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. (2005-01-07)
