Pi
The number pi (denoted with the lowercase Greek letter π) is a mathematical constant which occurs in many areas of mathematics and physics. It is also known as Archimedes' constant or Ludolph's number and is equal to the ratio of a circle's circumference to its diameter in Euclidean geometry. Alternatively, π can be defined as the area of a circle of radius 1, or as the smallest positive number x for which sin(x) = 0.The value of π to the first sixtyfour digits is:
3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37510 58209 74944 592...
Table of contents 
2 Formulae involving π 3 Computing the value of pi 4 Open questions 5 The nature of π 6 Pi culture 7 Related articles 8 External Links 
Properties
The number π is an irrational number: that is, it cannot be written as the ratio of two integers. This was proved in 1761 by Johann Heinrich Lambert. In fact, the number is transcendental, as was proved by Ferdinand Lindemann in 1882. This means that there is no polynomial with integer (or rational) coefficients of which π is a root. As a consequence, it is impossible to express π using only a finite number of integers, fractions and their roots.
This result establishes the impossibility of squaring the circle: it is impossible to construct, using ruler and compass alone, a square whose area is equal to the area of a given circle. The reason is that the coordinates of all points that can be constructed with ruler and compass are special algebraic numbers.
Formulae involving π
 Circumference of circle of radius r: C = 2 π r
 Area of circle of radius r: A = π r^{2}
 Area of ellipse with semiaxes a and b: A = π ab
 Volume of sphere of radius r: V = (4/3) π r^{3}
 Surface area of sphere of radius r: A = 4 π r^{2}
 Angles: 180 degrees is equivalent to π radians
 (Leibniz' formula)
 (Wallis' product)
 (Euler)


 (Stirling's formula)
 (Euler's identity, also called "The most remarkable formula in the world")
 The probability that two randomly chosen integers are relatively prime is 6/π^{2}.
 The probability that a randomly chosen integer is squarefree is 6/π^{2}.
 The average number of ways to write a positive integer as the sum of two perfect squares (order matters) is π/4.

 for almost every x_{0} in [0, 1] where the x_{i} are iterates of the Logistic map for r=4.
 (Heisenberg's uncertainty principle)
 (Einstein's field equation of general relativity)
 (The probability density function for the normal distribution.)
Computing the value of pi
Due to the transcendental nature of π there are no nice closed expressions for π. Therefore calculations have to use approximations to the number. For many purposes, 3.14 or 22/7 is close enough, although engineers often use 3.1416 (5 significant figures) or 3.14159 (6 significant figures) for more accuracy. 355/113, with two each of the first 3 odd digits, is a simple and easily memorised fraction which is good for 7 significant figures.
An Egyptian scribe called Ahmes is the source of the oldest known text to give an approximate value for π. The Rhind Papyrus dates from the 17th century BC and describes the value in such a way that the result obtained comes out to 256 divided by 81 or 3.160.
Liu Hui computed π to 3.141014 (incorrect in the fourth decimal digit) in 263 A.D. and suggested that 3.14 was a good approximation.
Ludolph van Ceulen (circa 1600) computed the first 35 decimals. He was so proud of this accomplishment that he had them inscribed on his tombstone.
The Slovene mathematician Jurij Vega in 1789 calculated the first 140 decimal places for π of which 137 were correct and held the world record for over 50 years at that time. He improved John Machin's formula from 1706 and his method is still mentioned today.
None of the formulas given above can serve as an efficient way of approximating π. For fast calculations, one may use formulas like Machin's:
 (5+i)^{4} · (239 + i) = 114244114244 i.
Extremely large numbers of digits of π are typically computed with the GaussLegendre algorithm and Borwein's algorithm; the SalaminBrent algorithm which was invented in 1976 has also been used in the past.
The first one million digits of π and 1/π are available from Project Gutenberg (see external links below). The current record (December 2002) stands at 1,241,100,000,000 digits, which were computed in September 2002 on a 64node Hitachi supercomputer with 1 Terabyte of main memory, which carries out 2 trillion operations per second, nearly twice as many as the computer used for the previous record (206 billion digits); the following Machinlike formulas were used for this:
These approximations have so many digits that they are no longer of any practical use, except for testing new supercomputers.
In 1996 David H. Bailey, together with Peter Borwein and Simon Plouffe, discovered a new formula for π as an infinite series:
Other formulas that have been used to compute pi include:
 (Ramanujan)
 (David Chudnovsky and Gregory Chudnovsky)
Open questions
The most pressing open question about π is whether it is normal, i.e. whether any digit block occurs in the expansion of π just as often as one would statistically expect if the digits had been produced completely randomly. This should be true in any base, not just in base 10.
It isn't even known which of the digits 0,...,9 occur infinitely often in the decimal expansion of π.
Bailey and Crandall showed in 2000 that the existence of the above mentioned BaileyBorweinPlouffe formula and similar formulas imply that the normality in base 2 of π and various other constants can be reduced to a plausible conjecture of chaos theory. See Bailey's above mentioned web site for details.
The nature of π
In nonEuclidean geometry the sum of the angles of a triangle may be more or less than π, and the ratio of a circle's circumference to its diameter may also differ from π. This doesn't change the value of π, but does affect many formulae in which it appears. So in particular, the shape of the universe does not affect the value of π at all: it is a mathematical constant, not a physical value.
Pi culture
There is an entire field of humorous yet serious study that involves the use of mnemonic techniques to remember the digits of π, which is known as Piphilology. This is obviously a play on Pi itself and the linguistic field of philology.
The most common mnemonic technique is to memorise a sentence in which the number of letters in each word in turn is equal to the corresponding digit of π. The most famous example of this is from Isaac Asimov:
 How I want a drink, alcoholic of course, after the heavy lectures involving quantum mechanics!
 How I wish I could recollect pi easily today!
There are piphilologists who have written poems which encode 100s of digits. This is an example of constrained writing.
March 14 marks Pi Day which is celebrated by many lovers of π. On July 22, Pi Approximation Day is celebrated (22/7 is a popular approximation of π).
Related articles
 Greek letter Pi
 Pi (movie)
 Calculus
 Geometry
 Trigonometric function
 Pi through experiment
 pi meson
 Proof that π is transcendental
 A simple proof that 22/7 exceeds &pi
External Links
 Wikisource  Pi to 1,000 Places
 Wikisource  Pi to 10,000 Places
 Wikisource  Pi to 100,000 Places
 Project Gutenberg EText containing a million digits of Pi
 Statistics about the first 1.2 trillion digits of Pi
 J J O'Connor and E F Robertson: A history of Pi. Mac Tutor project
 Andreas P. Hatzipolakis: PiPhilology. A site with hundreds of examples of π mnemonics
 From the Wolfram Mathematics site lots of formulae for π
 Finding the value of Pi
 PlanetMath: Pi
 The pihacks Yahoo! Group