# factorial

The factorial of a positive integer is the product of all the positive integers less than or equal to it. The
factorial of *n* is usually denoted *n*! The
first few are 1!=1, 2!=2, 3!=6, 4!=24, 5!=120, and 6!=720. We also define 0! to be 1 (in mathematics, an
empty product has the value one).

The number *n*! is the number of permutations of *n* items (how many ways *n* distinct items can be arranged in a row). The number of ways you can pick
*r* or *n* identical items is the binomial
coefficient *n*!/(*r*!(*n*-*r*)!).

**See Also:** FactorialPrime, StirlingsFormula, WilsonsTheorem

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