Factorial number

Factorial numbers are numbers of the form n!, where n! is defined recursively as n*(n-1)! and 0! is defined as 1.

One interesting property of factorials is that n! can be written as n*(n-1)*(n-2)*...*3*2*1.

The first few factorial numbers are:

0! = 1

1! = 1

2! = 2*1 = 2

3! = 3*2! = 6

4! = 4*3! = 24

5! = 5*4! = 120

6! = 6*5! = 720