The Universe of Disco

Tue, 17 Oct 2006

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:

I asked for an explanantion. Thanks to the Wonders of the Internet, an explanantion was not long in coming. In short, the artist screwed up. He used a table of digits in the following format:


  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

The digits are meant to be read across, so that after the "535" in the first group, the next digits are "8979..." on the same line. But instead, the artist has skipped down to the first group in the second line, "8214...". Whoops.

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.

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