**Factorial**

The product of a given integer and all smaller positive integers. The factorial of *n* is written *n*! and is read aloud "*n* factorial".

Note: By definition, 0! = 1.

Formula: | n! = n·(n – 1)·(n – 2) · · · 3·2·1 |

Example: | 6! = 6·5·4·3·2·1 = 720 |

**See also**

Permutation formula, combination formula, binomial coefficient

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