head. This strategy is the primary contributor to my success as a programmer; it is somewhat superior to the typical programmer's response, which is to swing into action, overthink the problem, and come up with an elaborate solution. Aron Nimzovitch once said that the problem chess novices have is the irrepressible urge to always be doing something. Programmers are similar. They are all very bright people, very good at solving problems, and they solve problems all the time, even the ones that don't need to be solved.

I seem to be digressing. How unusual. In any case, this problem really did have to be solved. One wants the database object to flush out its pending changes at the time it becomes inacessible. If the object is never garbage collected, then the programmer must always remember to flush out the changes manually. Miss one call to flush, and your updates are lost. This is unacceptable. The primary purpose of a database is to record the updates. So I had to take my head out from under the covers, like it or not.

I thought about several solutions, and even tried one out, but it was too complicated and got me into a horrible tar pit, so I threw it away and started over. (That is another superior strategy that programmers don't exercise as often as they should. As Erik Naggum says, they will drive a hundred miles through a forest, stopping every five feet to cut down another tree, instead of pausing to wonder if maybe they shouldn't have driven off the road in the first place.)

Then I got the bright idea to use weak references, which seemed like just the thing. That's what weak references are for: breaking dependency loops so that things that need to be garbage collected can be. Fortunately, it was time to go, so I walked home instead of diving into the chyme-filled swimming pool of weak references.

With the weak references, you need to decide which reference to weaken. There is a reference to the record object, in the change log inside the database object. And there is a reference to the database object, in the record object. Which do you weaken?

If you weaken the reference to the record, you get a disaster:

        {
          my ($rec) = $db->lookup(...);
          $rec->set_color("purple");
        }
        $db->flush;

What if you weaken the reference inside the record, the one that points back to the database? That is hardly any better:

        my $rec;
        {
          my $db = FlatFile->new(...);
          ($rec) = $db->lookup(...);
        }
        $rec->set_color("purple");

Conclusion: I've heard it before, and it wasn't funny the first time.

On the walk home, I realized something else: actually storing the database data inside the record objects is a bad move. The general advice under which this is a bad move is something like Don't store the same data in two places. The specific problems in this instance are exemplified by this:

        my ($a) = $db->lookup(unique_id => "142857");
        my ($b) = $db->lookup(unique_id => "142857");
        $a->set_color("red");
        $b->set_color("purple");
        $a->color eq "purple";  # True or false?

To work properly, $a and $b should be identical objects. One way to do this is to store an object in memory for every record in the database, and hand out these preconstructed objects as needed; then both calls to lookup return the same object. This is time- and memory-intensive. Another way to do this is to cache the record objects as they are constructed, and arrange for lookup to return the cached objects when appropriate. This is more complicated.

A simpler solution is not to store the data in memory at all. Record objects are always created as needed, but contain nothing but a database handle and some sort of locator information that says how to get the record data, should it be asked for. ("Any problem can be solved by another layer of indirection," they say, although it's not really true. Still, there are several classes of problems that can be solved by adding another layer of indirection, and this particular object identity problem could serve well as an exemplar of one of those classes.) Then modifications don't go into the record objects themselves. Instead, they go into the database object as an instruction to modify a certain record in a certain way.

This solution, however, presupposes that there is a good way to build locator information for a flat file and update it as needed. Fortunately, there is. I did a really good job of solving this problem a few years ago when I wrote the Tie::File module. It represents a text file as a Perl array, so a record locator can simply be an index into the array, and a record object then becomes something like:

        {
          db => $db,
          recno => 37,
        }

        { 0 => no change,
          1 => no change,
          2 => "color" field was set to "purple",
          3 => no change,
          4 => "size" field was set to "unusually large",
          ...
        }        

Using Tie::File also eliminates a lot of I/O issues that I had solved before, and gets all the I/O code out of the database module. I had already been thinking about getting rid of the explicit I/O and having the database module depend on Tie::File, and when I recognized the lurking record object identity problem, I was convinced that it had to happen sooner rather than later. Having done it, I'm really pleased with the outcome.

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On design
I'm writing this Perl module called FlatFile, which is supposed to provide lightweight simple access to flat-file databases, such as the Unix password file. An interesting design issue came up, and since I think that understanding is usually best served by minuscule examination of specific examples, that's what I'm going to do.

The basic usage of the module is as follows: You create a database object that represents the entire database:

        my $db = FlatFile->new(FILE => "/etc/passwd", 
                               FIELDS => ['username', 'password', 'uid', 'gid',
                                          'gecos', 'homedir', 'shell'],
                               FIELDSEP => ':',
                              ) or die ...;

        my @roots = $db->lookup(uid => 0);

        for my $root (@roots) {
          if ($root->username eq 'root') {
            $root->set_shell('/bin/false');
          } else {
            $root->delete;
          }
        }

Since lookup returns all the matching records, there is the question of what this should do:

        my $root = $db->lookup(uid => 0);

Every Perl function needs to make a decision about this issue. The function could be called in list context or in scalar context, and you need to choose the two behaviors sensibly. Here are some possibilities for what lookup might do if called in scalar context:

1. die unconditionally

2. return the number of matching records, analogous to the builtin grep function or the @array syntax

3. return the single matching record, if there is only one, and die if there is more than one.

4. return the first matching record, and discard the others

5. return a reference to an array of all matching records

6. return an iterator object which can be used to access all the matching records

How to decide on the best behavior? This is the kind of problem that I really enjoy. What will people expect? What will they want? What do they need?

Two important criteria are:

1. Difficulty: Whatever I provide should be something that's not easy to get any other way.

2. Usefulness: Whatever I provide should be something that people will use a lot.

        my $ref = [ $db->lookup(...) ];

        sub lookup_ref {
          my $self = shift;
          [ $self->lookup(@_) ];
        }

        my $N_recs = () = $db->lookup(...);

I ran the question by the folks on IRC, and reaction against this design was generally negative. Folks said that it's not the module's job to try to discern the programmer's intention and enforce this inference by committing suicide.

I can certainly get behind that point of view. I once wrote an article complaining bitterly about modules that call die. I said it was like when you're having tea and crumpets on your 112-piece Spode china set, and you accidentally chip the teacup, and the butler comes running in, crying "Don't worry, Master! I'll take care of that for you!" and then he whips out a hammer and smashes all 112 pieces of china to tiny bits.

I don't think the point applies here, though. I had mentioned it in connection with the Text::ParseWords module, which would throw an exception if the input string was unparseable, hardly an uncommon occurrence, and one that was entirely unavoidable: if I knew that the string would be unparseable, I wouldn't be calling Text::ParseWords to parse it.

Folks on IRC said that when the method might call die, you have to wrap every call to it in an exception handler, which I certainly agree is a pain in the ass. But in this example, you do not have to do that. Here, to prevent the function from dying is very easy: just call it in list context; then it will never die. If what you want is behavior #4, to have it discard all the records but the first one, that is easy to get, regardless of the design I adopt for scalar context behavior:

        my ($rec) = $db->lookup(...);

        sub lookup_first {
          my $self = shift;
          my ($rec) = $self->lookup(@_);
          return $rec;
        }

I was almost persuaded by the strong opinion in favor of #4, but then Roderick Schertler spoke up in favor of #3, for basically the reasons I set forth. I consider M. Schertler to have higher-than-normal reliability on matters of this type, so his opinion counterbalances several of the counteropinions on the other side. #3 is not too difficult to get, but still scores higher than most of the others on the difficulty scale. There doesn't seem to be a trivial inline expression of it, as there was with #2, #4, and #5. You would have to actually write a method, or else do something nasty like:

        (my ($rec) = $db->lookup(...)) < 2 or die ...;

#6 (return an iterator object) is very tempting, because it is the only one that scores high on the difficulty criterion scale: it is difficult or impossible to do this any other way, so by providing it, I am providing a real service to users of the module, rather than yet another way to do the same thing. The module's user cannot implement a good iterator interface as a wrapper around lookup, because lookup always searches the entire database before it returns, and allocates enough memory to store every returned record, whereas a good iterator interface will search only as far as is necessary to find the next matching record, and will store only one record at a time.

This performance argument would be more important if we expected the databases to be very large. But since this is a module for manipulating plain text files, we can expect that they will not be too big, and perhaps the time and memory costs of searching them will be relatively small, so perhaps this design will score fairly low on the usefulness scale.

I still haven't made up my mind, although writing this article has pushed me strongly toward #6. I would be glad to receive email on the matter.

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Contravariant types
I just had a slightly frustrating discussion with some colleagues, involving a small matter of object-oriented design. I don't want to get into the details of the problem here, except to say that it involved a class A and its derived class B; I was asking for advice about the design of a "demote" method that would take a B object and turn it into an A object.

The frustrating part was that about half of the people in the conversation were confused by my use of the word "demotion" and about whether A was inheriting from B or vice versa. I had intended for B to inherit from A. The demotion, as I said, takes a B object and gives you back an equivalent but stripped-down A object.

To me, this makes perfect sense, logically and terminologically. Demotion implies movement downward. Downward is toward the base class; that's why it's the "base" class. A is the base class here, so the demotion operation takes a B and gives you back an A.

Or, to make the issue clearer with an example, suppose that the two classes are Soldier and General. Which inherits from the other? Obviously, General inherits from Soldier, and not vice-versa. Soldiers support methods for marching, sleeping, and eating. Generals inherit all these methods, and support additional methods for ordering attacks and for convening courts martial. What does a demotion method do? It turns a General into a Soldier. It turns an object of the derived class into an object of the base class.

So how could people get this mixed up? I'm not sure, but I think one possibility is that they were thinking of subclasses and superclasses. The demotion method takes an object in the subclass and returns an object in the superclass. The terminology here is backwards. There are lots and lots of people, me included, who never use the terms "subclass" and "superclass", for precisely this reason. Even if my colleagues weren't thinking of these terms, they were probably thinking of the conventional class inheritance diagram, in which the base class, contrary to its name, is at the top of the diagram, with the derived classes hanging under it. The demotion operation, in this picture, pushes an object upwards, toward the base class.

The problem with "subclass" and "superclass" runs deeper. Mathematical terminology for sets is well-established and intuitive: A is a "subset" of B if set A is entirely contained in set B, if every element of A is an element of B. For example, the set of generals is a subset of the set of soldiers. The converse relation is that B is a superset of A: the set of soldiers is a superset of the set of generals. We expect from the names that a subset will be a smaller set than its superset, and so it is. There are fewer generals than soldiers.

Now let's consider programming language types. A type can be considered to be just a set of values. For example, the int type is the set of all integer values. The real type is the set of all real number values. Since every integer is also a real number, we might say that the int type is a subset of the real type. In fact, the word we usually use is that int is a subtype of real. But "subtype" means no more and no less than "subset".

Now let's consider the types General and Soldier of all objects of classes General and Soldier respectively. Clearly, General is a subtype of Soldier, since every General is a Soldier. This matches the OOP terminology also: General is a subclass of Soldier.

The confusing thing for data types, I think, is that there are two ways in which a type can be a "subtype" of another. A could be a smaller set than B, in which case we use the words "subtype" and "subclass", in accordance with mathematical convention. But A could also support a smaller set of operations than B; in OOP-world we would say that A is a base class and B a derived class. But then B is a subclass of A, which runs counter to the terminological implication that A is at the "base".

(It's tempting to add a long digression here about how computer scientists always draw their trees with the root at the top and the leaves at the bottom, and then talk about how many nodes are under the root of the tree. I will try to restrain myself.)

Anyway, this contravariance is what I really wanted to get at. If we adopt the rule of thumb that most values support few operations, and a few values support some additional operations, then the containment relation for functionality is contravariant to the containment relation for sets. Large sets, like Soldier, support few operations, such as eat and march; smaller sets support more operations, such as convene_court_martial.

The thing that struck me about this is that functions themselves are contravariant. Suppose A and B are types. Now consider the type A×B of pairs of values where the first component is an A and the second is a B. This pairing operation is covariant in A and B. By this I mean that if A' is a subtype of A, then A'×B is a subtype of A×B. Similarly, if B' is a subtype of B, then A×B' is a subtype of A×B.

Similarly, +, the type sum operation, is also covariant in both of its arguments.

But function types are different. Suppose A → B is the type of functions whose arguments have type A and whose return values are type B. Then A → B' is a subtype of A → B. Here's a simple example: Let A and B be real, and let A' and B' be int. Then every int → int—that is, every function from integers to integers—is also an example of a int → real; it can be considered as a function that takes an int and returns a real. That's because it is actually returning an int, and an int is a kind of real.

But A' → B is not a subtype of A → B. Just the opposite: A → B is a subtype of A' → B.

I remember standing on a train platform around 1992 and realizing this for the first time, that containment of function types was covariant in the second component but contravariant in the first component. I was quite surprised.

I suspect that the use of "covariant" and "contravariant" here suggests some connection with category theory, and with the notions of covariant and contravariant functors, but I don't know what the connection is.

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484848 is excellent
Brad Murray wrote to request a proof that the number 4848...4848 is excellent:

So, are all concatenations of odd numbers of "48" excellent? I demand a proof!

First let's define the items we're talking about:

• a₀ = 4, a_n+1 = 100a_n + 84, so that the a_n are 4, 484, 48484, etc.

• Similarly, b₀ = 8, b_n+1 = 100b_n + 48, so that the b_n are 8, 848, 84848, etc.

• Similarly, c₀ = 48, c_n+1 = 10000c_n + 4848, so that the c_n are 48, 484848, 4848484848, etc.

• It's obvious, or even if not,
• it follows from an easy inductive proof, which
• is left as an exercise for the reader.

In order to show that c_n is excellent, it then suffices to show that c_n = b_n² - a_n² for all n.

First we'll prove the following lemma: for all n, 4b_n - 7a_n = 4. This follows easily by induction. For n=0, we have 4·8 - 7·4 = 32 - 28 = 4. Now suppose the lemma is proved for n=i; we want to show that it is true for n=i+1. That is, we want to calculate:

4bi+1 - 7ai+1	=
4(100bi+48) - 7(100ai+84)	=
400bi + 192 - 700ai - 588	=
400bi + 700ai + 192 - 588	=
100(4bi - 7ai) - 396	=
100·4 - 396	=
4

And we are done.

Now the main theorem, again by induction. We want to show that:

b_n² - a_n² = c_n

bi+12 - ai+12	=
(100bi + 48)2 - (100ai + 84)2	=
10000bi2 + 9600bi + 2304 - 10000ai2 - 16800ai - 7056	=
10000(bi2 - ai2) + 2400(4bi - 7ai) + 2304 - 7056	=
10000ci + 2400(4bi - 7ai) - 4752	=
10000ci + 2400·4 - 4752	=
10000ci + 4848	=
ci+1

Q.E.D.

I may have more to say about this later. I have a half-written article that complains about homework questions of the form "Solve problem X using technique Y," where Y is something like induction. The article was inspired by a particularly odious problem of this type:

if n + 1 balls are put inside n boxes, then at least one box will contain more than one ball. prove this principle by induction.

