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Mon, 23 Oct 2023 Katara is taking a Data Structures course this year. The most recent assignment gave her a lot of trouble, partly because it was silly and made no sense, but also because she does not yet know an effective process for writing programs, and the course does not attempt to teach her. On the day the last assignment was due I helped her fix the remaining bugs and get it submitted. This is the memo I wrote to her to memorialize the important process issues that I thought of while we were working on it.
Something we discussed that I forgot to include in the memo that we discussed is: After you fix something significant, or add significant new functionality, make a checkpoint copy of the entire source code. This can be as simple as simply copying it all into separate folder. That way, when you are fixing the next thing, if you mess up and break everything, it's easy to get back to a known-good state. The computer is really clumsy to use for many tasks, but it's just great at keeping track of information, so exploit that when you can. I think CS curricula should have a class that focuses specifically on these issues, on the matter of how do you actually write software? But they never do. [Other articles in category /prog] permanent link When I first set up this blog I didn't know how long I would do it, so instead of thinking about how I wanted it to look, I just took the default layout that came with Blosxom, and figured that I would change it when I got around to it. Now I am getting around to it. The primary problem is that the current implementation (with nested tables!) performs badly, especially on mobile devices and especially on pages with a lot of MathJax. A secondary issue is that it's troublesome to edit. People sometimes laugh at how it looks like a 2006 design (which it is). I don't much care about this. I like the information-dense layout, which I think is distinctive and on-brand. I would like a redesign that fixes these two problems. The primary goal is to get rid of the nested tables and replace the implementation with something that browsers can handle better, probably something based on CSS Flexbox. It doesn't have to look very different, but it does need to be straightforward enough that I can make the next ten years of changes without a lot of research and experimentation. If you know someone interested in doing this, please email me a referral or have them get in touch with me. If you are interested in doing it yourself, please send me a proposal. Include a cost estimate, as this would be paid work. Please do not advertise this on Hacker News, as that would run the risk of my getting 100 proposals, and that would be 50 times as many as I need. Thanks. [Other articles in category /meta] permanent link Sun, 22 Oct 2023We used to have a cat named Chase. To be respectful we would sometimes refer to him as “Mr. Cat”. And sometimes I amused myself by calling him “Señor Gato”. Yesterday I got to wondering: Where did Spanish get “gato”, which certainly sounds like “cat”, when the Latin is fēles or fēlis (like in “feline’)? And similarly French has chat. Well, the real question is, where did Latin get fēles? Because Latin also has cattus, which I think sounds like a joke. You're in Latin class, and you're asked to translate cat, but you haven't done your homework, so what do you say? “Uhhhh… ‘cattus’?” But cattus is postclassical Latin, replacing the original word fēles no more than about 1500 years ago. The word seems to have wandered all over Europe and Western Asia and maybe North Africa, borrowed from one language into another, and its history is thoroughly mixed up. Nobody is sure where it came from originally, beyond “something Germanic”. The OED description of cat runs to 600 words and shrugs its shoulders. I learned recently that such words (like brinjal, the eggplant) are called Wanderworts, wandering words. [Other articles in category /lang/etym] permanent link Sat, 21 Oct 2023The other day I was looking into vindaloo curry and was surprised to learn that the word “vindaloo” is originally Portuguese vin d'alho, a wine and garlic sauce. Amazing. In Japanese, squashes are called kabocha. (In English this refers to a specific type of squash associated with Japan, but in Japanese it's more generic.) Kabocha is from Portuguese again. The Portuguese introduced squashes to Japan via Cambodia, which in Portuguese is Camboja. [Other articles in category /lang/etym] permanent link Fri, 20 Oct 2023
The discrete logarithm, shorter and simpler
I recently discussed the “discrete logarithm” method for multiplying integers, and I feel like I took too long and made it seem more complicated and mysterious than it should have been. I think I'm going to try again. Suppose for some reason you found yourself needing to multiply a lot of powers of !!2!!. What's !!4096·512!!? You could use the conventional algorithm: $$ \begin{array}{cccccccc} & & & & 4 & 0 & 9 & 6 \\ × & & & & & 5 & 1 & 2 \\ \hline % & & & & 8 & 1 & 9 & 2 \\ & & & 4 & 0 & 9 & 6 & \\ & 2 & 0 & 4 & 8 & 0 & & \\ \hline % & 2 & 0 & 9 & 7 & 1 & 5 & 2 \end{array} $$ but that's a lot of trouble, and a simpler method is available. You know that $$2^i\cdot 2^j = 2^{i+j}$$ so if you had an easy way to convert $$2^i\leftrightarrow i$$ you could just convert the factors to exponents, add the exponents, and convert back. And all that's needed is a simple table: \begin{array}{rr} 0 & 1\\ 1 & 2\\ 2 & 4\\ 3 & 8\\ 4 & 16\\ 5 & 32\\ 6 & 64\\ 7 & 128\\ 8 & 256\\ 9 & 512\\ 10 & 1\,024\\ 11 & 2\,048\\ 12 & 4\,096\\ 13 & 8\,192\\ 14 & 16\,384\\ 15 & 32\,768\\ 16 & 65\,536\\ 17 & 131\,072\\ 18 & 262\,144\\ 19 & 524\,288\\ 20 & 1\,048\,576\\ 21 & 2\,097\,152\\ \vdots & \vdots \\ \end{array} We check the table, and find that $$4096\cdot512 = 2^{12}\cdot 2^9 = 2^{12+9} = 2^{21} = 2097152.