*pi*, the ratio between a circle's circumference and its diameter. (Assuming we're talking about Euclidean geometry. Which we are.)

Outside the US, some people like to celebrate the 22nd of July instead -- because in everyone else's notation, that looks like '22/7', and 22/7 is a very good approximation for π.

A few incidental bits of trivia about π:

- π is an
*irrational*number. This means it cannot be expressed as a ratio of two integers. Put another way: the circumference and diameter of a circle are*incommensurable*. Or put yet another way: if you write out π in decimal notation, it will never ever repeat. - π is also a
*transcendental*number. This means that it cannot be expressed as the solution to a polynomial equation with integer coefficients. That is: given an equation*a*x^{n}+*b*x^{n-1}+*c*x^{n-2}... +*z*x^{0}= 0, where*a*,*b*,*c*... are integers, a transcendental number is any number that x cannot be. - π is widely suspected to be a
*normal*number. This is not known for sure. A normal number is, roughly, one whose decimal expansion shows no patterns, where every digit is equally likely, and every finite sequence of digits is equally likely. This sounds pretty limiting; at present no one really has any idea how to prove that a given number is normal with 100% certainty. But if you look at it statistically, almost all real numbers are irrational; almost all irrational numbers are transcendental; and almost all transcendental numbers are normal. If you randomly pick a number on the real number line, the probability that it will be normal is 1. So, pretty good odds that π is normal, then. - If you know π to 39 decimal places -- 3.14159 26535 89793 23846 26433 83279 50288 4197 -- then you know it precisely enough to measure a circle the size of the observable universe to a precision finer than the width of an atom.

OK, here's one myth.

The first recorded algorithm for rigorously calculating the value of π was a geometrical approach using polygons, devised around 250 BC by the Greek mathematician Archimedes.It is true that Archimedes used this method to calculate π. But it is

-- Wikipedia, 'Pi'

*not*true that he devised the method. He just did it with a bit more precision than anyone had done previously. He made an advance, but it was an incremental advance, not something revolutionary. You can find Archimedes' full exposition in a surviving work, the

*Measurement of the circle*.

The 'exhaustion method'. If you draw regular polygons inside and outside the circle, then the more sides the polygons have, the more closely they approximate the actual circumference of the circle. (source: Wikimedia.org) |

The illustration shows how the exhaustion method works. Using 96-sided polygons, Archimedes narrowed down the value of π to between 3 10/71 and 3 10/70 -- that is, he found that π is somewhere between 3.1408... and 3.1429...

But the method was already in use 200 years earlier. Antiphon of Athens (ca. 480-411 BCE), Bryson of Heraclea Pontica (ca. 400 to after 340 BCE), and Eudoxus of Cnidus (ca. 391-338 BCE) had all used a similar method to calculate π long before Archimedes came along.

Antiphon, the earliest of the bunch, only used inscribed polygons -- that is, he only drew one shape, inside the circle, but not outside. As a result he only had one bound for the value of π. We don't know much about Eudoxus' effort. We do know that Bryson guessed (wrongly) that π would be given by the arithmetic mean of the inner and outer perimeters; and that Antiphon and Bryson were working on the area of the circle, not its perimeter. It was Eudoxus who showed that the area and perimeter were linked by the square of the radius.

The

*New Pauly*encyclopaedia reports (subscription needed) that it was Eudoxus, not Archimedes, whose influence led to the widespread use of exhaustion for all problems involving infinitesimals. Archimedes' work on π was just a refinement of Eudoxus.

Here's another myth, from a

*Time*article published on 'Pi Day' this year.

However, not too many generations after [Archimedes'] lifetime, the world experienced a "real decline in math," according to John Conway, mathematics professor emeritus at Princeton University who once won the school's Pi Day pie-eating contest. "Math and science in general went into a great decline from roughly the year zero to the year 1,000, and then the Arabs developed lots of math after that, like trigonometry."Oooh, do I detect a note of a renowned world expert saying something a little bit silly about another field? I think I do!

What, no love for all those Alexandrian mathematicians of the Roman era? No love for Heron, whose

*Metrica*has recently been published in a new French translation? Or Menelaus, whose work on spherical geometry was foundational for Arabic, Hebrew, and western astronomers for over a thousand years? Not to mention Diophantus, whose work laid down the parameters for the modern study of polynomials, and whose notation foreshadowed the development of algebra?

And then there are many other figures who are, admittedly, lesser, but still made important contributions: Sporus of Poros, who demolished earlier mathematicians' reliance on a curve called the 'quadratrix' in problems to do with squaring the circle; Ptolemy, who in the early 100s CE gained the world record for closest approximation of π (3 + 8/60 + 30/3600, = 3.141666...); and commentators like Pappus, Theon, Hypatia, Proclus, and Eutocius, whose work on Euclid, Ptolemy, and Archimedes were colossally useful in helping later mathematicians to understand the impenetrable language of their predecessors.

I guess it is fair to speak of a decline in Greek mathematics -- but Archimedes was not the be-all and end-all. If there was a decline, it was after the time of Diophantus. Archimedes has a curiously inflated reputation. I suppose that's because there are lots of

*good stories*about him: the story of his death ray; the dramatic story of his death that we find in Plutarch and Valerius Maximus; the story of the bathtub and the running around naked shouting '

*Hēurēka*!'; and the story of the Cattle Problem, whose solution involves a number with over 200,000 digits (ca. 7.76 × 10

^{206544}). Everything about him

*sounds*tremendously exciting. But hey, let's not forget later giants like Hipparchus, Menelaus, and Diophantus, all right?