The factorial of a number, N, is equal to the product of all positive integers less than and equal to N.
N! = N x (N-1) x (N-2) x … x 3 x 2 x1
The factorial of 0 is equal to 1.
Symbolically, 0! = 1
According to the convention of empty product, the result of multiplying no factors is a nullary product. It means that the convention is equal to the multiplicative identity.