$$ Easy-peasy. That is all very well but how often do you find yourself having to multiply a lot of powers of !!2!!? This was a lovely algorithm but with very limited application. What Napier (the inventor of logarithms) realized was that while not every number is an integer power of !!2!!, every number is an integer power of !!1.00001!!, or nearly so. For example, !!23!! is very close to !!1.00001^{313\,551}!!. Napier made up a table, just like the one above, except with powers of !!1.00001!! instead of powers of !!2!!. Then to multiply !!x\cdot y!! you would just find numbers close to !!x!! and !!y!! in Napier's table and use the same algorithm. (Napier's original table used powers of !!0.9999!!, but it works the same way for the same reason.) There's another way to make it work. Consider the integers mod !!101!!, called !!\Bbb Z_{101}!!. In !!\Bbb Z_{101}!!, every number is an integer power of !!2!!! For example, !!27!! is a power of !!2!!. It's simply !!2^7!!, because if you multiply out !!2^7!! you get !!128!!, and !!128\equiv 27\pmod{101}!!. Or: $$\begin{array}{rcll} 14 & \stackrel{\pmod{101}}{\equiv} & 10\cdot 101 & + 14 \\ & = & 1010 & + 14 \\ & = & 1024 \\ & = & 2^{10} \end{array} $$ Or: $$\begin{array}{rcll} 3 & \stackrel{\pmod{101}}{\equiv} & 5844512973848570809\cdot 101 & + 3 \\ & = & 590295810358705651709 & + 3 \\ & = & 590295810358705651712 \\ & = & 2^{69} \end{array} $$ Anyway that's the secret. In !!\Bbb Z_{101}!! the silly algorithm that quickly multiplies powers of !!2!! becomes more practical, because in !!\Bbb Z_{101}!!, every number is a power of !!2!!. What works for !!101!! works in other cases larger and more interesting. It doesn't work to replace !!101!! with !!7!! (try it and see what goes wrong), but we can replace it with !!107, 797!!, or !!297779!!. The key is that if we want to replace !!101!! with !!n!! and !!2!! with !!a!!, we need to be sure that there is a solution to !!a^i=b\pmod n!! for every possible !!b!!. (The jargon term here is that !!a!! must be a “primitive root mod !!n!!”. !!2!! is a primitive root mod !!101!!, but not mod !!7!!.) Is this actually useful for multiplication? Perhaps not, but it does have cryptographic applications. Similar to how multiplying is easy but factoring seems difficult, computing !!a^i\pmod n!! for given !!a, i, n!! is easy, but nobody knows a quick way in general to reverse the calculation and compute the !!i!! for which !!a^i\pmod n = m!! for a given !!m!!. When !!n!! is small we can simply construct a lookup table with !!n-1!! entries. But if !!n!! is a !!600!!-digit number, the table method is impractical. Because of this, Alice and Bob can find a way to compute a number !!2^i!! that they both know, but someone else, seeing !!2^i!! can't easily figure out what the original !!i!! was. See Diffie-Hellman key exchange for more details. [ Also previously: Percy Ludgate's weird variation on this ] [Other articles in category /math] permanent link Sun, 15 Oct 2023[ Addendum 20231020: This came out way longer than it needed to be, so I took another shot at it, and wrote a much simpler explanation of the same thing that is only one-third as long. ] A couple days ago I discussed the weird little algorithm of Percy Ludgate's, for doing single-digit multiplication using a single addition and three scalar table lookups. In Ludgate's algorithm, there were two tables, !!T_1!! and !!T_2!!, satisfying the following properties: $$ \begin{align} T_2(T_1(n)) & = n \tag{$\color{darkgreen}{\spadesuit}$} \\ T_2(T_1(a) + T_1(b)) & = ab. \tag{$\color{purple}{\clubsuit}$} \end{align} $$ This has been called the “Irish logarithm” method because of its resemblance to ordinary logarithms. Normally in doing logarithms we have a magic logarithm function !!\ell!! with these properties: $$ \begin{align} \ell^{-1}(\ell(n)) & = n \tag{$\color{darkgreen}{\spadesuit}$} \\ \ell^{-1}(\ell(a) + \ell(b)) & = ab. \tag{$\color{purple}{\clubsuit}$} \end{align} $$ (The usual notation for !!\ell(x)!! is of course “!!\log x!!” or “!!\ln x!!” or something of that sort, and !!\ell^{-1}(x)!! is usually written !!e^x!! or !!10^x!!.) The properties of Ludgate's !!T_1!! and !!T_2!! are formally identical, with !!T_1!! playing the role of the logarithm function !!\ell!! and !!T_2!! playing the role of its inverse !!\ell^{-1}!!. Ludgate's versions are highly restricted, to reduce the computation to something simple enough that it can be implemented with brass gears. Both !!T_1!! and !!T_2!! map positive integers to positive integers, and can be implemented with finite lookup tables. The ordinary logarithm does more, but is technically much more difficult. With the ordinary logarithm you are not limited to multiplying single digit integers, as with Ludgate's weird little algorithm. You can multiply any two real numbers, and the multiplication still requires only one addition and three table lookups. But the cost is huge! The tables are much larger and more complex, and to use them effectively you have to deal with fractional numbers, perform table interpolation, and worry about error accumulation. It's tempting at this point to start explaining the history and use of logarithm tables, slide rules, and so on, but this article has already been delayed once, so I will try to resist. I will do just one example, with no explanation, to demonstrate the flavor. Let's multiply !!7!! by !!13!!.
