"!" It's called FACTORIAL ...
In mathematics, the factorial of a non-negative integer n, denoted by n!, is theproduct of all positive integers less than or equal to n. For example,
The value of 0! is 1, according to the convention for an empty product.
The factorial operation is encountered in many areas of mathematics, notably incombinatorics, algebra, and mathematical analysis. Its most basic occurrence is the fact that there are n! ways to arrange n distinct objects into a sequence (i.e., permutations of the set of object