In more detail
5! = 5 × 4 × 3 × 2 × 1 = 120, and by convention 0! = 1. Factorials grow very fast: 20! is already about 2.4 × 10¹⁸.
Glossary
Math & Statistics
Factorial
The factorial of n, written n!, is the product of all positive whole numbers up to n, and counts the ways to order n distinct items.
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See also
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Glossary
Permutation
A permutation is an ordered arrangement of items; `P(n, k) = n! / (n − k)!` counts the ways to arrange k items chosen from n.
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Glossary
Combination
A combination is a selection of items where order does not matter; `C(n, k) = n! / (k! (n − k)!)` counts the ways to choose k from n.