N! = N * (N-1) * (N-2) * ... * 1.The factorial of zero is one because it is an empty product. Factorial can be defined recursively as
0! = 1 N! = N * (N-1)! , N > 0The gamma function is the equivalent for real numbers. For example, the number of ways of shuffling 52 playing cards is 52! or nearly 10^68. 52 Factorial.
Last updated: 2012-11-23