!!)
Factorial2[n]n!!The double factorial or semifactorial of a number n, is the product of all the integers from 1 up to n that have the same parity (odd or even) as n.
5!!
Factorial2[-3]
Factorial2 accepts Integers, Rationals, Reals, or Complex Numbers:
I!! + 1
Irrationals can be handled by using numeric approximation:
N[Pi!!, 6]