If I were multiplying !!7.236!! by !!13.877!!, I would be willing to accept all these costs, and generations of scientists and engineers did accept them. But for !!7.0000×13.000 = 91.000!! the process is ridiculous. One might wonder if there wasn't some analogous technique that would retain the small, finite tables, and permits multiplication of integers, using only integer calculations throughout. And there is! Now I am going to demonstrate an algorithm, based on logarithms, that exactly multiplies any two integers !!a!! and !!b!!, as long as !!ab ≤ 100!!. Like Ludgate's and the standard algorithm, it will use one addition and three lookups in tables. Unlike the standard algorithm, the tables will be small, and will contain only integers. Here is the table of the !!\ell!! function, which corresponds to Ludgate's !!T_1!!: $$ \begin{array}{rrrrrrrrrrr} {\tiny\color{gray}{1}} & 0, & 1, & \color{darkblue}{69}, & 2, & 24, & 70, & \color{darkgreen}{9}, & 3, & 38, & 25, \\ {\tiny\color{gray}{11}} & 13, & \color{darkblue}{71}, & \color{darkgreen}{66}, & 10, & 93, & 4, & 30, & 39, & 96, & 26, \\ {\tiny\color{gray}{21}} & 78, & 14, & 86, & 72, & 48, & 67, & 7, & 11, & 91, & 94, \\ {\tiny\color{gray}{31}} & 84, & 5, & 82, & 31, & 33, & 40, & 56, & 97, & 35, & 27, \\ {\tiny\color{gray}{41}} & 45, & 79, & 42, & 15, & 62, & 87, & 58, & 73, & 18, & 49, \\ {\tiny\color{gray}{51}} & 99, & 68, & 23, & 8, & 37, & 12, & 65, & 92, & 29, & 95, \\ {\tiny\color{gray}{61}} & 77, & 85, & 47, & 6, & 90, & 83, & 81, & 32, & 55, & 34, \\ {\tiny\color{gray}{71}} & 44, & 41, & 61, & 57, & 17, & 98, & 22, & 36, & 64, & 28, \\ {\tiny\color{gray}{81}} & \color{darkred}{76}, & 46, & 89, & 80, & 54, & 43, & 60, & 16, & 21, & 63, \\ {\tiny\color{gray}{91}} & 75, & 88, & 53, & 59, & 20, & 74, & 52, & 19, & 51, & 50\hphantom{,} \\ \end{array} $$ (If we only want to multiply numbers with !!1\le a, b \le 9!! we only need the first row, but with the full table we can also compute things like !!7·13=91!!.) Like !!T_2!!, this is not really a two-dimensional array. It just a list of !!100!! numbers, arranged in rows to make it easy to find the !!81!!st number when you need it. The small gray numerals in the margin are a finding aid. If you want to look up !!\ell(81)!! you can see that it is !!\color{darkred}{76}!! without having to count up !!81!! elements. This element is highlighted in red in the table above. Note that the elements are numbered from !!1!! to !!100!!, whereas all the other tables in these articles have been zero-indexed. I wondered if there was a good way to fix this, but there really isn't. !!\ell!! is analogous to a logarithm function, and the one thing a logarithm function really must do is to have !!\log 1 = 0!!. So too here; we have !!\ell(1) = 0!!. We also need an !!\ell^{-1}!! table analogous to Ludgate's !!T_2!!: $$ \begin{array}{rrrrrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, & 27, & 54, & 7, \\ {\tiny\color{gray}{10}} & 14, & 28, & 56, & 11, & 22, & 44, & 88, & 75, & 49, & 98, \\ {\tiny\color{gray}{20}} & 95, & 89, & 77, & 53, & 5, & 10, & 20, & 40, & 80, & 59, \\ {\tiny\color{gray}{30}} & 17, & 34, & 68, & 35, & 70, & 39, & 78, & 55, & 9, & 18, \\ {\tiny\color{gray}{40}} & \color{darkblue}{36}, & 72, & 43, & 86, & 71, & 41, & 82, & 63, & 25, & 50, \\ {\tiny\color{gray}{50}} & 100, & 99, & 97, & 93, & 85, & 69, & 37, & 74, & 47, & 94, \\ {\tiny\color{gray}{60}} & 87, & 73, & 45, & 90, & 79, & 57, & 13, & 26, & 52, & 3, \\ {\tiny\color{gray}{70}} & 6, & 12, & 24, & 48, & 96, & \color{darkgreen}{91}, & \color{darkred}{81}, & 61, & 21, & 42, \\ {\tiny\color{gray}{80}} & 84, & 67, & 33, & 66, & 31, & 62, & 23, & 46, & 92, & 83, \\ {\tiny\color{gray}{90}} & 65, & 29, & 58, & 15, & 30, & 60, & 19, & 38, & 76, & 51\hphantom{,} \\ \end{array} $$ Like !!\ell^{-1}!! and !!T_2!!, this is just a list of !!100!! numbers in order. As the notation suggests, !!\ell^{-1}!! and !!\ell!! are inverses. We already saw that the first table had !!\ell(81)=\color{darkred}{76}!! and !!\ell(1) = 0!!. Going in the opposite direction, we see from the second table that !!\ell^{-1}(76)= \color{darkred}{81}!! (again in red) and !!\ell^{-1}(0)=1!!. The elements of !!\ell!! tell you where to find numbers in the !!\ell^{-1}!! table. Where is !!17!! in the second table? Look at the !!17!!th element in the first table. !!\ell(17) = 30!!, so !!17!! is at position !!30!! in the second table. Before we go too deeply into how these were constructed, let's try the !!7×13!! example we did before. The algorithm is just !!\color{purple}{\clubsuit}!!: $$ \begin{align} % \ell^{-1}(\ell(a) + \ell(b)) & = ab\tag{$\color{purple}{\clubsuit}$} \\ 7·13 &= \ell^{-1}(\ell(7) + \ell(13)) \\ &= \ell^{-1}(\color{darkgreen}{9} + \color{darkgreen}{66}) \\ &= \ell^{-1}(75) \\ &= \color{darkgreen}{91} \end{align} $$ (The relevant numbers are picked out in green in the two tables.) As promised, with three table lookups and a single integer addition. What if the sum in the middle exceeds !!99!!? No problem, the !!