The mathematical constant PI is the number you get when you divide a circle’s circumference by its diameter. It can't be expressed exactly as a ratio of whole numbers. Starting with 3.14159265358979323846. . ., the decimal digits of PI go on forever. Statistically, the digits appear to behave like a sequence of random numbers.
Researchers have expended a great deal of effort computing as many of those digits as computer technology and mathematical methods allow. Last year PI was calculater to 206,158,430,000 decimal digits.
