Pi

Pi, sixteenth letter of the Greek alphabet, symbol p (lower case). The Greek alphabet dates from 1000-900 bc. The ancient Greeks also used the letter to represent the number 80.

Today p is used in mathematics as the symbol for the ratio of the circumference of a circle to its diameter. The Greek mathematician Archimedes correctly asserted that the value was between 3 10/70 and 3 10/71. The symbol p for the ratio was first used in 1706 by the English mathematician William Jones, but it became popular only after its adoption by the Swiss mathematician Leonhard Euler in 1737. In 1882 the German mathematician Ferdinand Lindemann proved that p is a transcendental number—that is, it is not the root of any polynomial equation with rational coefficients. Consequently, Lindemann was able to demonstrate that it is impossible to square the circle algebraically or by use of a ruler and compass.

Although pi is an irrational number, that is, it has an infinite number of decimal places, it can be calculated as accurately as is desired using the series,

Computers have calculated pi to 100 million decimal places, although this has no practical value.