Factorial

The factorial of a positive integer, n, denoted "n!", is the product of the positive integers less than or equal to n. E.g.,

: 5! = 5 * 4 * 3 * 2 * 1 = 120

: 10! = 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1 = 3628800

It is convenient for many applications to define 0! as 1. There is also the related gamma function, which is defined over the domain of non-negative real numbers in such a way that gamma(x) = x! when x is a positive integer.