Counting when order matters: The factorial function occurs often in probability and statistics. For a non-negative integer n, the factorial is denoted n! (which is read “n factorial”) and is defined...

Counting when order matters: The factorial function occurs often in probability and statistics. For a non-negative integer n, the factorial is denoted n! (which is read “n factorial”) and is defined as follows: First, 0! is defined to be 1. Next, if n is 1 or larger, then n!

means n(n − 1)(n − 2)··· 3 × 2 × 1. Thus 3! = 3 × 2 × 1 =