A student faced with this kind of question will conclude (correctly) that he or she is being forced to jump through a pointless hoop, and may conclude (incorrectly) that induction is useless. And students are frequently confused by pointless applications of principles. People learn better when they understand why things are happening; when students feel that they don't understand the point of what is being done, they feel that they don't understand the mechanics either.

In the real world—by which I mean what real scientists, mathematicians, and engineers do, in addition to what people in the grocery store do—I am excluding only homework assignments—we almost never get a problem of the form "solve X using technique Y". Problems we face in the real world always have the form "solve X, by hook or by crook." The closest we ever see to a prescribed technique are mere suggestions like "Well, Y might work here, so you could try that."

Questions that prescribe techniques are either lazy pedagogy or bad curriculum design. If technique Y—say, induction—is a useful technique, then it is because there is some problem X such that Y is superior to all other techniques for solving X. If all such X are outside the scope of the class, then Y is outside the scope of the class too. If, on the other hand, there is some X that is in the scope of the class, it is the instructor's job to find it and present it to the students, as an instructive example. To fail in this, and to make up a contrived and irrelevant problem in place of X, is a failure of the instructor's principal duty, which is to illustrate the subject matter by realistic and relevant examples.

For the theorem above about 484848, induction is clearly a good way to solve it; to solve the problem by direct calculation is painful.

There are other things to learn from the demonstration above. It serves as a wonderful example of what is wrong with standard mathematical style for writing up proofs. A student seeing this proof might well ask "where the heck did you get that lemma about 4b - 7a = 4? Is that something you knew from before? Did you just guess? Was it in the book somewhere?" But no, I did not guess, I did not know that before, and I did not get it from the book.

The answer is that I did the main demonstration first, starting with b_i² - a_i² and trying to get from there to c_i by using algebraic manipulations and the definitions of a, b, and c. And just when everything seemed to be going along well, I got stuck. I had:

10000c_i + 2400(4b_i - 7a_i) - 4752

10000c_i + 4848

So if it was going to work, I needed to have:

2400(4b_i - 7a_i) - 4752 = 4848

2400(4b_i - 7a_i) = 9600

4b_i - 7a_i = 4.

But a quick check of a couple of examples shows that 4b_i - 7a_i = 4 does work, at least for i=0 and 1, so maybe it would worth trying to prove in the general case. And indeed, the proof went through fine, and I won.

But in the presentation of the proof, everything is backwards: I pull the mystery lemma out of my ass at the beginning for no apparent reason, and then later on it happens to be what what I need at the crucial moment. Almost as if I knew beforehand what was going to happen!

There are a lot of things wrong with mathematics pedagogy, and those were two of them: artifically prescribed techniques to solve homework problems, and the ass-extraction of lemmas backwards in time.

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1+1=2

Buy Principia Mathematica (through section 56) from Bookshop.org (with kickback) (without kickback)

Principia Mathematica is an odd book, worth looking into from a historical point of view as well as a mathematical one. It was written around 1910, and mathematical logic was still then in its infancy, fresh from the transformation worked on it by Peano and Frege. The notation is somewhat obscure, because mathematical notation has evolved substantially since then. And many of the simple techniques that we now take for granted are absent. Like a poorly-written computer program, a lot of Principia Mathematica's bulk is repeated code, separate sections that say essentially the same things, because the authors haven't yet learned the techniques that would allow the sections to be combined into one.

For example, section ∗22, "Calculus of Classes", begins by defining the subset relation (∗22.01), and the operations of set union and set intersection (∗22.02 and .03), the complement of a set (∗22.04), and the difference of two sets (∗22.05). It then proves the commutativity and associativity of set union and set intersection (∗22.51, .52, .57, and .7), various properties like !!\alpha\cap\alpha = \alpha!! (∗22.5) and the like, working up to theorems like ∗22.92: !!\alpha\subset\beta \rightarrow \alpha\cup(\beta - \alpha)!!.

Section ∗23 is "Calculus of Relations" and begins in almost exactly the same way, defining the subrelation relation (∗23.01), and the operations of relational union and intersection (∗23.02 and .03), the complement of a relation (∗23.04), and the difference of two relations (∗23.05). It later proves the commutativity and associativity of relational union and intersection (∗23.51, .52, .57, and .7), various properties like !!\alpha\dot\cap\alpha = \alpha!! (∗22.5) and the like, working up to theorems like ∗23.92: !!\alpha\dot\subset\beta \rightarrow \alpha\dot\cup(\beta \dot- \alpha)!!.

The section ∗24 is about the existence of sets, the null set !!\Lambda!!, the universal set !!{\rm V}!!, their properties, and so on, and then section ∗24 is duplicated in ∗25 in a series of theorems about the existence of relations, the null relation !!\dot\Lambda!!, the universal relation !!\dot {\rm V}!!, their properties, and so on.

That is how Whitehead and Russell did it in 1910. How would we do it today? A relation between S and T is defined as a subset of S × T and is therefore a set. Union, intersection, difference, and the other operations are precisely the same for relations as they are for sets, because relations are sets. All the theorems about unions and intersections of relations, like , just go away, because we already proved them for sets and relations are sets. The null relation is the null set. The universal relation is the universal set.

A huge amount of other machinery goes away in 2006, because of the unification of relations and sets. Principia Mathematica needs a special notation and a special definition for the result of restricting a relation to those pairs whose first element is a member of a particular set S, or whose second element is a member of S, or both of whose elements are members of S; in 2006 we would just use the ordinary set intersection operation and talk about R ∩ (S×B) or whatever.

Whitehead and Russell couldn't do this in 1910 because a crucial piece of machinery was missing: the ordered pair. In 1910 nobody knew how to build an ordered pair out of just logic and sets. In 2006 (or even 1956), we would define the ordered pair <a, b> as the set {{a}, {a, b}}. Then we would show as a theorem that <a, b> = <c, d> if and only if a=c and b=d, using properties of sets. Then we would define A×B as the set of all p such that p = <a, b> ∧ a ∈ A ∧ b ∈ B. Then we would define a relation on the sets A and B as a subset of A×B. Then we would get all of ∗23 and ∗25 and a lot of ∗33 and ∗35 and ∗36 for free, and probably a lot of other stuff too.

(By the way, the {{a}, {a, b}} thing was invented by Kuratowski. It is usually attributed to Norbert Wiener, but Wiener's idea, although similar, was actually more complicated.)

There are no ordered pairs in Principia Mathematica, except implicitly. There are barely even any sets. Whitehead and Russell want to base everything on logic. For Whitehead and Russell, the fundamental notion is the "propositional function", which is a function φ whose output is a truth value. For each such function, there is a corresponding set, which they denote by !!\hat x\phi(x)!!, the set of all x such that φ(x) is true. For Whitehead and Russell, a relation is implied by a propositional function of two variables, analogous to the way that a set is implied by a propositional function of one variable. In 2006, we dispense with "functions of two variables", and just talk about functions whose (single) argument is an ordered pair; a relation then becomes the set of all ordered pairs for which a function is true.

Russell is supposed to have said that the discovery of the Sheffer stroke (a single logical operator from which all the other logical operators can be built) was a tremendous advance, and would change everything. This seems strange to us now, because the discovery of the Sheffer stroke seems so simple, and it really doesn't change anything important. You just need to append a note to the beginning of chapter 1 that says that ∼p and p∨q are abbreviations for p|p and p|p.|.q|q, respectively, prove the five fundamental axioms, and leave everything else the same. But Russell might with some justice have said the same thing about the discovery that ordered pairs can be interpreted as sets, a simple discovery that truly would have transformed the Principia Mathematica into quite a different work.

Anyway, with that background in place, we can discuss the Principia Mathematica proof of 1+1=2. This occurs quite late in Principia Mathematica, in section ∗110. My abridged version only goes to ∗56, but that is far enough to get to the important precursor theorem, ∗54.43, scanned below:

Now I should explain the notation, which has changed somewhat since 1910. First off, Principia Mathematica uses Peano's "dots" notation to disambiguate precedence, where we now use parentheses instead. The dot notation takes some getting used to, but has some distinct advantages over the parentheses. The idea is just that you indicate grouping by putting in dots, so that (1+2)×(3+4)×(5+6) is written as 1+2.×.3+4.×.5+6. The middle sub-formula is between a pair of dots. The (1+2) sub-formula is between a pair of dots also, but the dot on the left end is superfluous, and we omit it; similarly, the sub-formula (5+6) is delimited by a dot on the left and by the end of the formula on the right.

What if you need to nest parentheses? Then you use more dots. A double dot (:) is like a single dot, but stronger. For example, we write ((1+2)×3)+4 as 1+2 . × 3 : + 4, and the double dot isolates the entire 1+2 . × 3 expression into a single sub-formula to which the +4 applies.

Sometimes you need more levels of precedence, and then you use triple dots (.: and :.) and quadruple (::). This formula, as you see, has double and triple dots. Translating the dots into standard parenthesis notation, we have $$\ast54.43. \vdash ((\alpha, \beta \in 1 ) \supset (( \alpha\cap\beta = \Lambda) \equiv (\alpha\cup\beta \in 2)))$$. This is rather more cluttered-looking than the version with the dots, and in complicated formulas you can have trouble figuring out which parentheses match with which. With the dots, it's always easy. So I think it's a bit unfortunate that this convention has fallen out of use.

The !!\vdash!! symbol has not changed; it means that the formula to which it applies is asserted to be true. !!\supset!! is logical implication, and !!\equiv!! is logical equivalence. Λ is the empty set, which we write nowadays as ∅. ∩ ∪ and ∈ have their modern meanings: ∩ and ∪ are the set intersection and the union operators, and x∈y means that x is an element of set y.

The remaining points are semantic. α and β are sets. 1 denotes the set of all sets that have exactly one element. That is, it's the set { c : there exists a such that c = { a } }. Theorems about 1 include, for example:

• that Λ∉1 (∗52.21),

• that if α∈1 then there is some x such that α = {x} (∗52.1), and

• that {x}∈1 (∗52.22).

So here is theorem ∗54.43 again:

The proof, which appears in the scan above following the word "Dem." (short for "demonstration") goes like this:

Maybe the thing to notice here is how very small the steps are. ∗54.26, on which this theorem heavily depends, is almost the same; it asserts that {x}∪{y} ∈ 2 if and only if x≠y. ∗54.26, in turn, depends on ∗54.101, which says that α has 2 elements if and only if there exist x and y, not the same, such that α = {x} ∪ {y}. ∗54.101 is just a tiny bit different from the definition of 2. Theorem ∗51.231 says that {x} and {y} are disjoint if and only if x and y are different. ∗52.1 is a basic property of 1; we saw it before.

The other theorems cited in the demonstration are very tiny technical matters. ∗11.54 says that you can take an assertion that two things exist and separate it into two assertions, each one asserting that one of the things exists. ∗11.11 is even slimmer: it says that if φ(x, y) is always true, then you can attach a universal quantifier, and assert that φ(x, y) is true for all x and y. ∗13.12 concerns the substitution of equals for equals: if x and y are the same, then x possesses a property ψ if and only if y does too.

I haven't seen the later parts of Principia Mathematica, because my copy stops after section ∗56, and the arithmetic stuff is much later. But this theorem clearly has the sense of 1+1=2 in it, and the later theorem (∗110.643) that actually asserts 1+1=2 depends strongly on this one.

Although I am not completely sure what is going to happen later on (I've wasted far too much time on this already to put in more time to get the full version from the library) I can make an educated guess. Principia Mathematica is going to define the number 17 as being the set of all 17-element sets, and similarly for every other number; the use of the symbol 2 to represent the set of all 2-element sets prefigures this. These sets-of-all-sets-of-a-certain-size will then be identified as the "cardinal numbers".

The Principia Mathematica will define the sum of cardinal numbers p and q something like this: take a representative set a from p; a has p elements. Take a representative set b from q; b has q elements. Let c = a∪b. If c is a member of some cardinal number r, and if a and b are disjoint, then the sum of p and q is r.

With this definition, you can prove the usual desirable properties of addition, such as x + 0 = x, x + y = y + x, and 1 + 1 = 2.

In particular, 1+1=2 follows directly from theorem ∗54.43; it's just what we want, because to calculate 1+1, we must find two disjoint representatives of 1, and take their union; ∗54.43 asserts that the union must be an element of 2, regardless of which representatives we choose, so that 1+1=2.

Post scriptum: Peter Norvig says that the circumflex in the Principia Mathematica notation is the ultimate source of the use of the word lambda to denote an anonymous function in the Lisp and Python programming languages. I am sure you know that these languages get "lambda" from the use of the Greek letter λ by Alonzo Church to represent function abstraction in his "λ-calculus": In Lisp, (lambda (u) B) is a function that takes an argument u and returns the value of B; in the λ-calculus, λu.B is a function that takes an argument u and returns the value of B. (Recall also that Church was the doctoral advisor of John McCarthy, the inventor of Lisp.) Norvig says that Church was originally planning to write the function λu.B as û.B, but his printer could not do circumflex accents. So he considered moving the circumflex to the left and using a capital lambda instead: Λu.B. The capital Λ looked too much like logical and ∧, which was confusing, so he used lowercase lambda λ instead.

Post post scriptum: Everyone always says "Russell and Whitehead". Google results for "Russell and Whitehead" outnumber those for "Whitehead and Russell" by two to one, for example. Why? The cover and the title page say "Alfred North Whitehead and Bertrand Russell, F.R.S.". How and when did Whitehead lose out on top billing?

[ Addendum 20060913: I figured out how and when Whitehead lost out on top billing: 10 December 1950. ]

[ Addendum 20171107: W. Ethan Duckworth sent me a translation of this proposition into modern notation, which I discussed in a new article. ]

[ Addendum 20211028: The duplication of “calculus of classes” as “calculus of relations” was fixed the following year by Norbert Wiener. ]

[ Addendum 20230329: I discussed Wiener's invention of the ordered pair in 2021: [1] [2] ]

[ Addendum 20230328: I guessed “Principia Mathematica will define the sum of cardinal numbers p and q something like this…”. This is indeed more or less how it goes. Cardinal addition is introduced in ∗110. The 1+1=2 theorem looks like this:

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Excellent numbers
Another programmer presented the Philadelphia Perl Mongers with this problem, apropos of something else; he had gotten the problem from a friend of his. The problem is to find "excellent numbers". A number n is excellent if it has an even number of digits, and if when you chop it into a front half a and a back half b, you have b² - a² = n. For example, 48 is excellent, because 8² - 4² = 48, and 3468 is excellent, because 68² - 34² = 4624 - 1156 = 3468.

This other programmer had written a program to do brute-force search, which wasn't very successful, because there aren't very many excellent numbers. He was presenting it to the Perl Mongers because in the course of trying to speed up the program, he had learned something interesting: using the Memoize module to memoize the squaring function makes a program slower, not faster. But that is another matter for another article.

