The mathematical function that takes a natural number, N, and returns the product of N and all smaller positive integers. This is written

 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 > 0

The 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

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