\ell^{-1}!! table wraps around, so that element !!100!! is the same as element !!0!!: $$ \begin{align} % \ell^{-1}(\ell(a) + \ell(b)) & = ab\tag{$\color{purple}{\clubsuit}$} \\ 3·12 &= \ell^{-1}(\ell(3) + \ell(12)) \\ &= \ell^{-1}(\color{darkblue}{69} + \color{darkblue}{71}) \\ &= \ell^{-1}(140) \\ &= \ell^{-1}(40) &\text{(wrap around)}\\ &= \color{darkblue}{36} \end{align} $$ How about that. (This time the relevant numbers are picked out in blue.) I said this only computes !!ab!! when the product is at most !!100!!. That is not quite true. If you are willing to ignore a small detail, this algorithm will multiply any two numbers. The small detail is that the multiplication will be done mod !!101!!. That is, instead of the exact answer, you get one that differs from it by a multiple of !!101!!. Let's do an example to see what I mean when I say it works even for products bigger than !!100!!: $$ \begin{align} % \ell^{-1}(\ell(a) + \ell(b)) & = ab\tag{$\color{purple}{\clubsuit}$} \\ 16·26 &= \ell^{-1}(\ell(16) + \ell(26)) \\ &= \ell^{-1}(4 + 67) \\ &= \ell^{-1}(71) \\ &= 12 \end{align} $$ This tell us that !!16·26 = 12!!. The correct answer is actually !!16·26 = 416!!, and indeed !!416-12 = 404!! which is a multiple of !!101!!. The reason this happens is that the elements of the second table, !!\ell^{-1}!!, are not true integers, they are mod !!101!! integers. Okay, so what is the secret here? Why does this work? It should jump out at you that it is often the case that an entry in the !!\ell^{-1}!! table is twice the previous entry: $$\ell^{-1}(1+n) = 2\cdot \ell^{-1}(n)$$ In fact, this is true everywhere, if you remember that the numbers are not ordinary integers but mod !!101!! integers. For example, the number that follows !!64!!, in place of !!64·2=128!!, is !!27!!. But !!27\equiv 128\pmod{101}!! because they differ by a multiple of !!101!!. From a mod !!101!! point of view, it doesn't matter wther we put !!27!! or !!128!! after !!64!!, as they are the same thing. Those two facts are the whole secret of the !!\ell^{-1}!! table:
Certainly !!\ell^{-1}(0) = 2^0 = 1!!. And every entry in the !!\ell^{-1}!! is twice the previous one, if you are thinking in mod !!101!!. The two secrets are actually one secret: $$\ell^{-1}(n) = 2^n\pmod{101}.$$ This is why the multiplication algorithm works. Say we want to multiply !!7!! and !!13!! again. We look up !!7!! and !!13!! in !!\ell!!, and find !!\ell(7)=9!! and !!\ell(13)=66!!. What this is really telling us is that $$ \begin{align} 7 & = 2^{9\hphantom6} \pmod{101} \\ 13 & = 2^{66} \pmod{101} \\ \end{align} $$ so that multiplying !!7\cdot13!! mod !!101!! is the same as multiplying $$2^9\cdot 2^{66}.$$ But multiplying exact powers of !!2!! is easy, since you just add the exponents: !!2^9\cdot2^{66} = 2^{9+66} = 2^{75}!!, whether you are doing it in regular numbers or mod !!101!! numbers. And the !!\ell^{-1}!! table tells us directly that !!2^{75} = 91\pmod{101}!!. The !!\ell!! function, which is analogous to the regular logarithm, is called a discrete logarithm. What's going on with Percy Ludgate's algorithm? It's a sort of compressed, limited version of the discrete logarithm. I had a hope that maybe we could reimplement Ludgate's thing by basing it more directly on discrete logarithms. Say we had the !!\ell^{-1}!! table encoded in a wheel of some sort, with the !!100!! entries in order around the rim. There's a “current position” !!p!!, initially !!0!!, and a “current number” !!2^p!!, initially !!1!!. On the same axle as the wheel, mount a gear with exactly 100 teeth. We can easily turn the wheel exactly !!q!! positions by taking a straight bar with !!q!! teeth and using it to turn the gear, which turns the wheel. We easily multiply the current number by !!2!! just by turning the wheel one position clockwise. Multiplying by !!7!! isn't too hard, just turn the wheel !!9!! positions clockwise. We can do this by constructing a short bar with exactly !!9!! teeth and using it to turn the gear. Or maybe we have a meshing gear with !!9!! teeth, on another axle, which we give one full turn. Either way, if the current number was !!5!! before, it's !!35!! after. Multiplying by !!3!! is rather more of a pain, because we have to turn the wheel !!69!! positions, so we need a bar or a meshing gear with 69 teeth. (We could get away with one with only !!31!! teeth, if we could turn the wheel the other way, but that seems like it might be more complicated. Hmm, I suppose it would work to use a meshing gear with 31 teeth that engages a second gear (with any number of teeth) that engages the main gear.) Anyway I took a look to see if there were any better tables do use, and the answer is: maybe! If, instead of a table of !!2^n!!, we use a table of !!26^n!!, then the brass wheel approach performs a little better: $$ \begin{array}{rrrrrrrrrr} {\tiny\color{gray}{0}} & 1, & 26, & 70, & 2, & 52, & 39, & 4, & 3, & 78, & 8, \\ {\tiny\color{gray}{10}}& 6, & 55, & 16, & 12, & 9, & 32, & 24, & 18, & 64, & 48, \\ {\tiny\color{gray}{20}}& 36, & 27, & 96, & 72, & 54, & 91, & 43, & 7, & 81, & 86, \\ {\tiny\color{gray}{30}}& 14, & 61, & 71, & 28, & 21, & 41, & 56, & 42, & 82, & 11, \\ {\tiny\color{gray}{40}}& 84, & 63, & 22, & 67, & 25, & 44, & 33, & 50, & 88, & 66, \\ {\tiny\color{gray}{50}}& 100, & 75, & 31, & 99, & 49, & 62, & 97, & 98, & 23, & 93, \\ {\tiny\color{gray}{60}}& 95, & 46, & 85, & 89, & 92, & 69, & 77, & 83, & 37, & 53, \\ {\tiny\color{gray}{70}}& 65, & 74, & 5, & 29, & 47, & 10, & 58, & 94, & 20, & 15, \\ {\tiny\color{gray}{80}}& 87, & 40, & 30, & 73, & 80, & 60, & 45, & 59, & 19, & 90, \\ {\tiny\color{gray}{90}}& 17, & 38, & 79, & 34, & 76, & 57, & 68, & 51, & 13, & 35\hphantom{,} \\ \end{array} $$ Multiplying by !!2!! is no longer as simple as turning the wheel one notch clockwise; you have to turn it !!3!! positions counterclockwise. But that seems pretty easy. Multiplying by !!3!! is also rather easy: just turn the wheel !!7!! positions. If the table above is !!T_2!!, then the analogue of Ludgate's !!T_1!! table is: $$ \begin{array}{cccccccccc} \tiny\color{gray}{1} & \tiny\color{gray}{2} & \tiny\color{gray}{3} & \tiny\color{gray}{4} & \tiny\color{gray}{5} & \tiny\color{gray}{6} & \tiny\color{gray}{7} & \tiny\color{gray}{8} & \tiny\color{gray}{9} \\ 0 & 3 & 7 & 6 & 72 & 10 & 27 & 9 & 14 \\ \end{array} $$ That is, if you want to compute !!3·6!!, you start with the wheel in position !!0!!, then turn it by !!T_1(3) = 7!! positions, then by !!T_1(6) = 10!!, and now it's at position !!17!!, where the current number is !!18!!. The numbers in the !!T_1!! table are all pretty small, except that to multiply by !!5!! you have to turn by !!72!! positions, which is kinda awful. Still it's only a little worse than in the powers-of-2 version where to multiply by !!3!! you would have to turn the wheel by !!69!! positions. And overall the powers-of-26 table is better: the sum of the !!9!! entries is only !!148!!, which is optimal; the corresponding sum of the entries for the powers-of-2 table is !!216!!. Who knows, it might work, and even if it didn't work well it might be pretty cool. [Other articles in category /math] permanent link Mon, 02 Oct 2023
Irish logarithm forward instead of backward
Yesterday I posted about the so-called “Irish logarithm”, Percy Ludgate's little algorithm for single-digit multiplication. Hacker News user sksksfpuzhpx said:
and referred to Brian Coghlan's aticle “Percy Ludgate's Logarithmic indices”. Whereas I was reverse-engineering Ludgate's tables with a sort of ad-hoc backtracking search, if you do it right you can do it it more easily with a simple greedy search. Uh oh, I thought, I will want to write this up before I move on to the thing I planned to do next, which made it all the more likely that I never would get to the thing I had planned to do next. But Shreevatsa R. came to my rescue and wrote up the Coghlan thing at least as well as I could have myself. Definitely check it out. Thank you, Shreevatsa! [ Update 20231015: A different kind of all-integer logarithm: the discrete logarithm. ] [ Update 20231020: Better explanation of the discrete logarithm. ] [Other articles in category /math] permanent link Sun, 01 Oct 2023The Wikipedia article on “Irish logarithm” presents this rather weird little algorithm, invented by Percy Ludgate. Suppose you want to multiply !!a!! and !!b!!, where both are single-digit numbers !!0≤a,b≤9!!. Normally you would just look it up on a multiplication table, but please bear with me for a bit. To use Ludgate's algorithm you need a different little table: $$ \begin{array}{rl} T_1 = & \begin{array}{cccccccccc} \tiny\color{gray}{0} & \tiny\color{gray}{1} & \tiny\color{gray}{2} & \tiny\color{gray}{3} & \tiny\color{gray}{4} & \tiny\color{gray}{5} & \tiny\color{gray}{6} & \tiny\color{gray}{7} & \tiny\color{gray}{8} & \tiny\color{gray}{9} \\ 50 & 0 & 1 & 7 & 2 & 23 & 8 & 33 & 3 & 14 \\ \end{array} \end{array} $$ and a different bigger one: $$ \begin{array}{rl} T_2 = & % \left( \begin{array}{rrrrrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, & 3, & 6, & 12, \\ {\tiny\color{gray}{10}} & 24, & 48, & 0, & 0, & 9, & 18, & 36, & 72, & 0, & 0, \\ {\tiny\color{gray}{20}} & 0, & 27, & 54, & 5, & 10, & 20, & 40, & 0, & 81, & 0, \\ {\tiny\color{gray}{30}} & 15, & 30, & 0, & 7, & 14, & 28, & 56, & 45, & 0, & 0, \\ {\tiny\color{gray}{40}} & 21, & 42, & 0, & 0, & 0, & 0, & 25, & 63, & 0, & 0, \\ {\tiny\color{gray}{50}} & 0, & 0, & 0, & 0, & 0, & 0, & 35, & 0, & 0, & 0, \\ {\tiny\color{gray}{60}} & 0, & 0, & 0, & 0, & 0, & 0, & 49\hphantom{,} \end{array} % \right) \end{array} $$ I've formatted !!T_2!! in rows for easier reading, but it's really just a zero-indexed list of !!101!! numbers. So for example !!T_2(23)!! is !!5!!. The tiny gray numbers in the margin are not part of the table, they are counting the elements so that it is easy to find element !!23!!. Ludgate's algorithm is simply: $$ ab = T_2(T_1(a) + T_1(b)) $$ Let's see an example. Say we want to multiply !!4×7!!. We first look up !!4!! and !!7!! in !!T_1!!, and get !!2!! and !!33!!, which we add, getting !!35!!. Then !!T_2(35)!! is !!28!!, which is the correct answer. This isn't useful for paper-and-pencil calculation, because it only works for products up to !!9×9!!, and an ordinary multiplication table is easier to use and remember. But Ludgate invented this for use in a mechanical computing engine, for which it is much better-suited. The table lookups are mechanically very easy. They are simple one-dimensional lookups: to find !!T_1(6)!! you just look at entry !!6!! in the !!T_1!! table, which can be implemented as a series of ten metal rods of different lengths, or something like that. Looking things up in a multiplication table is harder because it is two-dimensional. The single addition in Ludgate's algorithm can also be performed mechanically: to add !!T_1(a)!! and !!T_1(b)!!, you have some thingy that slides up by !!T_1(a)!! units, and then by !!T_1(b)!! more, and then wherever it ends up is used to index into !!T_2!! to get the answer. The !!T_2!! table doesn't have to be calculated on the fly, it can be made up ahead of time, and machined from brass or something, and incorporated directly into the machine. (It's tempting to say “hardcoded”.) The tables look a little uncouth at first but it is not hard to figure out what is going on. First off, !!T_1!! is the inverse of !!T_2!! in the sense that $$T_2(T_1(n)) = n\tag{$\color{darkgreen}{\spadesuit}$}$$ whenever !!n!! is in range — that is when !!0≤ n ≤ 9!!. !!T_2!! is more complex. We must construct it so that $$T_2(T_1(a) + T_1(b)) = ab.