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The factorial of a number n written n! is the product of n along with all the natural numbers smaller than n (not including zero !!!). Therefore, n! = n x (n - 1) x (n - 2) ... 3 x 2 x 1 For example, 5! = 5 x 4 x 3 x 2 x 1 = 120 Here are some facts about the factorial: * Stirling's formula gives an approximation of the factorial: ** EQN:n!{\approx}n^ne^{-n}\sqrt{2\pi{n}} * There is also a version of the factorial that is defined for (some) non-integers: ** EQN:n!=\int_0^\infty{e^{-x}}x^n.dx See the Gamma Function * Wilson's Theorem says that EQN:(n-1)!\equiv-1\quad(mod\quad{n}) if and only if n is a prime number. For information about EQN:\quad(mod\quad{n}) see Modulo Arithmetic ---- Enrichment task * How many zeros are there at the end of 2009! * What is Zero Factorial? * Use Wilson's theorem to test whether 6 and 7 are prime numbers. ---- * http://en.wikipedia.org/wiki/Factorial * http://www.google.com/search?q=Factorial