# Factorial

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.

 n n! n n! 1 1 9 362,880 2 2 10 3,628,800 3 6 11 39,916,800 4 24 12 479,001,600 5 120 13 6,227,020,800 6 720 14 87,178,291,200 7 5,040 15 1,307,674,368,000 8 40,320 16 20,922,789,888,000