(C) 2012-2013 David Liao (lookatphysics.com)
How many ways can we arrange n distinct objects in n slots? The answer is n (n - 1) (n - 2) . . . 3 * 2 * 1. Because this kind of calculation appears often in the study of probabilities, we give it a symbol called the factorial: n! = n (n - 1) (n -2) . . . 3 * 2 * 1.