What is PI ? How many digits of PI ?

PI is a very old number and the 16th letter of the Greek alphabet. Egyptians and Babylonians discovered PI nearly 4000 years ago. The first calculation of PI was done by an Ancient Greek mathematician, physicist, inventor, astronomer and engineer Archimedes of Syracuse (287-212 BC). In 1706, William Jones the Welch mathematician first used a symbol to represent the ratio of the circumference of a circle to its diameter, approximately equal to 3.14159. He was a close friend of Sir Isaac Newton and Sir Edmund Halley. PI is a famous irrational number. There has 5 trillion digits of PI !