Factorial

Returns the factorial of a number.

Integer!

Given an integer n in the interval left square bracket 1 comma 65535 right square bracket, returns the factorial, usually denoted by n factorial, defined as the product of all integers between 1 and n: n factorial equals 1 times 2 times 3 midline horizontal ellipsis left parenthesis n minus 1 right parenthesis times n. By convention, 0 factorial equals 1.

factorial(Integer, Integer)

Given two integers n and k, returns the following computation: n times left parenthesis n minus k right parenthesis times left parenthesis n minus 2 k right parenthesis midline horizontal ellipsis a comma text  with  end text 1 less or equal than a less than k