\tag{$\color{purple}{\clubsuit}$}$$ for all !!a!! and !!b!! of interest, which means that !!0\le a, b\le 9!!. If you look over the table you should see that the entry !!n!! is often followed by !!2n!!. That is, !!T_2(i+1) = 2T_2(i)!!, at least some of the time. In fact, this is true in all the cases we care about, where !!2n = ab!! for some single digits !!a, b!!. The second row could just as well have started with !!24, 48, 96, 192!!, but Ludgate doesn't need the !!96, 192!! entries, so he made them zero, which really means “I don't care”. This will be important later. The algorithm says that if we want to compute !!2n!!, we should compute $$ \begin{align} 2n & = T_2(T_1(2) + T_1(n)) && \text{Because $\color{purple}{\clubsuit}$} \\ & = T_2(1 + T_1(n)) \\ & = 2T_2(T_1(n)) && \text{Because moving one space right doubles the value}\\ & = 2n && \text{Because $\color{darkgreen}{\spadesuit}$} \end{align} $$ when !!0≤n≤9!!. I formatted !!T_2!! in rows of !!10!! because that makes it easy to look up examples like !!T_2(35) = 28!!, and because that's how Wikipedia did it. But this is very misleading, and not just because it makes !!T_2!! appear to be a !!10×10!! table when it's really a vector. !!T_2!! is actually more like a compressed version of a !!7×4×3×3!! table. Let's reformat the table so that the rows have length !!7!! instead of !!10!!: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 5, & 10, & 20, & 40, & 0, \\ {\tiny\color{gray}{28}} & 81, & 0, & 15, & 30, & 0, & 7, & 14, \\ {\tiny\color{gray}{35}} & 28, & 56, & 45, & 0, & 0, & 21, & 42, \\ {\tiny\color{gray}{42}} & 0, & 0, & 0, & 0, & 25, & 63, & 0, \\ {\tiny\color{gray}{49}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{56}} & 35, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{63}} & 0, & 0, & 0, & 49 \\ \end{array} $$ We have already seen that moving one column right usually multiplies the entry by !!2!!. Similarly, moving down by one row is seen to triple the !!T_2!! value — not always, but in all the cases of interest. Since the rows have length !!7!!, moving down one row from !!T_2(i)!! gets you to !!T_2(i+7)!!, and this is why !!T_1(3) = 7!!: to compute !!3n!!, one does: $$ \begin{align} 3n & = T_2(T_1(3) + T_1(n)) && \text{Because $\color{purple}{\clubsuit}$} \\ & = T_2(7 + T_1(n)) \\ & = 3T_2(T_1(n)) && \text{Because moving down triples the value}\\ & = 3n && \text{Because $\color{darkgreen}{\spadesuit}$} \end{align} $$ Now here is where it gets clever. It would be straightforward easy to build !!T_2!! as a stack of !!5×7!! tables, with each layer in the stack having entries quintuple the layer above, like this: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{28}} & 81, & 0, & 0, & 0, & 0, & 0, & 0, \\ \\ {\tiny\color{gray}{35}} & 5, & 10, & 20, & 40, & 0, & 0, & 0, \\ {\tiny\color{gray}{42}} & 15, & 30, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{49}} & 45, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{56}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{63}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ \\ {\tiny\color{gray}{70}} & 25, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{77}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{84}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{91}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{98}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ \end{array} $$ This works, if we make !!T_1(5)!! the correct offset, which is !!7·5 = 35!!. But it wastes space, and the larger !!T_2!! is, the more complicated and expensive is the brass thingy that encodes it. The last six entries of the each layer in the stack are don't-cares, so we can just omit them: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{28}} & 81, \\ \\ {\tiny\color{gray}{29}} & 5, & 10, & 20, & 40, & 0, & 0, & 0, \\ {\tiny\color{gray}{36}} & 15, & 30, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{43}} & 45, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{50}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{57}} & 0, \\ \\ {\tiny\color{gray}{58}} & 25, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{65}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{72}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{79}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{86}} & 0\hphantom{,} \\ \end{array} $$ And to compensate we make !!T_1(5) = 29!! instead of !!35!!: you now move down one layer in the stack by skipping !!29!! entries forward, instead of !!35!!. The table is still missing all the multiples of !!7!!, but we can repeat the process. The previous version of !!T_2!! can now be thought of as a !!29×3!! table, and