How is the symbol ! used in mathematics? I was told of its use before, but I seem to have forgotten the definition.

when you have some positive integer n, n! = n * (n-1) * (n-2) * ..... 3*2*1

also, 0! = 1 I forgot why but they never told me that in high school so it's worth noting.

6! = 1*2*3*4*5*6

is that good enough :)

Guest......here's an informal proof of why 0!   = 1

Note :

[n] !  = [n + 1 ] !  / [ n + 1]   ......so......

2!  =   3! / 3     =  2

1!  =   2! / 2    =  1

0!  = 1!  /  1     = 1