What is the "!" for?

"!"  means  "factorial"

 

0!  =  1

1!  =  1

2!  =  2x1  =  2

3!  =  3x2x1  =  6

4!  =  4x3x2x1  =  24

5!  =  5x4x3x2x1  =  120

 

The fact that  0! = 1  might seem strange, but it's necessary for some formulas.

 

If  n > 1,  then  n!  =  n·(n - 1)·(n - 2)·(n - )·...·3·2·1