A slightly less brutal brute-force search locates excellent numbers quite easily. Suppose the number n has 2k digits, and that it is the concatenation of a and b, each with k digits. We want b² - a² = n. Since n is the concatenation of a and b, it is equal to a·10^k + b. So we want:

a·10^k + b = b² - a²

a·(10^k + a) = b² - b

That is, instead of brute-forcing n, which has 2k digits, we need only do a brute-force search on a, which has only k digits. This is a lot faster. To find all 10-digit excellent numbers, we are now searching only 90,000 values of a instead of 9,000,000,000 values of n. This is feasible, whereas the larger search isn't.

All we need is some way to determine whether a given number q is rectangular, and that is not very hard to do. One way is simply to precompute a table of all rectangular numbers up to a certain size, and look up q in the table.

But another way is to notice that it is sufficient to be able to extract the "rectangular root" of q. That's the number b such that b² - b = q. If we can make a good guess about what b might be, then we can calculate b²-b and see if we get back q. This is analogous to checking whether a number s is a perfect square by guessing its square root r and then checking to see if r² = s. (Of course, it only works if you can guess right!)

But how can we guess the rectangular root of q? The computer has a built-in square root function, but no rectangular root function. But that's okay, because they are very nearly the same thing. Rectangular numbers are very nearly squares: when b is large, b² - b is relatively very close to b². So if we are given some number q, and we want to know if it has the form b² - b, we can get a very good estimate of the size of b just by taking √q.

For example, is 29750 a rectangular number? √29750 = 172.48, so if 29750 is rectangular, it will be 173² - 173 = 29756. So, no.

Is 1045506 rectangular? √1045506 = 1022.4998, so if 1045506 is rectangular, it will be 1023²-1023 = 1045506—check!

The only possible problem is that our assumption that b²-b and b² will be close may not hold when b is too small. So we might need to put a special case in our program to use a different test for rectangularity when q is too small. But it turns out that the test works fine even for q as small as 2: Is 2 rectangular? √2 = 1.4, so if 2 is rectangular, it will be 2²-2—check!

So we code up an is_rectangular function:

        sub is_rectangular {
          my $q = shift;
          my $b = 1 + int(sqrt($q));  # Guess
          if ($b * $b - $b == $q) {   # Check guess
            return $b;                # Aha!
          } else {
            return;                   # Not rectangular
          }
        }

Now we need a function that tells us whether we have some number a that might be the upper half of an excellent number:

        my $k = shift || 3;
        my $p = 10**$k;

        sub is_upper_half_of_excellent_number {
          my $a = shift;
          return is_rectangular($a * ($p + $a));
        }

We could infer k from a itself, but we'll hardwire it for two reasons. One reason is speed. The other is that we might want to accept a number like 0003514284 as being excellent, where here a has some leading zeroes; fixing k is one way to do this. We also precalculate the constant p = 10^k, which we need for the calculation.

Now we just write the brute-force loop:

        for my $a (0 .. $p-1) {
          if(my $b = is_upper_half_of_excellent_number($a)) {
            print "$a$b\n";
          }
        }

        01
        10101
        16128
        34188
        140400
        190476
        216513
        300625
        334668
        416768
        484848
        530901
        6401025
        8701276

The ones at the top of the list are correct, if you remember about the leading zeroes: 188² - 034² = 35344 - 1156 = 034188, and 001² - 000² = 1 - 0 = 000001.

The seven-digit numbers at the bottom don't work, because the program has a bug: we forgot to enforce the restriction that b must also have exactly k digits; the last two numbers in the display have four-digit b's. So we need to add another test to the main loop:

        for my $a (0 .. $p-1) {
          if(my $b = is_upper_half_of_excellent_number($a)) {
            print "$a$b\n" if length($b) == $k;
          }
        }

The ten-digit solutions are:

        0101010101
        3333466668
        4848484848
        4989086476

The other thing that I think is interesting is that the other programmer was trying to solve his performance problem with optimization tricks. But when you are searching over 9,000,000,000 cases, optimization tricks don't work. At 1,000 cases per second, 9,000,000,000 cases takes about 104 days to finish. If you can optimize your program to speed it up by 50%, it will still take 52 days to finish. But cutting the 9,000,000,000 cases to 90,000 cuts the run time from 104 days to 90 seconds.

Not to say that optimizing programs never works, of course. But you have to know when optimization might work and when it can't. In this case, micro-optimization wasn't going to help; the only way to fix an algorithm that does a brute-force search of 9,000,000,000 cases is to find a better algorithm. The point of this article is to show that sometimes finding a better algorithm can be pretty easy.

[ Addendum 20060620: I wrote a followup article about how it's not a coincidence that 4848484848 is excellent. ]

[ Addendum 20160105: brian d foy has a whole website about his work on this problem. ]

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The envelope paradox
[Addendum 20151130: I have since written this up in greater and more formal detail. If you find the details here lacking, please consult my math.se post about it.]

This is on my mind because someone asked about it in IRC yesterday and I was surprised at how coherently I was able to explain it on the spur of the moment. There are several versions of this paradox. My favorite version goes like this: you're going to play a game with an adversary. The adversary writes two different numbers on slips of paper and puts them in an envelope. The numbers are completely arbitrary; they could be absolutely any numbers whatsoever: zero, or π, or -1428573901823.00013, or anything else.

You pick one slip at random from the envelope and examine the number written on it. You then make a prediction about whether the other number is larger or smaller. If your prediction is correct, you win a dollar; if it is incorrect, you lose a dollar.

Clearly, you can break even in the long run simply by making your prediction at random. And it seems just just as clear that there is no strategy you can use that does better than breaking even. But this is the paradox: there is a strategy you can use that does better than breaking even. (This is what W.V.O. Quine calls a "veridical paradox": it's something that seems impossible, but is nevertheless true.)

Spoilers follow, so you might want to stop reading here for twenty-four hours and try to figure out a winning strategy yourself.

Here's your winning strategy. Before you see A, choose a random number R. If A < R, then conclude that A is "small", and predict that B is larger. If A > R, make the opposite prediction.

There are three possibilities. Either (1) A and B are both less than R, or (2) they are both greater than R, or else (3) one is less than R and one is greater.

In case 1, you predict that B > A, and you have a 50% probability of being correct. In case 2, you predict that B < A, and you have a 50% probability of being correct.

But in case 3, you win every time! If A < R < B then you see A, conclude that A is "small", and predict that B > A, which is correct; if B < R < A then you see A, conclude that A is "large", and predict that B < A, which is correct.

Since you're breaking even in cases 1 and 2, and you have a guaranteed win in case 3, you have a better-than-even chance of winning overall. There's some positive probability p (which depends on the method you use to choose R) that you have case 3, and if so, then your expected positive return on the game is p dollars per game.

The paradoxical part is that it initially seems as though you can't get any idea, just from looking at A, of whether it's larger or smaller than the unknown number B. But you can get such an idea, because you can tell from looking at A how big it is, and big numbers are more likely to be larger than B than small numbers are.

What you've done with R is to invent a definition of "large" and "small" numbers: numbers larger than R are "large" and those smaller than R are "small". It's an arbitrary definition, and it doesn't always succeed in distinguishing large from small numbers—it thinks that R+1 and 1000000R+1000000 are both "large"—but it can distinguish some large numbers from some small numbers, and it never gets confused and concludes that x is large and y is small when x is actually smaller than y. So it may be arbitrary, and extremely coarse, but it is never actually wrong.

In the cases where this very coarse method of deciding "large" from "small" fails to distinguish A from B, you get no new information, but that's okay, because you can still break even. But if you get lucky and the adversary has chosen numbers that you can distinguish, then you win.

Another way to look at the paradox is like this: suppose the adversary is required to choose his two numbers at random. Then you have a simple winning strategy: if A is positive, predict that B is smaller, and if A is negative, predict that B is larger. Even when both numbers are positive or both are negative, you win half the time; if one is positive and one is negative, you are guaranteed a win.

If the adversary knows that this is what you are doing, he can cut you back to merely breaking even, by limiting himself to always choosing positive numbers. But you can foil this strategy of his by choosing your "positive" and "negative" classes to be divided somewhere other than at 0: instead of "positive" being "> 0" and "negative" being "< 0", you make them mean "> R" and "< R". The adversary still wants to choose two numbers that are always positive, but since he doesn't know how big R is, hw doesn't know how large he has to make his own numbers to get them both to be "positive".

Still, this suggests the best strategy for the adversary: choose two very very large numbers that are close together. By doing this, he can make your expected win close to zero.

The envelope paradox is often presented in a different form: you are given two envelopes. One contains a bunch of money, say x dollars. The other contains twice as much. You open one envelope at random and examine its contents. Then you choose one envelope to keep.

A naïve analysis goes like this: I open the first envelope and see x. I can keep this envelope and collect amount x. If I switch, I have a 50% chance of ending up with 2x and a 50% chance of ending up with x/2, for an expected outcome of 5x/4. Since 5x/4 > x, I should always switch.

This is what Quine calls a "falsidical paradox": the reasoning seems good, but leads to an impossible conclusion. The strategy of always switching can't possibly be correct, because you could apply it with without even seeing what is in the envelope. You could keep switching back and forth all day, never opening either envelope, and increasing your expected winnings to infinity.

The tricky part, again, is that having seen x in the envelope, you cannot conclude that there is exactly a 50% chance of x being the larger of the two amounts. You get some information from the size of x, and if x is a large amount of money, then the probability that x is the larger of the two amounts is thereby greater than 0.5.

To do a full analysis, one has to ask the question of how the original amounts were selected. Say that the two amounts are b and 2b; let's call b the "base amount". How did the adversary select b? Let's say that the probability of the base amount being any particular amount x is P(x). It is impossible that b has an equal probability of being every number, because $$\int_{-\infty}^\infty P(x) dx$$ is required to be 1, and if P(b) is the same for every possible base amount b, then it is a constant function, and constant functions do not have the required property.

When you see x in the envelope, you know that one of two situations occurred. Either x is the base amount, and so is smaller, which occurs with probability P(x), or x/2 is the base amount, and x itself is the larger, which occurs with probability P(x/2).

Since these are the only possibilities, the a posteriori likelihood that x is the smaller number is P(x)/(P(x/2) + P(x)). This is equal to 1/2 only if P(x) = P(x/2). Although this can occur for particular values of x, it can't be true for every x. As x increases, P(x) approaches zero, so for sufficiently large x, we must have P(x/2) > P(x), so P(x/2) + P(x) > 2P(x), and P(x)/(P(x/2) + P(x)) < P(x)/2P(x) = 1/2.

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Just put it in the damn object!
The Perl Program Repair Shop book, despite outward appearances, is coming along. A few months ago in Pittsburgh I gave a talk about someone's module that I didn't think was very well-written, for various reasons. Up to that point, the book had been about a lot of small-scale stuff: repeated code, unnecessary variables, making two passes over a data structure when only one was necessary, C-style for loops, and such. But unlike the previous examples, this one had been written by someone who was actually competent. So the problems I found were competent-programmer examples, rather than incompetent-programmer examples. I need some more chapters of that kind of examples, or the book will not be of much interest to competent programmers.

What kind of mistakes do competent programmers make? They make a lot of errors of object-oriented design.

This module's purpose was to emulate some Java library that lets you register an "observer" of an object. Let's say that the "observer" object is a Guard, and the object it is observing is an "Alarm". The idea is that the Alarm object registers the Guard as being an observer of the Alarm, and then whenever the Alarm calls a notify_observers method, the notify method in the Guard object is called back. Actually the Java people didn't make the names that sensible; instead of notify_observers calling the notify method in the observers, it calls the update method. Why wasn't notify_observers called update_observers, then? I dunno. It's Java. You want me to explain Java?

Okay, so for each Alarm A you need somewhere to store the list of observers of A. Where do you put that?

The author of the module put it in a global hash. I don't think it's immediately clear that this was a mistake. But I do think it was a mistake. Big problems arise; the module had a lot of bugs, mostly related to garbage collection. As a result of putting the Guard objects into this hash, they are never garbage collected. Well, not quite. The author used a weak reference in the hash, so the objects there are garbage collected.

Weak references are one of those technical solutions that fits really well into the formula "A programmer had a problem. So he used weak references. Then he had two problems." (Non-greedy regexes are another example. Some people say Perl itself is an example.) Weak references do solve a couple of very specific problems, mostly having to do with caching. For anything else, they turn out to be a bigger problem than the thing you were trying to solve. In this case, the weak references cause this delightful problem:

  
        my $alarm = Alarm->new();
        $alarm->add_observer(Guard->new);
        $alarm->notify_observers("I like pie!");


        Failed to send observation from 'Alarm=HASH(0x8113f74)' to '':
        Can't call method "update" on an undefined value at
        lib/Class/Observable.pm line 95.

This was only one of several problems caused by this design, to store the observers in a global hash. Some of these could possibly be avoided if Perl were a better language, but others I think not.

My suggestion here was the following advice: Whenever you're trying to store information about an object, there is a right place to store it and a wrong place to store it. The right place: inside the object. The wrong place: anywhere else.

Rationale: What is an object? It's a place where you store all the pertinent information about some entity. So here we have some pertinent information about this entity, the alarm, namely the list of guards who are watching it. Where should we store this information? Well, we have this data structure, the Alarm object, which is supposed to store all the pertinent information about the alarm. The list of guards is part of this, so that's where it should go. Entia non sunt multiplicanda praeter necessitatem.

Now, this choice may be obvious, but it has a fair share of problems too, which is why I think the other programmer might not have made it. The add_observers method in the base class of Alarm is called upon to add the Guard to the list of observers of the Alarm. If the list of observers is stored in the Alarm object, add_observersmust make some assumptions about how the Alarm is implemented. But it turns out that once you start thinking about it, the problems turn out to have fairly simple solutions. For example, the base class needs to know what list to append the Guard to. It could assume that the list will be stored in $alarm->{Observers}, but that's iffy: it assumes that the Alarm will be made from a hash, it assumes that that hash key is not used for something else, and so on. So instead, you give Alarm a method that returns a reference to the array in which the Guard is supposed to place itself. Normally, that method is inherited from the base class itself, and returns a reference to $self->{Observers}, but the Alarm class can override that if it needs to. It can even get back the original global-variable implementation by overriding the method to return a reference to an element in a global hash.

To summarize, both designs appeared to have significant technical problems, but the problems in the design I suggested turned out to be a lot easier to solve well then the problems that arose from the design that the author chose. At least in this example, the advice that object data should be stored in the object turned out pretty well.

(Cynical advice to people wishing to become famous experts: phrase your advice so that it sounds inevitable. Anyone wishing to argue against advice like object data should be stored in the object will have an uphill battle, and risks looking like an idiot.)

Now today I was in the shower, thinking about this other piece of software I wrote recently, in which I was having, surprise, a garbage collection problem. The software is a module that is intended to provide lightweight access to flat-file databases, which is something that has been inexplicably overlooked by CPAN.

