The factorial operation says to multiply the designated number by every positive integer smaller than that number.
n! = n (n – 1) (n – 2) 3 2 1
When using the operation in the formulas for the number of permutations or combinations of n things taken k at a time, factorial values need to be inserted into the numerator and denominator of the fraction. The first sixteen factorial values are given here. And, by definition, 0! = 1.