we can stack another !!29×3!! table below it, with all the entries in the new layer being !!7!! times the original one: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{28}} & 81, \\ \\ {\tiny\color{gray}{29}} & 5, & 10, & 20, & 40, & 0, & 0, & 0, \\ {\tiny\color{gray}{36}} & 15, & 30, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{43}} & 45, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{50}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{57}} & 0, \\ \\ {\tiny\color{gray}{58}} & 25, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{65}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{72}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{79}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{86}} & 0, \\ \\ \hline \\ {\tiny\color{gray}{87}} & 7, & 14, & 28, & 56, & 0, & 0, & 0, \\ {\tiny\color{gray}{94}} & 21, & 42, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{101}} & 63, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{108}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{115}} & 0, \\ \\ {\tiny\color{gray}{116}} & 35, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{123}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{130}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{137}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{144}} & 0, \\ \\ {\tiny\color{gray}{145}} & 0, & 0, & 0, & 0, & 0, & 0, & \ldots \\ \\ \hline \\ {\tiny\color{gray}{174}} & 49\hphantom{,} \\ \end{array} $$ Each layer in the stack has !!29·3 = 87!! entries, so we could take !!T_1(7) = 87!! and it would work, but the last !!28!! entries in every layer are zero, so we can discard those and reduce the layers to !!59!! entries each. $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{28}} & 81, \\ \\ {\tiny\color{gray}{29}} & 5, & 10, & 20, & 40, & 0, & 0, & 0, \\ {\tiny\color{gray}{36}} & 15, & 30, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{43}} & 45, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{50}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{57}} & 0, \\ \\ {\tiny\color{gray}{58}} & 25, \\ \\ \hline \\ {\tiny\color{gray}{59}} & 7, & 14, & 28, & 56, & 0, & 0, & 0, \\ {\tiny\color{gray}{66}} & 21, & 42, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{73}} & 63, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{80}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{87}} & 0, \\ \\ {\tiny\color{gray}{88}} & 35, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{95}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{102}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{109}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{116}} & 0, \\ \\ {\tiny\color{gray}{117}} & 0, \\ \\ \hline \\ {\tiny\color{gray}{118}} & 49\hphantom{,} \\ \end{array} $$ Doing this has reduced the layers from !!87!! to !!59!! elements each, but Ludgate has another trick up his sleeve. The last few numbers in the top layer are !!45, 25,!! and a lot of zeroes. If he could somehow finesse !!45!! and !!25!!, he could trim the top two layers all the way back to only 38 entries each: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{28}} & 81, \\ \\ {\tiny\color{gray}{29}} & 5, & 10, & 20, & 40, & 80, & 0, & 0, \\ {\tiny\color{gray}{36}} & 15, & 30, \\ \\ \hline \\ {\tiny\color{gray}{38}} & 7, & 14, & 28, & 56, & 0, & 0, & 0, \\ {\tiny\color{gray}{45}} & 21, & 42, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{52}} & 63, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{59}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{66}} & 0, \\ \\ {\tiny\color{gray}{67}} & 35, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{74}} & 0, & 0, \\ \hline \\ {\tiny\color{gray}{76}} & 49\hphantom{,} \\ \end{array} $$ We're now missing !!25!! and we need to put it back. Fortunately the place we want to put it is !!T_1(5) + T_1(5) = 29+29 = 58!!, and that slot contains a zero anyway. And similarly we want to put !!45!! at position !!14+29 = 43!!, also empty: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{28}} & 81, \\ \\ {\tiny\color{gray}{29}} & 5, & 10, & 20, & 40, & 0, & 0, & 0, \\ {\tiny\color{gray}{36}} & 15, & 30, \\ \\ \hline \\ {\tiny\color{gray}{38}} & 7, & 14, & 28, & 56, & 0, & \color{purple}{45}, & 0, \\ {\tiny\color{gray}{45}} & 21, & 42, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{52}} & 63, & 0, & 0, & 0, & 0, & 0, & \color{purple}{25}, \\ {\tiny\color{gray}{59}} & 0, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{66}} & 0, \\ \\ {\tiny\color{gray}{67}} & 35, & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{74}} & 0, & 0, \\ \\ \hline \\ {\tiny\color{gray}{76}} & 49\hphantom{,} \\ \end{array} $$ The arithmetic pattern is no longer as obvious, but property !!