You use the module like this:

        my $db = FlatFile->new(FILE => ..., FIELDS => [...], ...);

        my @red = $db->lookup(color => "red");

        $red[0]->set_color("blue");  # paint it blue

        $red[1]->delete;

The way I implemented this was to have a class variable in the Record class, $Record::DB, which contained the database object. But then the database object is never garbage-collected, which means it is never finalized, which means that changes to the database are not automatically written to the disk when the database object becomes inaccessible, which means that you have to call $db->flush to make sure the changes are written out, which opens the possibility that you will forget, and the module will bugger your database with a badger.

What I realized in the shower was that I had better take my own advice, that object data should be stored in the object. If the source database is a pertinent piece of information about a record, then I had better store the source database as a piece of member data in the record object, and not try this stupid thing involving a global variable.

If the advice is good, this will be a better design. I haven't done it yet, so I'm not sure. But it certainly looks like a better design.

I want to say I don't know why I used the global variable in the first place, but I do know: my original design for the record object was that it had nothing in it but the data from the record; no metadata at all. The data members were named after the fields in the database, so there was no namespace left over for metadata. That didn't work out too well, and although I resisted putting metadata into the record objects long enough to screw myself regarding where to store the owning database and some other stuff, I eventually got backed into a corner and had to redesign the object to have space for metadata. But the effects of original misdesign persisted, because I was still storing a bunch of the object's metadata in stupid global variables, until I took that shower.

That's two successes so far for my advice, which is two good signs. Now all I need is a good counterexample. Every piece of advice in this book is going to have a counterexample. That's the big problem with computer programming advice books: no counterexamples. But that's another article for another day.

[Other articles in category /prs] permanent link

Worst mathematical notation ever
All my blog posts lately seem to be turning into enormous meandering rants, which isn't so bad in itself, but I never finish any of them. So I thought I'd just cut this one short and stick to one point instead of seventeen. But someday you're all going to have to read the full article, which complains about how 1960s computer scientists, math grad students, and other insecure people who hunger for legitimacy sometimes try to get it by doing everything with as much notation and pseudo-formal language as possible.

$$\equiv_{i_0+1} = \equiv_{i_0} = \equiv$$

The explanation is that the author is constructing a sequence of equivalence relations, each one ≡_n derived from the previous one ≡_n-1. Eventually (after i₀ iterations of the procedure), the construction has nothing left to do, and the new relation is the same as the one from which it was constructed. At this point the constructed relation turns out to be a certain one ≡ with desirable properties.

[ Addendum 20220123: I've tried a couple of times to find these notes again, with no success. It's a tough thing to search for. ]

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Irreducible polynomials

A puzzle for high school math students

Solution

This implies, however, that there are no irreducible real polynomials of degree higher than 2. Every real polynomial of degree 3 or higher factors into simpler factors.

For odd-degree polynomials, this is not surprising, since every such polynomial has a real zero. But I remember the day, surprisingly late in my life, when I realized that it also implies that x⁴ + 1 must be factorable. I was so used to x² + 1 being irreducible, and I had imagined that x⁴ + 1 must be irreducible also. x² + bx + 1 factors if and only if |b| ≥ 2, and I had imagined that something similar would hold for x⁴ + 1—But no, I was completely wrong.

x⁴ + 1 has four zeroes, which are the fourth roots of -1, or, if you prefer, the two square roots of each of i and -i. When I was in elementary school I used to puzzle about the square root of i; I hadn't figured out yet that it was ±(1 + i)/√2; let's call these two numbers j and -j. The square roots of -i are the conjugates of j and -j: ±(1 - i)/√2, which we can call k and -k because my typewriter doesn't have an overbar.

So x⁴ + 1 factors as (x + j)(x - j)(x + k)(x - k). But this doesn't count because j and k are complex. We get the real factorization by remembering that j and k are conjugate, so that (x - j)(x - k) is completely real; it equals x² - √2x + 1. And similarly (x + j)(x + k) = x² + √2x + 1.

So x⁴ + 1 factors into (x² - √2x + 1)(x² + √2x + 1). Isn't that nice?

[ Addendum 20220221: I accidentally wrote the same article a second time. ]

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My favorite math problem

Introduction

Descartes' theorem

We can repeat this process. If a₁, a₂, ... a_n are the zeroes of some nth-degree polynomial P, we can factor P as (x-a₁)(x-a₂)...(x-a_n).

The only possibly tricky thing here is that some of the a_i might be the same. x² - 2x + 1 has a zero of 1, but twice, so it factors as (x-1)(x-1). x³ - 4x² + 5x - 2 has a zero of 1, but twice, so it factors as (x-1)(x² - 3x + 2), where (x² - 3x + 2) also has a zero of 1, but this time only once.

Descartes' theorem has a number of important corollaries: an nth-degree polynomial has no more than n zeroes. A polynomial that has a zero can be factored. When coupled with the so-called fundamental theorem of algebra, which says that every polynomial has a zero over the complex numbers, it implies that every nth-degree polynomial can be factored into a product of n factors of the form (x-z) where z is complex, or into a product of first- and second-degree polynomials with real coefficients.

The square roots of -1

If we want to find the square roots of -1, we need to factor x² + 1. There are no real numbers that do this, so we call the polynomial irreducible. Instead, we make up some square roots for it, which we will call i and j. We want (x - i)(x - j) = x² + 1. Multiplying through gives x² - (i + j)x + ij = x² + 1. So we have some additional relationships between i and j: ij = 1, and i + j = 0.

Because i + j = 0, we always dispense with j and just call it -i. But it's essential to realize that neither one is more important than the other. The two square roots are identical twins, different individuals that are completely indistinguishable from each other. Our choice about which one was primary was completely arbitrary. That doesn't even do justice to the arbitrariness of the choice: i and j are so indistinguishable that we still don't know which one got the primary notation. Every property enjoyed by i is also enjoyed by j. i³ = j, but j³ = i. e^iπ = -1, but so too e^jπ = -1. And so on.

In fact, I once saw the branch of mathematics known as Galois theory described as being essentially the observation that i and -i were indistinguishable, plus the observation that there are no other indistinguishable pairs of complex numbers.

Anyway, all this is to set up the main point of the article, which is to describe my favorite math problem of all time. I call it the "train problem" because I work on it every time I'm stuck on a long train trip.

We are going to investigate irreducible polynomials; that is, polynomials that do not factor into products of simpler polynomials. Descartes' theorem implies that irreducible polynomials are zeroless, so we are going to start by looking for polynomials without zeroes. But we are not going to do it in the usual world of the real numbers. The answer in that world is simple: x² + 1 is irreducible, and once you make up a zero for this polynomial and insert it into the universe, you are done; the fundamental theorem of algebra guarantees that there are no more irreducible polynomials. So instead of the reals, we're going to do this in a system known as Z₂.

Z₂

+ 0 1 0 0 1 1 1 0

× 0 1 0 0 0 1 0 1

The 1+1=0 oddity is the one and only difference between Z₂ and ordinary algebra. All the other laws remain the same. To be explicit, I will list them below:

• a + 0 = a
• a + b = b + a
• (a + b) + c = a + (b + c)
• a × 0 = 0
• a × 1 = a
• a × b = b × a
• (a × b) × c = a × (b × c)
• a × (b + c) = a × c + b × c

Algebra in Z₂

Because a + a = 0, we have a = -a for all a, and this means that in Z₂, addition and subtraction are the same thing. If you don't like subtraction—and really, who does?—then Z₂ is the place for you.

As a consequence of this, there are some convenient algebraic manipulations. For example, suppose we have a + b = c and we want to solve for a. Normally, we would subtract b from both sides. But in Z₂, we can add b to both sides, getting a + b + b = c + b. Then the b + b on the left cancels (because b + b = 0) leaving a = c + b. From now on, I'll call this the addition law.

If you suffered in high school algebra because you wanted (a + b)² to be equal to a² + b², then suffer no more, because in Z₂ they are equal. Here's the proof:

(a + b)2	=
(a + b)(a + b)	=
a(a + b) + b(a + b)	=
(a2 + ab) + (ba + b2)	=
a2 + ab + ab + b2	=
a2 + b2

Because the ab + ab cancels in the last step.

  This happy fact, which I'll call the square law, also holds true for sums of more than two terms: (a + b + ... + z)² = a² + b² + ... + z². But it does not hold true for cubes; (a + b)³ is a³ + a²b + ab² + b³, not a³ + b³.

Irreducible polynomials in Z₂

There are four second-degree polynomials:

1. x²
2. x² + 1
3. x² + x
4. x² + x + 1

Polynomial	Zeroes	Factorization
x2	0 (twice)	x2
x2 + 1	1 (twice)	(x+1)2
x2 + x	0, 1	x(x+1)
x2 + x + 1	none

Note in particular that, unlike in the real numbers, x² + 1 is not irreducible in Z₂. 1 is a zero of this polynomial, which, because of the square law, factors as (x+1)(x+1). But we have found an irreducible polynomial, x² + x + 1. The other three second-degree polynomials all can be factored, but x² + x + 1 cannot.

Like the fictitious mathematicians who invented i, we will rectify this problem, by introducing into Z₂ a new number, called b, which has the property that b² + b + 1 = 0. What are the consequences of this?

The first thing to notice is that we haven't added only one new number to Z₂. When we add i to the real numbers, we also get a lot of other stuff like i+1 and 3i-17 coming along for the ride. Here we can't add just b. We must also add b+1.

The next thing we need to do is figure out how to add and multiply with b. The addition and multiplication tables are extended as follows:

+ 0 1 b b+1 0 0 1 b b+1 1 1 0 b+1 b b b b+1 0 1 b+1 b+1 b 1 0

× 0 1 b b+1 0 0 0 0 0 1 0 1 b b+1 b 0 b b+1 1 b+1 0 b+1 1 b

The only items here that might require explanation are the entries in the lower-right-hand corner of the multiplication table. Why is b×b = b+1? Well, b² + b + 1 = 0, by definition, so b² = b + 1 by the addition law. Why is b × (b+1) = 1? Well, b × (b+1) = b² + b = (b + 1) + b = 1 because the two b's cancel. Perhaps you can work out (b+1) × (b+1) for yourself.

From here there are a number of different things we could investigate. But what I think is most crucial is to discover the value of b³. Let's calculate that. b³ = b·b². b², we know, is b+1. So b³ = b(b+1) = 1. Thus b is a cube root of 1.

People with some experience with this sort of study will not be surprised by this. The cube roots of 1 are precisely the zeroes of the polynomial x³ + 1 = 0. 1 is obviously a cube root of 1, and so Descartes' theorem says that x³ + 1 must factor into (x+1)P, where P is some second-degree polynomial whose zeroes are the other two cube roots. P can't be divisible by x, because 0 is not a cube root of 1, so it must be either (x+1)(x+1), or something else. If it were the former, then we would have x³ + 1 = (x+1)³, but as I pointed out before, we don't. So it must be something else. The only candidate is x² + x + 1. Thus the other two cube roots of 1 must be zeroes of x² + x + 1.

The next question we must answer without delay is: b is one of the zeroes of x² + x + 1; what is the other? There is really only one possibility: it must be b+1, because that is the only other number we have. But another way to see this is to observe that if p is a cube root of 1, then p² must also be a cube root of 1:

b3	=	1
(b3)2	=	12
b6	=	1
(b2)3	=	1

So we now know the three zeroes of x³ + 1: they are 1, b, and b². Since x³ + 1 factors into (x+1)(x²+x+1), the two zeroes of x²+x+1 are b and b². As we noted before, b² = b+1.

This means that x² + x + 1 should factor into (x+b)(x+b+1). Multiplying through we get x² + bx + (b+1)x + b(b+1) = x² + x + 1, as hoped.

Actually, the fact that both b and b² are zeroes of x²+x+1 is not a coincidence. Z₂ has the delightful property that if z is a zero of any polynomial whose coefficients are all 0's and 1's, then z² is also a zero of that polynomial. I'll call this Theorem 1. Theorem 1 is not hard to prove; it follows from the square law. Feel free to skip the demonstration which follows in the rest of the paragraph. Suppose the polynomial is P(x) = a_nxⁿ + a_n-1x^n-1 + ... + a₁x + a₀, and z is a zero of P, so that P(z) = 0. Then (P(z))² = 0 also, so (a_nzⁿ + a_n-1z^n-1 + ... + a₁z + a₀)² = 0. By the square law, we have a_n²(zⁿ)² + a_n-1²(z^n-1)² + ... + a₁²z² + a₀² = 0. Since all the a_i are either 0 or 1, we have a_i² = a_i, so a_n(zⁿ)² + a_n-1(z^n-1)² + ... + a₁z² + a₀ = 0. And finally, (z^k)² = (z²)^k, so we get a_n(z²)ⁿ + a_n-1(z²)^n-1 + ... + a₁z² + a₀ = P(z²) = 0, which is what we wanted to prove.

Now you might be worried about something: the fact that b is a zero of x² + x + 1 implies that b² is also a zero; doesn't the fact that b² is a zero imply that b⁴ is also a zero? And then also b⁸ and b¹⁶ and so forth? Then we would be in trouble, because x² + x + 1 is a second-degree polynomial, and, by Descartes' theorem, cannot have more than two zeroes. But no, there is no problem, because b³ = 1, so b⁴ = b·b³ = b·1 = b, and similarly b⁸ = b², so we are getting the same two zeroes over and over.

Symmetry again

Third-degree polynomials

Polynomial				Zeroes	Factorization
x3				0, 0, 0	x3
x3			+ 1	1, b, b+1	(x+1)(x2+x+1)
x3		+ x		0, 1, 1	x(x+1)2
x3		+ x	+ 1	none
x3	+ x2			0, 0, 1	x2(x+1)
x3	+ x2		+ 1	none
x3	+ x2	+ x		0, b, b+1	x(x2+x+1)
x3	+ x2	+ x	+ 1	1, 1, 1	(x+1)3

There are two irreducible third-degree polynomials. Let's focus on x³ + x + 1, since it seems a little simpler. As before, we will introduce a new number, c, which has the property c³ + c + 1 = 0. By the addition law, this implies that c³ = c + 1. Using this, we can make a table of powers of c, which will be useful later:

c1	=		c
c2	=	c2
c3	=		c +	1
c4	=	c2 +	c
c5	=	c2 +	c +	1
c6	=	c2 +		1
c7	=			1

To calculate the entries in this table, just multiply each line by c to get the line below. For example, once we know that c⁵ = c² + c + 1, we multiply by c to get c⁶ = c³ + c² + c. But c³ = c + 1 by definition, so c⁶ = (c + 1) + c² + c = c² + 1. Once we get as far as c⁷, we find that c is a 7th root of 1. Analogous to the way b and b² were both cube roots of 1, the other 7th roots of 1 are c², c³, c⁴, c⁵, c⁶, and 1. For example, c⁵ is a 7th root of 1 because (c⁵)⁷ = c³⁵ = (c⁷)⁵ = 1⁵ = 1.

We've decided that c is a zero of x³ + x + 1. What are the other two zeroes? By theorem 1, they must be c² and (c²)² = c⁴. Wait, what about c⁸? Isn't that a zero also, by the same theorem? Oh, but c⁸ = c·c⁷ = c, so it's really the same zero again.

