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##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
(n! means n × (n-1) × ... × 1)
Factorial of n, written n!, is the product of all positive integers from 1 to n.
5! = 5 × 4 × 3 × 2 × 1 = 120
Special cases: 0! = 1 (by definition, this makes formulas work), 1! = 1.
Factorials grow fast. 10! = 3,628,800. This is the number of ways to arrange 10 distinct items.