\color{purple}{\clubsuit}!! still holds We're not done yet! The table still has a lot of zeroes we can squeeze out. If we change !!T_1(5)!! from !!29!! to !!23!!, the !!5,10,20,40!! group will slide backward to just after the !!54!!, and the !!15, 30!! will move to the row below that. We will also have to move the other multiples of !!5!!. The !!5!! itself moved back by six entries, and so did everything after that in the table, including the !!35!! (from position !!32+29!! to !!32+23!!) and the !!45!! (from position !!14+29!! to !!14+23!!) so those are still in the right places. Note that this means that !!7!! has moved from position !!38!! to position !!32!!, so we now have !!T_1(7) = 32!!. But the !!25!! is giving us trouble. It needed to move back twice as far as the others, from !!29+29 = 58!! to !!23+23 = 46!!, and unfortunately it now collides with !!63!! which is currently at position !!7+7+32 = 46!!. $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & \color{purple}{5}, & \color{purple}{10}, & \color{purple}{20}, & \color{purple}{40}, & 0, \\ {\tiny\color{gray}{28}} & 81, & 0, & \color{purple}{15} & \color{purple}{30}, \\ \\ \hline \\ {\tiny\color{gray}{32}} & 7, & 14, & 28, & 56, & 0, & \color{darkgreen}{45}, & 0, \\ {\tiny\color{gray}{39}} & 21, & 42, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{46}} & {63\atop\color{darkred}{¿25?}} & 0, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{53}} & 0, & 0, & \color{darkgreen}{35}, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{60}} & 0, & 0, & 0, & 0, \\ \\ \hline \\ {\tiny\color{gray}{64}} & 49\hphantom{,} \\ \end{array} $$ We need another tweak to fix !!25!!. !!7!! is currently at position !!32!!. We can't move !!7!! any farther back to the left without causing more collisions. But we can move it forward, and if we move it forward by one space, the !!63!! will move up one space also and the collision with !!25!! will be solved. So we insert a zero between !!30!! and !!7!!, which moves up !!7!! from position !!32!! to !!33!!: $$ \begin{array}{rrrrrrrr} {\tiny\color{gray}{0}} & 1, & 2, & 4, & 8, & 16, & 32, & 64, \\ {\tiny\color{gray}{7}} & 3, & 6, & 12, & 24, & 48, & 0, & 0, \\ {\tiny\color{gray}{14}} & 9, & 18, & 36, & 72, & 0, & 0, & 0, \\ {\tiny\color{gray}{21}} & 27, & 54, & 5, & 10, & 20, & 40, & 0, \\ {\tiny\color{gray}{28}} & 81, & 0, & 15 & 30, \\ \\ \hline \\ {\tiny\color{gray}{32}} & \color{purple}{0}, & \color{darkgreen}{7}, & \color{darkgreen}{14}, & \color{darkgreen}{28}, & \color{darkgreen}{56}, & 45, & 0, \\ {\tiny\color{gray}{39}} & 0, & \color{darkgreen}{21}, & \color{darkgreen}{42}, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{46}} & 25,& \color{darkgreen}{63}, & 0, & 0, & 0, & 0, & 0, \\ {\tiny\color{gray}{53}} & 0, & 0, & 0, & \color{darkgreen}{35}, & 0, & 0, & 0, \\ {\tiny\color{gray}{60}} & 0, & 0, & 0, & 0, \\ \\ \hline \\ {\tiny\color{gray}{64}} & \color{purple}{0}, & \color{purple}{0}, & \color{darkgreen}{49}\hphantom{,} \\ \end{array} $$ All the other multiples of !!7!! moved up by one space, but not the non-multiples !!25!! and !!45!!. Also !!49!! had to move up by two, but that's no problem at all, since it was at the end of the table and has all the space it needs. And now we are done! This is exactly Ludgate's table, which has the property that $$T_2(p + 7q + 23r + 33s) = 2^p3^q5^r7^s$$ whenever !!2^p3^q5^r7^s = ab!! for some !!0≤a,b≤9!!. Moving right by one space multiplies the !!T_2!! entry by !!2!!, at least for the entries we care about. Moving right by seven spaces multiplies the entry by !!3!!. To multiply by !!5!! or !!7!! we move right by or !!23!! or by !!33!!, respectively. These are exactly the values in the !!T_1!! table: $$\begin{align} T_1(2) & = 1\\ T_1(3) & = 7\\ T_1(5) & = 23\\ T_1(7) & = 33 \end{align}$$ The rest of the !!T_1!! table can be obtained by remembering !!\color{darkgreen}{\spadesuit}!!, that !!T_2(T_1(n)) = n!!, so for example !!T_1(6) = 8!! because !!T_2(8) = 6!!. Or we can get !!T_1(6)!! by multiplication, using !!\color{purple}{\clubsuit}!!: multiplying by !!6!! is the same as multiplying by !!2!! and then by !!3!!, which means you move right by !!1!! and then by !!7!!, for a total of !!8!!. Here's !!T_1!! again for reference: $$ \begin{array}{rl} T_1 = & \begin{array}{cccccccccc} 0 & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 \\ \hline 50 & 0 & 1 & 7 & 2 & 23 & 8 & 33 & 3 & 14 \\ \end{array} \end{array} $$ (Actually I left out a detail: !!T_1(0) = 50!!. Ludgate wants !!T_2(T_1(0) + T_1(b)) = 0!! for all !!b!!. So we need !!T_2(T_1(0) + k) = 0!! for each !!k!! in !!T_1!!. !!T_1(0) = 50!! is the smallest value that works. This is rather painful, because it means that the !!66!!-item table aboveis not sufficient. Ludgate has to extend !!T_2!! all the way out to !!101!! items in order to handle the seemingly trivial case of !!0\cdot 0 = T_2(50 + 50)!!. But the last 35 entries are all zeroes, so the the brass widget probably doesn't have to be too much more complicated.) Wasn't that fun? A sort of mathematical engineering or a kind that has not been really useful for at least fifty years. But actually that was not what I planned to write about! (Did you guess that was coming?) I thought I was going to write this bit as a brief introduction to something else, but the brief introduction turned out to be 2500 words and a dozen complicated tables. We can only hope that part 2 is forthcoming. I promise nothing. [ Update 20231002: Rather than the ad-hoc backtracking approach I described here, one can construct !!T_1!! and !!T_2!! in a simpler and more direct way. Shreevatsa R. explains. ] [ Update 20231015: Part 2 has arrived! It discusses a different kind of all-integer logarithm called the “discrete” logarithm. ] [ Update 20231020: I think this is a clearer explanation of the discrete logarithm. Shorter, anyway. ] [Other articles in category /math] permanent link |