What about c³, c⁵, and c⁶? Those are the zeroes of the other irreducible third-degree polynomial, x³ + x² + 1. For example, (c³)³ + (c³)² + 1 = c⁹ + c⁶ + 1 = c² + (c²+1) + 1 = 0.

So when we take Z₂ and adjoin a new number that is a solution of the previously unsolvable equation x³ + x + 1 = 0, we're actually forced to add six new numbers; the resulting system has 8, and includes three different solutions to both irreducible third-degree polynomials, and a full complement of 7th roots of 1. Since the zeroes of x³ + x + 1 and x³ + x² + 1 are all 7th roots of 1, this tells us that x⁷ + 1 factors as (x+1)(x³ + x + 1)(x³ + x² + 1), which might not have been obvious.

Onward

• We've seen cube roots and seventh roots of 1. Where did the fifth roots go?

  (The square theorem ensures that even-order roots are uninteresting. The fourth roots of 1, for example, are just 1, 1, 1, and 1. The sixth roots are the same as the cube roots, but repeated: 1, 1, b, b, b², and b².)

  The fifth roots, it turns out, appear later, as the four zeroes of the polynomial x⁴ + x³ + x² + x + 1. These four, and 1, are five of the 15 fifteenth roots of 1. Eight others appear as the zeroes of the other two irreducible fourth-degree polynomials, and the other two fifteenth roots are b and b². If we let d be a zero of one of the other irreducible fourth-degree polynomials, say x⁴ + x + 1, then the zeroes of this polynomial are d, d², d⁴, and d⁸, and the zeroes of the other are d⁷, d¹¹, d¹³, and d¹⁴. So we have an interesting situation. We can extend Z₂ by adjoining the zeroes of x³ + 1, to get a larger system with 4 elements instead of only 2. And we can extend Z₂ by adjoining the zeroes of x⁷ + 1, to get a larger system with 8 elements instead of only 2. But we can't do the corresponding thing with x⁵ + 1, because the fifth roots of 1 don't form a closed system. The sum of any two of the seventh roots of 1 was another seventh root of 1; for example, c³ + c⁴ = c⁶ and c + c² = c⁴. But the corresponding fact about fifth roots is false. Suppose, for example, that y⁵ = 1. We'd like for y + y² to be another fifth root of 1, but it isn't; it actually turns out that (y + y²)⁵ = b. For which n do the nth roots of 1 form a closed system?

• What do we get if we extend Z₂ with both b and c? What's bc? What's b + c? (Spoiler: b + c turns out to be zero of x⁶ + x⁵ + x³ + x² + 1, and is a 63rd root of 1, so we may as well call it f. bc = f⁴⁸.)

  The degree of a number is the degree of the simplest polynomial of which it is a zero. What's the degree of b + c? In general, if p and q have degrees d(p) and d(q), respectively, what can we say about the degrees of pq and p + q?

• Since there is an nth root of 1 for every odd n, that implies that for all odd n, there is some k such that n divides 2^k - 1. How are n and k related?

• For n = 2, 3, and 4, the polynomial xⁿ + x + 1 was irreducible. Is this true in general? (No: x⁵ + x + 1 = (x³ + x² + 1)(x² + x + 1).) When is it true? Is there a good way to find irreducible polynomials? How many irreducible polynomials are there of degree n? (The sequence starts out 0, 0, 1, 2, 3, 6, 9, 18, 30, 56, 99,...)

• The Galois theorem says that any finite field that contains Z₂ must have 2ⁿ elements, and its addition component must be isomorphic to a direct product of n copies of Z₂. Extending Z₂ with c, for example, gives us the field GF(8), obtained as the direct product of {0, 1}, {0, c}, and {0, c²}. The addition is very simple, but the multiplication, viewed in this light, is rather difficult, since it depends on knowing the secret identity c · c² = c + 1.

  Identify the element a₀ + a₁c + a₂c² with the number a₀ + 2a₁ + 4a₂, and let the addition operation be exclusive-or. What happens to the multiplication operation?

• Consider a linear-feedback shift register:

          # Arguments: N, a positive integer
          #            0 <= K <= 2**N-1
          MAXBIT = 2**N
          c = 2
          until c == 1:
            c *= 2
            if (c => MAXBIT):
              c &= ~MAXBIT
              c ^= K
            print c
  

  (Except that this function runs backward relative to the way that LFSRs usually work.)

  Does this terminate for all choices of N and K? (Yes.) For any given N, the maximum number of iterations of the until loop is 2^N - 1. For which choices of K is this maximum achieved?

• Consider irreducible sixth-degree polynomials. We should expect that adjoining a zero of one of these, say f, will expand Z₂ to contain 64 elements: 0, 1, f, f², ..., f⁶², and f⁶³ = 1. In fact, this is the case.

  But of these 62 new elements, some are familiar. Specifically, f⁹ = c, and f²¹ = b. Of the 62 powers of f, 8 are numbers we have seen before, leaving only 54 new numbers. Each irreducible 6th-degree polynomial has 6 of these 54 as its zeroes, so there are 9 such polynomials. What does this calculation look like in general?

• c¹, c², and c⁴ are zeroes of x³ + x + 1, one of the two irreducible third-degree polynomials. This means that x³ + x + 1 = (x + c¹)(x + c²)(x + c⁴). Multiplying out the right-hand side, we get: x³ + (c + c² + c⁴)x² + (c³ + c⁵ + c⁶)x + 1. Equating coefficients, we have c + c² + c⁴ = 0 and c³ + c⁵ + c⁶ = 1. c³, c⁵, and c⁶ are the zeroes of the other irreducible third-degree polynomial. Coincidence? (No.)

• The irreducible polynomials appear to come in symmetric pairs: if a_nxⁿ + a_n-1x^n-1 + ... + a₁x + a₀ is irreducible, then so is a₀xⁿ + a₁x^n-1 + ... + a_n-1x + a_n. Why?

• c¹, c², and c⁴ are the zeroes of one of the irreducible third-degree polynomials. 1, 2, and 4 are all the three-bit binary numbers that have exactly one 1 bit. c³, c⁵, and c⁶ are the zeroes the other irreducible third-degree polynomial. 3, 5, and 6 are all the three-bit binary numbers that have exactly one 0 bit. Coincidence? (No; theorem 1 is closely related here.)

  Now consider the analogous case for fourth-degree polynomials. The 4-bit numbers that have one 1 bit are 0001, 0010, 0100, and 1000 (1, 2, 4, and 8) which are all zeroes of one of the irreducible 4th-degree polynomials. The 4-bit numbers that have one 0 bit are 1110, 1101, 1011, and 0111 (7, 11, 13, and 14) which are all zeroes of another of the irreducible 4th-degree polynomials.

  The 4-bit numbers with two 1 bits and two 0 bits fall into two groups. In one group we have the numbers where similar bits are adjacent: 0011, 0110, 1100, and 1001, or 3, 6, 9, and 12. These are the zeroes of the third irreducible 4th-degree polynomial. The remaining 4-bit numbers are 0101 and 1010, which correspond to b and b².

  There is, therefore, a close relationship between the structure an extension of Z₂ and the so-called "necklace patterns" of bits, which are patterns that imagine that the bits are joined into a circle with no distinguishable starting point.

  The exceptional values (b and b²) corresponded to necklace patterns that did not possess the full n-fold symmetry. This occurs for composite values of n. But we might also expect to find exceptions at n = 11, since then 2ⁿ-1 = 2047 is composite. However, 2047 is exceptional in another way: 2047 = 23 · 89, and neither 23 nor 89 is a divisor of 2ⁿ-1 for any smaller value of n. Can we prove from this that when p is prime and 2^p-1 is not, the divisors of 2^p-1 do not divide 2^q-1 for any q < p?

• Since cⁱ are the seventh roots of 1, we have x⁷ + 1 = (x + 1)(x + c)(x + c²)...(x + c⁶). After dividing out the trivial factor of x+1, we are left with:

  x⁶ + x⁵ + ... + 1 = (x + c)(x + c²)...(x + c⁶)

  Multiplying through on the right side and equating coefficients gives:

  c + c2 + ... + c6	=	1
  cc2 + cc3 + ... + c5c6	=	1
  cc2c3 + cc2c4 + ... + c4c5c6	=	1
  ...	=	...
  cc2c3c4c5c6	=	1

  What, if anything, can one make of this?

And then, when I get tired of Z₂, I can start all over with Z₃.

[Other articles in category /math] permanent link

Creeping featurism and the ratchet effect
"Creeping featurism" is a well-known phenomenon in the software world. It refers to the tendency of software to acquire more and more features, to the ultimate detriment of its usability. Software with more and more features is harder to learn to use; it's harder to document effectively. Perhaps most important, it is harder to maintain; the more complicated software is, the more likely it is to have bugs. Partly this is because the different features interact with one another in unanticipated ways; partly it is just that there is more stuff to spend the maintenance budget on.

But the concept of "creeping featurism" his wider applicability than just to program features. We can recognize it in other contexts.

For example, someone is reading the Perl manual. They read the section on the unpack function and they find it confusing. So they propose a documentation patch to add a couple of sentences, explicating the confusing point in more detail.

It seems like a good idea at the time. But if you do it over and over—and we have—you end up with a 2,000 page manual—and we did.

The real problem is that it's easy to see the benefit of any proposed addition. But it is much harder to see the cost of the proposed addition, that the manual is now 0.002% larger.

The benefit has a poster child, an obvious beneficiary. You can imagine a confused person in your head, someone who happens to be confused in exactly the right way, and who is miraculously helped out by the presence of the right two sentences in the exact right place.

The cost has no poster child. Or rather, the poster child is much harder to imagine. This is the person who is looking for something unrelated to the two-sentence addition. They are going to spend a certain amount of time looking for it. If the two-sentence addition hadn't been in there, they would have found what they were looking for. But the addition slowed them down just enough that they gave up without finding what they needed. Although you can grant that such a person might exist, they really aren't as compelling as the confused person who is magically assisted by timely advice.

Even harder to imagine is the person who's kinda confused, and for whom the extra two sentences, clarifying some obscure point about some feature he wasn't planning to use in the first place, are just more confusion. It's really hard to understand the cost of that.

But the benefit, such as it is, comes in one big lump, whereas the cost is distributed in tiny increments over a very large population. The benefit is clear, and the cost is obscure. It's easy to make a specific argument in favor of any particular addition ("people might be confused by X, so I'm going to explain it in more detail") and it's hard to make such an argument against the addition. And conversely: it's easy to make the argument that any particular bit of text should stay in, hard to argue that it should be removed.

As a result, there's what I call a "ratchet effect": you can make the manual bigger, one tiny notch at a time, and people do. But having done so, you can't make it smaller again; someone will object to almost any proposed deletion. The manual gets bigger and bigger, worse and worse organized, more and more unusable, until finally it collapses under its own weight and all you can do is start over again.

You see the same thing happen in software, of course. I maintain the Text::Template Perl module, and I frequently get messages from people saying that it should have some feature or other. And these people sometimes get quite angry when I tell them I'm not going to put in the feature they want. They're angry because it's easy to see the benefit of adding another feature, but hard to see the cost. "If other people don't like it," goes the argument, "they don't have to use it." True, but even if they don't use it, they still pay the costs of slightly longer download times, slightly longer compile times, a slightly longer and more confusing manual, slightly less frequent maintenance updates, slightly less prompt bug fix deliveries, and so on. It is so hard to make this argument, because the cost to any one person is so very small! But we all know where the software will end up if I don't make this argument every step of the way: on the slag heap.

This has been on my mind on and off for years. But I just ran into it in a new context.

Lately I've been working on a book about code style and refactoring in Perl. One thing you see a lot in Perl programs written by beginners is superfluous parentheses. For example:

                next if ($file =~ /^\./);
                next if !($file =~ (/[0-9]/));
                next if !($file =~ (/txt/));

        die $usage if ($#ARGV < 0);

So the situation is impossible, at least in principle, and yet people have to deal with it somehow. But the advice you often hear is "if you're not sure of the precedence, just put in the parentheses." I think that's really bad advice. I think better advice would be "if you're not sure of the precedence, look it up."

Because Perl's Byzantine operator table is not responsible for all the problems. Notice in the examples above, which are real examples, taken from real code written by other people: Many of the parentheses there are entirely superfluous, and are not disambiguating the precedence of any operators. In particular, notice the inner parentheses in:

                next if !($file =~ (/txt/));

                next if !($file =~ /txt/);

By saying "if you're not sure, just avoid the problem" we are encouraging this kind of fearful, superstitious approach to the issue. That approach would be appropriate if it were the only way to deal with the issue, but fortunately it is not. There is a more rational approach: you can look it up, or even try an experiment, and then you will know whether the parentheses are required in a particular case. Then you can make an informed decision about whether to put them in.

But when I teach classes on this topic, people sometimes want to take the argument even further: they want to argue that even if you know the precedence, and even if you know that the parentheses are not required, you should put them in anyway, because the next person to see the code might not know that.

And there we see the creeping featurism argument again. It's easy to see the potential benefit of the superfluous parentheses: some hapless novice maintenance programmer might misunderstand the expression if I don't put them in. It's much harder to see the cost: The code is fractionally harder for everyone to read and understand, novice or not. And again, the cost of the extra parentheses to any particular person is so small, so very small, that it is really hard to make the argument against it convincingly. But I think the argument must be made, or else the code will end up on the slag heap much faster than it would have otherwise.

Programming cannot be run on the convoy system, with the program code written to address the most ignorant, uneducated programmer. I think you have to assume that the next maintenance programmer will be competent, and that if they do not know what the expression means, they will look up the operator precedence in the manual. That assumption may be false, of course; the world is full of incompetent programmers. But no amount of parentheses are really going to help this person anyway. And even if they were, you do not have to give in, you do not have to cater to incompetence. If an incompetent programmer has trouble understanding your code, that is not your fault; it is their fault for being incompetent. You do not have to take special steps to make your code understandable even by incompetents, and you certainly should not do so at the expense of making it harder for competent programmers to read and understand, no, not to the tiniest degree.

The advice that one should always put in the parentheses seems to me to be going in the wrong direction. We should be struggling for higher standards, both for ourselves and for our associates. The conventional advice, it seems to me, is to give up.

[Other articles in category /prog] permanent link

Egyptian Fractions
The Ahmes papyrus is one of the very oldest extant mathematical documents. It was written around 3800 years ago. As I mentioned recently, a large part of it is a table of the values of fractions of the form !!\frac 2n!! for odd integers !!n!!. The Egyptians, at least at that time, did not have a generalized fraction notation. They would write fractions of the form \frac 1n, and they could write sums of these. But convention dictated that they could not use the same unit fraction more than once. So to express !!\frac 35!! they would have needed to write something like !!\frac 12 + \frac{1}{10}!!, which from now on I will abbreviate as !![2, 10]!!. (They also had a special notation for !!\frac 23!!, but I will ignore that for a while.) Expressing arbitrary fractions in this form can be done, but it is non-trivial.

