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Rockmore: Happy Pi Day!

03/14/12 7:55AM By Dan Rockmore
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(Host) Commentator Dan Rockmore is a professor of mathematics and computer science at Dartmouth College - and today he's in a festive mood!

(Rockmore) Happy Pi Day One and All, or, maybe I should say Three and a Little...

Yes, today is the day that we celebrate everybody's favorite little number whose decimal expansion starts Three-One-Four, short for three and one tenth and 4 one hundredths. Now I could go on, in fact, I could go on forever, because pi's entire decimal expansion goes on forever, never repeating, and seemingly without any patterns to be found anywhere in an endless stream of numbers. The fact that pi's decimal expansion never repeats means that pi can't be written as a simple fraction or ratio of whole numbers. This is why we call pi an irrational number.

The digits roll out, going on and on, and if you're willing to resort to a machine, then the current record for computing digits of pi on your home computer is 5 trillion digits!
Now, even though there may be nothing rational about it, some people devote quite a bit of time and energy to memorizing as much of this random sequence of digits as possible. The current world record is held by Lu Chao of China, who recited the first 67,890 digits of pi in just over 24 hours!

One of the best known tricks for memorizing digits of pi is to use a mnemonic in which a sequence of words encode the digits in the lengths of the words. For example, "Man I wish I liked spellings, in dreamy times and comfy dwelling."  So that works out to 3 for "man", 1 for "I" and so on and so on for the first twelve digits of pi! How's that for a little e-pi-gram!

All that memorizing might make you pretty hungry, so off to the kitchen you go and in fact, you're in a pretty good place to actually make pi! Here's what I mean: roll out a nice big slab of dough, say a little over a foot long on each side. And then take a piece of string that is exactly half a foot long.  Put a pin through one end of it and stick that pin into the center of the dough. Then pull the string tight and revolve it around the pin to trace out a circle. The diameter of that circle will be one foot and the distance around or the circumference will be exactly pi feet! Add some cheese and tomato sauce and a little garlic you'll really have a pizza pi!

This little cooking exercise gets at the universal nature of pi: take any circle anywhere, your dinner dishes, the hubcap on your car, or our perfectly crafted pizza... pie, then the ratio of its circumference to its diameter is always the number pi! You might say that pi is all around us. And that's enough to make even the most irreverent of us...  pious.
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