Factorial

The product of all positive integers from 1 up to n, written n!.

By convention 0! = 1, and n! = n × (n − 1)!. Factorials count the number of orderings of n distinct objects and appear in the formulas for permutations and combinations, the Taylor series and many probability distributions.

n! = 1 · 2 · 3 · ⋯ · n