A simple algorithm for calculating this so-called "Egyptian fraction representation" is the greedy algorithm: To represent !!\frac nd!!, find the largest unit fraction !!\frac 1a!! that is less than !!\frac nd!!. Calculate a representation for !!\frac nd -\frac 1a!!, and append !!\frac 1a!!. This always works, but it doesn't always work well. For example, let's use the greedy algorithm to find a representation for !!\frac 29!!. The largest unit fraction less than !!\frac 29!! is !!\frac 15!!, and !!\frac 29 - \frac 15 = \frac{1}{45}!!, so we get !!\frac 29 = \frac 15 + \frac{1}{45} = [5, 45]!!. But it also happens that !!\frac 29 = [6, 18]!!, which is much more convenient to calculate with because the numbers are smaller. Similarly, for !!\frac{19}{20}!! the greedy algorithm produces $$\begin{align} \frac{19}{20} & = [2] + \frac{9}{20}\\\\ & = [2, 3] + \frac{7}{60} \\\\ & = [2, 3, 9, 180]. \end{align}$$ But even !!\frac{7}{60}!! can be more simply written than as !![9, 180]!!; it's also !![10, 60], [12, 30]!!, and, best of all, !![15, 20]!!.

So similarly, for !!\frac 37!! this time, the greedy methods gives us !!\frac 37 = \frac 13 + \frac{2}{21}!!, and that !!\frac{2}{21}!! can be expanded by the greedy method as !![11, 231]!!, so !!\frac 37 = [3, 11, 231]!!. But even !!\frac{2}{21}!! has better expansions: it's also !![12, 84], [14, 42],!! and, best of all, !![15, 35],!! so !!\frac 37 = [3, 15, 35]!!. But better than all of these is !!\frac 37 = [4, 7, 28]!!, which is optimal.

Anyway, while I was tinkering with all this, I got an answer to a question I had been wondering about for years, which is: why did Ahmes come up with a table of representations of fractions of the form !!\frac 2n!!, rather than the representations of all possible quotients? Was there a table somewhere else, now lost, of representations of fractions of the form !!\frac 3n!!?

The answer, I think, is "probably not"; here's why I think so.

Suppose you want !!\frac 37!!. But !!\frac 37 = \frac 27 + \frac 17!!. You look up !!\frac 27!! in the table and find that !!\frac 27 = [4, 28]!!. So !!\frac 37 = [4, 7, 28]!!. Done.

OK, suppose you want !!\frac 47!!. You look up !!\frac 27!! in the table and find that !!\frac 27 = [4, 28]!!. So !!\frac 47 = [4, 4, 28, 28] = [2, 14]!!. Done.

Similarly, !!\frac 57 = [2, 7, 14]!!. Done.

To calculate !!\frac 67!!, you first calculate !!\frac 37!!, which is !![4, 7, 28]!!. Then you double !!\frac 37!!, and get !!\frac 67 = \frac 12 + \frac 27 + \frac{1}{14}!!. Now you look up !!\frac 27!! in the table and get !!\frac 27 = [4, 28]!!, so !!\frac 67 = [2, 4, 14, 28]!!. Whether this is optimal or not is open to argument. It's longer than !![2, 3, 42]!!, but on the other hand the denominators are smaller.

Anyway, the table of !!\frac 2n!! is all you need to produce Egyptian representations of arbitrary rational numbers. The algorithm in general is:

• To sum up two Egyptian fractions, just concatenate their representations. There may now be unit fractions that appear twice, which is illegal. If a pair of such fractions have an even denominator, they can be eliminated using the rule that !!\frac 1{2n} + \frac 1{2n} = \frac 1n!!. Otherwise, the denominator is odd, and you can look the numbers up in the !!\frac 2n!! table and replace the matched pair with the result from the table lookup. Repeat until no pairs remain.

• To double an Egyptian fraction, add it to itself as per the previous.

• To calculate !!\frac ab!!, when !!a = 2k!!, first calculate !!\frac kb!! and then double it as per the previous.

• To calculate !!\frac ab!! when !!a!! is odd, first calculate !!\frac{a-1}{b}!! as per the previous; then add !!\frac 1b!!.

• !!\frac{19}{20} = \frac{18}{20} + \frac{1}{20}!!
• !!\frac{19}{20} = \frac{9}{10} + \frac{1}{20}!!
  • !!\frac{9}{10} = \frac{8}{10} + \frac{1}{10}!!
  • !!\frac{9}{10} = \frac 45 + \frac{1}{10}!!
    • !!\frac 45 = \frac 25 + \frac 25!!
      • !!\frac 25 = [3, 15]!! (from the table)
    • !!\frac 45 = [3, 3, 15, 15]!!
    • !!\frac 45 = \frac 23 + \frac{2}{15}!!
      • !!\frac 23 = [2, 6]!! (from the table)
      • !!\frac{2}{15} = [12, 20]!! (from the table)
    • !!\frac 45 = [2, 6, 12, 20]!!
  • !!\frac{9}{10} = [2, 6, 10, 12, 20] !!
• !!\frac{19}{20} = [2, 6, 10, 12, 20, 20] !!
• !!\frac{19}{20} = [2, 6, 10, 10, 12] !!
• !!\frac{19}{20} = [2, 5, 6, 12] !!

An alternative algorithm is to expand the numerator as a sum of powers of !!2!!, which the Egyptians certainly knew how to do. For !!\frac{19}{20}!! this gives us $$\frac{19}{20} = \frac{16}{20} + \frac{2}{20} + \frac{1}{20} = \frac 45 + [10, 20].$$ Now we need to figure out !!\frac 45!!, which we do as above, getting !!\frac 45 = [2, 6, 12, 20]!!, or !!\frac 45 = \frac 23 + [12, 20]!! if we are Egyptian, or !!\frac 45 = [2, 4, 20]!! if we are clever. Supposing we are neither, we have $$\begin{align} \frac{19}{20} & = [2, 6, 12, 20, 10, 20] \\\\ & = [2, 6, 12, 10, 10] \\\\ &= [2, 6, 12, 5] \\\\ \end{align}$$ as before.

(It is not clear to me, by the way, that either of these algorithms is guaranteed to terminate. I need to think about it some more.)

Getting the table of good-quality representations of !!\frac 2n!! is not trivial, and requires searching, number theory, and some trial and error. It's not at all clear that !!\frac{2}{105} = [90, 126]!!.

Once you have the table of !!\frac 2n!!, however, you can grind out the answer to any division problem. This might be time-consuming, but it's nevertheless trivial. So Ahmes needed a table of !!\frac 2n!!, but once he had it, he didn't need any other tables.

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Addenda to recent articles 200604
Here are some notes on posts from the last month that I couldn't find better places for.

• Stan Yen points out that I missed an important aspect of the convenience of instant mac & cheese: it has a long shelf life, so it is possible to keep a couple of boxes on hand for when you want them, and then when you do want macaroni and cheese you don't have to go shopping for cheese and pasta. M. Yen has a good point. I completely overlooked this, because my eating habits are such that I nearly always have the ingredients for macaroni and cheese on hand.

  M. Yen also points out that some of the attraction of Kraft Macaroni and Cheese Dinner is its specific taste and texture. We all have occasional longings for the comfort foods of childhood, and for many people, me included, Kraft dinner is one of these. When you are trying to recreate the memory of a beloved food from years past, the quality of the ingredients is not the issue. I can sympathize with this: I would continue to eat Kraft dinner if it tasted the way I remember it having tasted twenty years ago. I still occasionally buy horrible processed American cheese slices not because it's a good deal, or because I like the cheese, but because I want to put it into grilled cheese sandwiches to eat with Campbell's condensed tomato soup on rainy days.

• Regarding the invention of the = sign, R. Koch sent me two papers by Florian Cajori, a famous historian of mathematics. One paper, Note on our sign of Equality, presented evidence that a certain Pompeo Bolognetti independently invented the sign, perhaps even before Robert Recorde did. The = sign appears in some notes that Bolognetti made, possibly before 1557, when Recorde's book The Whetstone of Witte was published, and certainly before 1568, when Bolognetti died. Cajori suggests that Bolognetti used the sign because the dash ----- was being used for both equality and subtraction, so perhaps Bolognetti chose to double the dash when he used it to denote equality. Cajori says "We have here the extraordinary spectacle of the same arbitrary sign having been chosen by independent workers guided in their selection by different considerations."

  Buy A History of Mathematical Notations from Bookshop.org (with kickback) (without kickback)

  The other paper M. Koch sent is Mathematical Signs of Equality, and traces the many many symbols that have been used for equality, and the gradual universal adoption of Recorde's sign. Introduced in England in 1557, the Recorde sign was first widely adopted in England. Cajori: "In the seventeenth century Recorde's ===== gained complete ascendancy in England." But at that time, mathematicians in continental Europe were using a different sign , introduced by René Descartes. Cajori believes that the universal adoption of Recorde's = sign in Europe was due to its later use by Leibniz. Much of this material reappears in Volume I of Cajori's book A History of Mathematical Notations. Thank you, M. Koch.

  Ian Jones at the University of Warwick has a good summary of Cajori's discussion of this matter.
  

• Regarding Rayleigh scattering in the atmosphere, I asserted:

  The sun itself looks slightly redder ... this effect is quite pronounced ... when there are particles of soot in the air ... .

  Neil Kandalgaonkar wrote to inform me that there is a web page at the site of the National Oceanic and Atmospheric Administration that appears at first to dispute this. I said that I could not believe that the NOAA was actually disputing this. I was in San Diego in October 2003, and when I went outside at lunchtime, the sun was red. At the time, the whole county was on fire, and anyone who wants to persuade me that these two events were entirely unrelated will have an uphill battle.

  Buy Light and Color in the Outdoors from Bookshop.org (with kickback) (without kickback)

  Fortunately, M. Kandalgaonkar and I determined that the NOAA web page not asserting any such silly thing as that smoke does not make the sun look red. Rather, it is actually asserting that smoke does not contribute to good sunsets. And M. Kandalgaonkar then referred me to an amazing book, M. G. Minnaert's Light and Color in the Outdoors.

  This book explains every usual and unusual phenomenon of light and color in the outdoors that you have ever observed, and dozens that you may have observed but didn't notice until they were pointed out. (Random example: "This explains why the smoke of a cigar or cigarette is blue when blown immediately into the air, but becomes white if it is been kept in the mouth first. The particles of smoke in the latter case are covered by a coat of water and become much larger.") I may report on this book in more detail in the future. In the meantime, Minnaert says that the redness of the sun when seen through smoke is due primarily to absorption of light, not to Rayleigh scattering:

  The absorption of carbon increases rapidly from the red to the violet of the spectrum; this characteristic is exemplified in the blood-red color of the sun when seen through the smoke of a house on fire.

• A couple of people have written to suggest that perhaps the one science question any high school graduate ought to be able to answer is "What is the scientific method?" Yes, I quite agree.

  Nathan G. Senthil has also pointed out Richard P. Feynman's suggestion on this topic. In one of the first few of his freshman physics lectures, Feynman said that if nearly all scientific knowledge were to be destroyed, and he were able to transmit only one piece of scientific information to future generations, it would be that matter is composed of atoms, because a tremendous amount of knowledge can be inferred from this one fact. So we might turn this around and suggest that every high school graduate should be able to give an account of the atomic theory of matter.

• I said that Pick's theorem Implies that every lattice polygon has an area that is an integer multiple of 1/2, "which I would not have thought was obvious." Dan Schmidt pointed out that it is in fact obvious. As I pointed out in the Pick's theorem article, every such polygon can be built up from right triangles whose short sides are vertical and horizontal; each such triangle is half of a rectangle, and rectangles have integer areas. Oops.

• Seth David Schoen brought to my attention the fascinating phenomenon of tetrachromacy. It is believed that some (or all) humans may have a color sensation apparatus that supports a four-dimensional color space, rather than the three-dimensional space that it is believed most humans have.

  As is usual with color perception, the complete story is very complicated and not entirely understood. In my brief research, I discovered references to at least three different sorts of tetrachromacy.

  1. In addition to three types of cone cells, humans also have rod cells in their retinas. The rod cells have a peak response to photons of about 500 nm wavelength, which is quite different from the peak responses of any of the cones. In the figure below, the dotted black line is the response of the rods; the colored lines are the responses of the three types of cones.

     

     So it's at least conceivable that the brain could make use of rod cell response to distinguish more colors than would be possible without it. Impediments to this are that rods are poorly represented on the fovea (the central part of the retina where the receptors are densest) and they have a slow response. Also, because of the way the higher neural layers are wired up, rod vision has poorer resolution than cone vision.

     I did not find any scientific papers that discussed rod tetrachromacy, but I didn't look very hard.

  2. The most common form of color blindness is deuteranomaly, in which the pigment in the "green" cones is "redder" than it should be. The result is that the subject has difficulty distinguishing green and red. (Also common is protanomaly, which is just the reverse: the "red" pigment is "greener" than it should be, with the same result. What follows holds for protanomaly as well as deuteranomaly.)

     Genes for the red and green cone pigments are all carried on the X chromosomes, never on the Y chromosomes. Men have only one X chromosome, and so have only one gene each for the red and green pigments. About 6-8% of all men carry an anomalous green pigment gene on their X chromosome instead of a normal one and suffer from deuteranomaly.

     Each of these men inherited his X chromosome from his mother, who must also therefore carry the anomalous gene on one of her two X chromosomes. The other X chromosome of such a woman typically carries the normal version of the gene. Since such a woman has genes for both the normal and the "redder" version of the green pigment, she might have both normal and anomalous cone cells. That is, she might have the normal "green" cones and also the "redder" version of the "green" cones. If so, she will have four different kinds of cones with four different color responses: the usual "red", "green" and "blue" cones, and the anomalous "green" cone, which we might call "yellow".

     The big paper on this seems to be A study of women heterozygous for colour deficiencies, by G. Jordan and J. D. Mollon, appeared in Vision Research, Volume 33, Issue 11, July 1993, Pages 1495-1508. I haven't finished reading it yet. Here's my summary of the abstract: They took 31 of women who were known to be carriers of the anomalous gene and had them perform color-matching tasks. Over a certain range of wavelengths, a tetrachromat who is trying to mix light of wavelengths a and b to get as close as possible to perceived color c should do it the same way every time, whereas a trichromat would see many different mixtures as equivalent. And Jordan and Mollon did in fact find a person who made the same color match every time.

     Another relevant paper with similar content is Richer color experience in observers with multiple photopigment opsin genes, by Kimberly A. Jameson, Susan M. Highnote, and Linda M. Wasserman, appeared in Psychonomic Bulletin & Review 2001, 8 (2), 244-261. Happily, this is available online for free.

     My own description is highly condensed. Ryan's Sutherland's article Aliens among us: Preliminary evidence of superhuman tetrachromats is clear and readable, much more so than my explanation above. Please do not be put off by the silly title; it is an excellent article.

  3. The website Processes in Biological Vision claims that the human eye normally contains a color receptor that responds to very short-wavelength violet and even ultraviolet light, but that previous studies have missed this because the lens tends to filter out such light and because indoor light sources tend not to produce it. The site discusses the color perception of persons who had their lenses removed. I have not yet evaluated these claims, and the web site has a strong stink of crackpotism, so beware.

