Factorial
The product of a given integer and
all smaller
positive integers. The factorial
of n is written n!
and is read aloud "n factorial".
Note: By definition,
0! = 1.
Formula: 
n! = n·(n – 1)·(n – 2) · · · 3·2·1 
Example: 
6! = 6·5·4·3·2·1 = 720 
See
also
Permutation
formula, combination
formula,
binomial coefficient