• In discussing Hero's formula, I derived the formula (2a²b² + 2a²c² + 2b²c² - a⁴ - b⁴- c⁴)/16 for the square of the area of a triangle with sides of lengths a, b, and c, and then wondered how to get from that mess to Hero's formula itself, which is nice and simple: p(p-a)(p-b)(p-c), where p is half the perimeter.

  François Glineur wrote in to show me how easy it is. First, my earlier calculations had given me the simpler expression 16A² = 4a²b² - (a²+b²-c²)², which, as he says, is unfortunately not symmetric in a, b and c. We know that it must be expressible in a symmetric form somehow, because the triangle's area does not know or care which side we have decided to designate as side a.

  But the formula above is a difference of squares, so we can factor it to obtain (2ab + a² + b² - c²)(2ab + c² - a² - b²), and then simplify the a² ±2ab + b² parts to get ((a+b)² - c²)(c² - (a-b)²). But now each factor is itself a difference of squares and can be factored, obtaining (a+b+c)(a+b-c)(c+a-b)(c-a+b). From here to Hero's formula is just a little step. As M. Glineur says, there are no lucky guesses or complicated steps needed. Thank you, M. Glineur.

  M. Glineur ended his note by saying:

  In my opinion, an even "better" proof would not break the symmetry between a, b and c at all, but I don't have convincing one at hand.

  Gareth McCaughan wrote to me with just such a proof; I hope to present it sometime in the next few weeks. It is nicely symmetric, and its only defect is that it depends on trigonometry.

• Carl Witty pointed out that my equation of the risk of Russian roulette with the risk of driving an automobile was an oversimplification. For example, he said, someone playing Russian roulette, even at extremely favorable odds, appears to be courting suicide in a way that someone driving a car does not; a person with strong ethical or religious beliefs against suicide might then reject Russian roulette even if it is less risky than driving a car. I hadn't appreciated this before; thank you, M. Witty.

  I am reminded of the story of the philosopher Ramon Llull (1235–1315). Llull was beatified, but not canonized, and my recollection was that this was because of the circumstances of his death: he had a habit of going to visit the infidels to preach loudly and insistently about Christianity. Several narrow escapes did not break him of this habit, and he was eventually he was torn apart by an angry mob. Although it wasn't exactly suicide, it wasn't exactly not suicide either, and the Church was too uncomfortable with it to let him be canonized.

  Then again, Wikipedia says he died "at home in Palma", so perhaps it's all nonsense.

• Three people have written in to contest my assertion that I did not know anyone who had used a gas chromatograph. By which I mean that three people I know have asserted that they have used gas chromatographs.

  It also occurred to me that my cousin Alex Scheeline is a professor of chemistry at UIUC, and my wife's mother's younger brother's daughter's husband's older brother's wife's twin sister is Laurie J. Butler, a professor of physical chemistry at the University of Chicago. Both of these have surely used gas chromatographs, so they bring the total to five.

  So it was a pretty dumb thing to say.

• In yesterday's Google query roundup, I brought up the following search query, which terminated at my blog:

               a collection of 2 billion points is completely enclosed
               by a circle.  does there exist a straight line having
               exactly 1 billion of these points on each side
  

  This has the appearance of someone's homework problem that they plugged into Google verbatim. What struck me about it on rereading is that the thing about the circle is a tautology. The rest of the problem does not refer to the circle, and every collection of 2 billion points is completely enclosed by a circle, so the clause about the circle is entirely unnecessary. So what is it doing there?

  All of my speculations about this are uncharitable (and, of course, speculative), so I will suppress them. I did the query myself, and was not enlightened.

  If this query came from a high school student, as I imagine it did, then following question probably has at least as much educational value:

          Show that for any collection of 2 billion points, there is a
          circle that completely encloses them.
  

  It seems to me that to answer that question, you must get to the heart of what it means for something to be a mathematical proof. At a higher educational level, this theorem might well be dismissed as "obvious", or passed over momentarily on the way to something more interesting with the phrase "since X is a finite set, it is bounded." But for a high school student, it is worth careful consideration. I worry that the teacher who asked the question does not know that finite sets are bounded. Oops, one of my uncharitable speculations leaked out.

[Other articles in category /addenda] permanent link

Google query roundup
Once again I am going to write up the interesting Google queries that my blog has attracted this month.

Probably the one I found most interesting was:

           1 if n + 1 are put inside n boxes, then at least one box
             will contain more than one ball. prove this principle by
             induction.

Another mathematical question that came up was:

           1 a collection of 2 billion points is completely enclosed
             by a circle.  does there exist a straight line having
             exactly 1 billion of these points on each side

What might happen in the middle is that you might have one billion minus n points on the left, and then suddenly the line intersects more than n points at once, so that the number of points on the left jumps up by a whole bunch, skipping right past one billion, instead of ticking up by one at a time. So what we really need is to ensure that this never happens. But that's no trouble. Taking the points two at a time, we can find the slope of the line that will pass through the two points. There are at most 499,999,999,500,000,000 such slopes. If we pick a line that has a slope different from one of these, then no matter where we put it, it cannot possibly intersect more than one of the points. Then as we slide the line from one side to the other, as above, the count of the number of points on the left never goes up by more than 1 at a time, and we win.

Another math query that did not come from Google is:

        Why can't there be a Heron's formula for an arbitrary quadrilateral

All three quadrilaterals have exactly the same edge lengths, but their areas are different, so there couldn't possibly be a formula to tell you the area from just the edge lengths.

The following query is a little puzzling:

           1 undecidable problems not in np

It might be that the querent was looking for decidable problems that are not in NP. Here the answer is more interesting. There are many possibilities, but surprisingly few known examples. The problem of determining whether there is any string that is matched by a given regex is known to require exponential time, if regular expressions are extended with a {2} notation so that a{2} is synonymous with aa. Normally, if someone asks you if there is any string that matches a regex, you can answer just by presenting such a string, and then the querent can check the answer by running the regex engine and checking (in polynomial time) that the string you have provided does indeed match.

But for regexes with the {2} notation, the string you would provide might have to be gigantic, so gigantic that it could not be checked efficiently. This is because one can build a relatively short regex that matches only enormous strings: a{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2}{2} is only 61 characters long, and it does indeed match one string, but the string it matches is 1,048,576 characters long.

Many problems involving finding the good moves in certain games are known to be decidable and believed to not be in NP, but it isn't known for sure.

Many problems that involve counting things are known to be decidable but are believed to not be in NP. For example, consider the NP-complete problem discussed here, called X3C. X3C is in NP. If Sesame Workshop presents you with a list of episodes and a list of approved topics for dividing the episodes into groups of 3, and you come up with a purported solution, Sesame Workshop can quickly check whether you did it right, whether each segment of Elmo's World is on exactly one video, and whether all the videos are on the approved list.

But consider the related problem in which Sesame Workshop comes to you with the same episodes and the same list of approved combinations, and asks, not for a distribution of episodes onto videotapes, but a count of the number of possible distributions. You might come back with an answer, say 23,487, but Sesame Workshop has no way to check that this is in fact the right number. Nobody has been able to think of a way that they might do this, anyhow.

Such a problem is clearly decidable: enumerate all possible distributions of episodes onto videos, check each one to see if it satisfies Sesame Workshop's criteria, and increment a counter if so. It is clearly at least as hard as the NP-complete problem of determining whether there is any legal distribution, because if you can count the number of legal distributions, there is a legal distribution if and only if the count of legal distributions is not 0. But it may well be outside of NP, because seems quite plausible that there is no quick way to verify a purported count of solutions, short of generating, checking, and recounting all possible distributions.

This is on my mind anyway, because this month I got email from two separate people both asking me for examples of problems that were outside of NP but still decidable, one on March 18:

I was hoping to get some information on a problem that is considered np-hard but not in NP (other than halting problem).

I was visiting your website in search of problems that are NP-Hard but not NP-Complete which are decision problems, but not the halting problem.

Here's one that was surprising:

           1 wife site:plover.com

           1 mathematical solution for eliminating debt

        [18/Jan/2006:09:10:46 -0500] eliminate debt using linear math
        [08/Feb/2006:13:48:13 -0500] linear math system eliminate debt
        [08/Feb/2006:13:53:28 -0500] linear math system eliminate debt
        [25/Feb/2006:15:12:01 -0500] how to get out of debt using linear math
        [23/Apr/2006:10:32:18 -0400] Mathematical solution for eliminating debt
        [23/Apr/2006:10:33:43 -0400] Mathematical solution for eliminating debt

           1 which 2 fraction did archimedes add together to write 3/4

There are several algorithms for writing any fraction as a sum of fractions of the form 1/n. The greedy algorithm suffices. Say you want to write 2/5. This is bigger than 1/3 and smaller than 1/2, so write 2/5 = 1/3 + x. x is now 1/15 and we are done. Had x had a numerator larger than 1, we would have repeated the process.

The greedy algorithm produces an Egyptian fraction representation of any number, but does not always do so conveniently. For example, consider 19/20. The greedy algorithm finds 19/20 = 1/2 + 1/3 + 1/9 + 1/180. But 19/20 = 1/2 + 1/4 + 1/5, which is much more convenient. So the problem of finding good representations for various numbers was of some interest, and the Ahmes papyrus, one of the very oldest mathematical manuscripts known, devotes a large amount of space to the representations of various numbers of the form 2/n.

           1 why does pi appear in both circle area and circumference?

           1 the only two things in our universe starting with m & e

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"Never heard of it," said the machine.

"What? But it's only sodium. You know, the metal, the element..."

"Sodium starts with an s, and I work only in n."

"But in Latin it's natrium."

"Look, old boy," said the machine, "if I could do everything starting with n in every possible language, I'd be a Machine That Could Do Everything in the Whole Alphabet, since any item you care to mention undoubtedly starts with n in one foreign language or another. It's not that easy. I can't go beyond what you programmed. So no Sodium."

I think The Cyberiad is uproarious. It is easily the funniest book I have ever read. I laugh out out every time I read it.

The most amazing thing about it is that it was originally written in Polish. The translator, Michael Kandel, is a genius. I would not have dared to translate a Polish story about a machine that can make anything that begins with n. It is an obvious death trap. Eventually, the story is going to call for the machine to manufacture something like napad rabunkowy ("stickup") that begins with n in Polish but not in English. Perhaps the translator will get lucky and find a synonym that begins with n in English, but sooner or later his luck will run out.

I once met M. Kandel, and I asked him if it had been n in the original, or if it had been some other letter that was easy in Polish but impossible in English, like z, or even ł. No, he said it had indeed been n. He said that the only real difficulty he had was that Trurl asks the machine to make nauka ("science") and he had to settle for "nature".

(Incidentally, I put the word napad rabunkowy into the dictionary lookup at poltran.com and it told me angrily "PLEASE TYPE POLISH WORDS USING CAPITAL LETTERS ONLY!" Yeah, that's a good advertisement for your software.)

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By the way, narghiles are like hookahs, and nankeens are trousers made of a certain kind of cotton cloth, also called nankeen.

The most intriguing query I got was:

           1 consciousness torus photon core

My favorite result from this query is unfortunately offline, available only from the Google cache:

The photon is actually composed of two tetrahedrons that are joined together, as we see in figure 4.6, and they then pass together through a cube that is only big enough to measure one of them at a time.

Also, I got 26 queries from people looking for the Indiana Pacers' cheerleaders, and 31 queries from people looking for examples of NP-complete problems.

This is article #100 on my blog.

[Other articles in category /google-roundup] permanent link

Google query roundup
A highly incomplete version of this month's Google query roundup appeared on the blog on Saturday by mistake. It's fixed now, but if you are reading this through an aggregator, the aggregator may not give you the chance to see the complete version.

The complete article is available here.

[Other articles in category /google-roundup] permanent link

Abbreviations in medieval manuscripts
In an earlier article I discussed my surprise at finding "examples" and "alteration" abbreviated to "exãples" and "alteratiõ" in Robert Recorde's 1557 book The Whetstone of Witte. Then later I quoted Jonathan Hoefler, an expert in typographical matters:

Diacritical marks have been used to abbreviate printed words ever since Gutenberg, and early English printers adopted the same conventions that Gutenberg used for Latin (a trick he picked up from medieval scribes.)

1. I can't make head or tail of most of it, but
2. the fourth line begins mella locustis.

Eventually I did what I should have done in the first place and plugged mella locustis into Google. The result was quite conclusive. The words here are from a very famous hymn about John the Baptist, attributed to Paulus Diaconus (c. 720 -799). The hymn is in three parts, and this is the beginning of the second part. The words here are:

Antra deserti teneris sub annis
civium turmas fugiens, petisti,
n levi saltim maculare vitam
famine posses.

Praebuit hirtum tegimen camelus,
artubus sacris strofium bidentis,
cui latex haustum, sociata pastum
mella locustis.

Caeteri tantum cecinere vatum
corde praesago iubar adfuturum;
...

The amount of abbreviation here is just amazing. In the first line, deserti is abbreviated deseti, and the s and the e are all squashed together, sub is abbreviated sb, annus is abbreviated ãnis, civium is abbreviated civiû and is illegible anyway, because the letters all look alike, as in Russian cursive. (I have a similar problem with cui on the third line.)

On the second line, artubus is written artub3; Hoefler had already pointed out to me that the 3 was a common notation in 16th-century printing. On the third line, pastum is written pa'tû, where the wiggly mark between the a and the t denotes an elided s. Or perhaps the scribe left it out by mistake and then went back to squeeze it in later.

Probably the most amazing abbreviations in the whole thing are in the fourth line. (I wonder if perhaps the scribe realized he was running out of room and wanted to squeeze in as much as possible.) The word caeteri is abbreviated to ceti, tantum to tm, and praesago to p'sago. (Also note uatû, which is an abbreviation for vatum; I had been wondering for some time what Uatu had to do with it.)

There are a number of other typographical features of interest. The third word in the second line is apparently hirtum. The hi in the manuscript is written as a sort of a V-shape. The r in corde on the fourth line (and elsewhere) is a form that was once common, but is now obsolete.

This hymn, by the way, is the one that gives us the names do, re, mi, fa, so, la, si for the notes of the major scale. The first part of the hymn begins:

Ut queant laxis resonare fibris
mira gestorum famuli tuorum,
solve polluti labii reatum,
sancte Iohannes!

The thing about the locusts and wild honey reminds me of something else. I was once on a business trip to Ottawa and found that there was a French Bible in my hotel room. And I discovered that, although I cannot read French, I could read the Bible in French, because I already knew what it was going to say. So I lay in bed and read the French Bible and enjoyed the rather strange sensation of being able to pretend to myself to be able to read French.

Two points struck me at the time. One was that when I read "Dieu dit: Que la lumière soit!" ("God said, 'Let there be light'") my instant reaction was to laugh at how absurd it was to suggest that God had spoken French when He created the universe. It's like that Reader's Digest joke about the guy who thinks the Spanish-speaking folks are silly for talking to the squirrels in the park in Spanish, because squirrels don't speak Spanish. I didn't know I had that in me, but there I was, laughing at the silly idea of God saying "Que la lumière soit!" You know, I still find it silly.

The other memorable occurrence was a little less embarrassing. The part in Matthew (excuse me; "Matthieu") about John the Baptist eating locusts and wild honey was "Il se nourrissait de sauterelles et de miel sauvage." I was impressed at how tasty it sounded, in French. It is not hard to imagine going into an expensive restaurant and ordering sauterelles et de miel sauvage off the menu. I concluded that food always sounds better in French, at least to an anglophone like me.

[ Addendum 20200706: Many years later, I learned where this illustration was from: it is Duc de Berry's Book of Hours, created between 1412 and 1416. I found this out in a happy way: the Metropolitan Museum of Art's was in the habit of tweeting pictures of objects from their collection, and one day they tweeted either the page above or one sufficiently similar to it that I was able to recognize it. ]

[ Addendum 20240817: Duc de Berry had at least two books of hours. This is not from the _Très Riches Heurs_ as I wrote above, but rather the [_Belles Heures_](WPREF/Belles_Heures_of_Jean_de_France,_Duc_de_Berry). It is in the Cloisters collection of the Metroploitan Museum of Art, which has a superb website with many scans](https://blog.metmuseum.org/artofillumination/). The page above is [folio 211r](https://blog.metmuseum.org/artofillumination/manuscript-pages/folio-211r/).]

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Yellow
Most of the academic part of high school is mercifully gone from my memory, dissolved together into a uniform blur of dullness. I wrote recently about one question I remember fondly from one final exam. I only remember a few specific exam questions, and probably that is the only one I remember because it was such a good question.

The one exam question that sticks most clearly in my mind was from my eighth-grade science class: What color do you see if you look at a yellow light through a monochromatic red filter?

I said it would look red, and was marked wrong. I argued my answer with the teacher, Mr. Goodman, but he would not give me credit. I puzzled over this for a long time, and eventually understood what had happened.

The word "yellow" is not a designation of physics alone; it refers to a certain experience, and is a perceptual phenomenon, not a purely physical one. It's possible to phrase the question to avoid this, but the question was not phrased in that way; it was expressly phrased in perceptual terms. A person who was achromatopsic might see a yellow light through a monochomatic red filter in a very unusual way.

To bring up the possibility of achromatopsia in the context of an exam is just nitpicking; I mention it only to point out that the question, as posed, must involve a consideration of human perception. And the objection I raised at the time is certainly not just nitpicking, because there are two entirely different physical phenomena that both go by the name of "yellow". The confusion of these two things occurs in the human retina.

The perception of color is a very complicated business, and I can't explain it in complete detail today. Partly this is because it isn't understood in complete detail, partly because I don't know everything that is understood, and partly it is because this article is about something else, and I want to try to come to the point eventually. So all my assertions about color perception in this article should be taken as metaphors for what is really happening. Although they give a correct general idea of a perceptual process that is something like what actually occurs, they are not accurate and are not intended to be accurate.

With that warning in place, I will now explain human color perception. Color is sensed by special "cone cells" in the retina. Different cone cells are sensitive to different frequencies of photons. Cone cells come in three types, which we will call "red", "green", and "blue", although all three of these are misnomers to one degree or another. The "red" cone cells are sensitive to red and yellow photons; the "green" cone cells to yellow and green photons. We will ignore the blue cones.

Photons with a wavelength of around 570 nanometers stimulate both the red and the green cone cells, and this stimulation is eventually perceived as the color yellow. But you can stimulate the cone cells the same way without using any light with a frequency around 570 nm. If you bombard the retina with photons of 650 nm, you stimulate only the red cones, and the light looks red; if you bombard the retina with photons of 520 nm, you stimulate only the green cones, and the light looks green. If you bombard the retina with both kinds of photons at once, both the red and green cones are stimulated, just as they were by the 570 nm photons. They have no way to know that they are being stimulated by two different groups of photons instead of by the same group, so the perception is the same.

This is why your computer monitor and your television can display the color yellow, despite having no source of yellow light. The monitor has little red phosphors and little green phosphors. When it activates both of them at once, the red and green photons stream out and get into your eye, where they stimulate the red and green cones, and you perceive the color yellow.

But from a purely physical point of view, this "yellow" phenomenon is not at all like the one that occurs when you look at a lemon or at a sodium vapor street light. The photons coming off the lemon are all of about the same frequency, around 570 nm. The photons coming off the computer monitor picture of the lemon are two different frequencies, some around 520 nm, and some around 650 nm. Your eye is not equipped to tell the difference.

Now, suppose you are looking at "yellow light" through a monochromatic red filter that passes only 650 nm photons. What do you see?

Well, if it was monochomatic yellow light, say from a lemon, then you see nothing, because the filter stops the 570 nm photons. This was Mr. Goodman's exam answer.

But if it was the yellow light that comes from a television picture of a lemon, then it contains some 520 nm photons, which are stopped by the filter, and some 650 nm photons, which are not stopped. You would see a red lemon. This was my exam answer.

The perception of mixed red and green light as yellow was part of the curriculum that year---we had had a classroom demonstration of it---and so it was fair game for the exam. Mr. Goodman's exam question, as posed, was genuinely ambiguous. There are two physical phenomena that are both described as "yellow", and he could have meant either one.

This confusion of two distinct physical phenomena by the retina is something we take for granted, but it is by no means inevitable. I often imagine our meeting with the aliens, and their surprise when they learn that all of us, every human on earth, are color-blind. They will find this out quickly, as soon as they see a television or a computer monitor. "Your monitor is broken," Zxaxgr will say.

"It looks all right to me," replies Flash Gordon.

"Is there something wrong with your eyes? The color adjustment for yellow is completely off. It is coming out as redgreen instead of as yellow."

"I see nothing wrong with the color adjustment for yellow," replies Flash.

"There must be something wrong with your eyes."

And yes, Zxaxgr is right. There is something wrong with our eyes. There is an intrinsic design flaw in our computer monitors, none of which can display yellow, and we don't care, because none of us can tell the difference between redgreen and yellow.

To empathize with Zxaxgr's puzzlement, imagine how strange it would be to learn that the alien televisions cannot display green; they display purple instead: purple trees, purple broccoli, purple frogs, purple flags of Saudi Arabia. And the aliens have never noticed the problem. You want to ask them about it, but your English-to-Alien dictionary doesn't have an entry for "purple". When you ask them about it, they say you're nuts, everything looks green, as it should. You learn that they have no word for purple; they just call it green, and eventually you find out that it's because they can't tell the difference between purple and green.

Whatever you're thinking now, that's what Zxaxgr is going to think.

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More on risk
My article on risk was one of the more popular articles so far; it went to the top of Reddit, and was widely commented on. It wasn't the best article I've written. I confused a bunch of important things. But now I find I have more to say on the topic, so I'm back to screw up again.

One big problem with the Reddit posting is that the guy who posted it there titled the post on risk, or why poor people might not be stupid to play the lottery. So a lot of the Reddit comments complained that I had failed to prove that poor people must not be stupid to play the lottery, or that I was wrong on that point. They argued that the dollar cost of a lottery ticket is more valuable to a poor person than to a rich one, and so on. But I didn't say anything about poor people. People read this into the article based on the title someone else had attached to it, and they couldn't get rid of this association even after I pointed out that the article had nothing to say about poor people.

Something I do a lot, in this blog, and in life, is point out fallacious arguments. You get some argument that X is true, because of P and Q and therefore R, and then I'll come along point out that P is false and Q is irrelevant, and anyway they don't imply R, and even if they did, you can't conclude X from R, because if you could, then you could also conclude Y and Z which are obviously false. For example, in a recent article I addressed the argument that:

You can double your workforce participation from 27% to 51% of the population, as Singapore did; you can't double it again.

The "risk" article was one of those. I wanted to refute one specific argument, which is that (a) the expected return on a lottery ticket is negative, so therefore (b) it's stupid to buy lottery tickets. My counter-argument was to point out that (a) the expected return on fire insurance is negative, but that you can't conclude that therefore (b) it's stupid to buy fire insurance. It might be stupid to buy lottery tickets, but if it is, it's not because the expected return is negative. Or at least it's not only because the expected return is negative. There must be more to it than that.

I really like that pattern of argument, and I use it a lot: A can't imply B, because if it did, then it would also imply B', and B' is false, or at least B' is a belief held only by dumbasses.

None of this addresses the question of whether or not I think it's stupid to buy lottery tickets. I have not weighed in on that matter. My only argument is that the argument from expected value is insufficient to prove the point.

People have a lot of trouble with second-order arguments like this, though. If I argue "that argument against B is no good," they are likely to hear it as an argument in favor of B. Several of the Reddit people made this mistake. The converse mistake is to interpret "that argument against B is no good, because it can be converted into an argument against B'" as an argument against B'! Some of the Reddit people made this mistake too, and disdainfully explained to me why buying fire insurance is not stupid.

Another problem with the article was that it followed my usual pattern of meandering digression. Although the main point of the article was to refute the argument from expected value, I threw in a bunch of marginally related stuff that I thought was fun and interesting: the stuff about estimating the value one ascribes to one's own life; the stuff about the surprisingly high chance of being killed by a meteor strike. Email correspondents and Reddit commenters mistook both of these for arguments about the lottery, and tried to refute them as such. Well, I have nobody to blame but myself for that. If you present a muddled, miscellaneous article, you can't complain when other people are confused by it.

If I were going to do the article again, one thing I'd try to fix is the discussion of utility. I think my biggest screwup was to confuse two things that are not the same. One is the utility, which decreases for larger amounts of money; your second million dollars has less value than your first million. But another issue, which I didn't separate in my mind, was the administration cost of money. There must be a jargon term for this, but I don't know what it is.

Economists like to pretend that money is perfectly fungible, and this is a reasonable simplifying assumption in most cases. But it's easy to prove that money isn't perfectly fungible. Imagine you've just won a prize. You can have one thousand dollars paid in hundred-dollar bills, or you can have a thousand and one dollars, paid in pennies. Anyone who really believes that money is perfectly fungible will take the pennies, even though they weigh six hundred pounds, because that way they get the one-dollar bonus.

Money has a physical manifestation, even when it's just numerals written in a ledger somewhere, and managing the physical manifestation of money has an associated cost. The cost of managing a penny is a significant fraction of the value of the penny, to the point that many people throw away pennies or dump them in jars just to avoid the cost of dealing with them. In some circumstances, like the lottery ticket purchase, the non-fungibility of money is important. Blowing one dollar on a lottery that pays a thousand dollars is not the same as blowing a thousand dollars on a lottery that pays a million dollars, and it's not the same as blowing your whole paycheck on a big stack of lottery tickets. Partly it's the risk issue, and partly it's this other issue, that I don't know the name of, that a single dollar is worth less than one one-thousandth of a thousand dollars, because the cost to administer and manage it is proportionately higher. I didn't make this clear in the original article because it wasn't clear in my mind. Oh well, I'm not yet a perfect sage.

One last point that has come up is that a couple of people have written to me to say that they would not take the Russian roulette bet for any amount of money at any odds. (Here's a blog post to that effect, for example.) One person even suggested that I only assumed he would take the bet at some odds because I'm an American, and I can't conceive of anyone refusing a big pot of money.

Well, maybe that's true, but I don't think that's why I assumed that everyone would take the bet for some amount of money. I assumed it because that is what I have observed people to do. I now know there are people who say that they would not play Russian roulette at any odds for any payoff. And I think those people are fooling themselves.

If you think you're one of those people, I have this question for you: Do you own a bicycle helmet? And if you do, did you buy the very top-of-the-line helmet? Or did you buy a mid-price model that might offer less protection? What, just to save money? I offered you a million dollars at million-to-one odds. Do you think that fifty dollars you saved on your bicycle helmet is paying you off for less risk than my million-to-one Russian roulette bet?

Well, maybe you don't own a bicycle, so you think you have no need of a helmet. But if the people who wrote to me were as risk-averse as some of them said they were, the lack of a bicycle wouldn't stop them from wearing helmets all the time anyway—another reason I think they are fooling themselves. I've met some of these people, and they don't go around in helmets and padded armor all the time.

Or maybe you do own the very safest helmet money can buy, since you have only one head, after all. But I bet you can find some other example? Have you ever flown in a plane? Did you refuse to fly anywhere not served by Qantas, like Raymond in Rain Man, because every other airline has had a crash? If you had a choice to pay double to fly with Qantas, would you take it? Or would you take the cheap flight and ignore the risk?

One comment that replies to the blog I cited above really hits the nail on the head, I think. It says: "you don't get paid a million dollars to get in your car and drive somewhere, but what are the chances you'll be killed in an auto accident?" My Russian roulette game is a much better deal than driving your car.

I'm going to end this article, as I did the last one, with an amusing anecdote about risk. My great-uncle Robert E. Machol was for a time the chief scientist of the Federal Aviation Administration. The regulations for infant travel were (and still are) that an infant may make an air trip on its parent's lap; parents do not need to buy a separate ticket and a seat for the infant.

In one air disaster, an infant that was being held on its parent's lap was thrown loose, hurtled to the end of the corridor, and died. The FAA was considering changing the rules for infants to require that they purchase a separate ticket, entitling them to their own seat, into which would be installed an FAA-approved safety car seat. Infants in their own restraint seats would be much safer than those held on their parents' laps.

Dr. Machol argued against this rule change, on the following grounds: If parents are required to buy separate tickets for their infants, air travel will be more expensive for them. As a result, some families will opt to take car trips instead of plane trips. Car trips are much more dangerous than plane trips; the fatalities per passenger per mile are something like twenty times higher. More babies can be expected to be killed in the resulting auto crashes than can be expected to be saved by the restraint seat requirement.

As before, this is not intended as an argument for or against anything in particular, except perhaps that the idea of risk is complex and hard to understand. Probably people will try to interpret it as an argument about the fungibility of money, or whatever the next Reddit person decides to put in the article title. You'd think I would have learned my lesson by now, but, as I said, I'm not yet a perfect sage.

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Counting squares
Let's take a bunch of squares, and put a big "X" in each one, dividing each square into four triangular wedges. Then let's take three colors of ink, say red, blue, and black, and ink in the wedges. How many different ways are there of inking up the squares? Well, that's easy. There are four wedges, and each one can be one of three colors, so the answer is 3⁴ = 81. No, wait, that's not right, because the two squares below are really the same:

So we have to decide what counts and what doesn't. If one square can be turned into the other by a one-quarter clockwise or counterclockwise rotation, or by a half-turn, then we'll say that the two squares are the same. So all the squares below are "the same":

What about turning the squares over? Are the two squares to the right "the same"? Let's say that the squares are inked on only one side, so that those two would not considered the same, even if we decided to allow squares with green wedges. Later on we will make the decision the other way and see how things change.

OK, so let's see. All four wedges might be the same color, and there are 3 colors, so there are 3 ways to do that, shown at right.