Double factorial

The double factorial is a version on the factorial, defined as $$n!! = n \cdot (n - 2) \cdot (n - 4) \cdot \ldots$$. In other words, it is made by multiplying all the odd numbers up to n if n is odd, or multiplying all the even numbers up to n if n is even. The first few values of $$n!!$$ for $$n = 0$$, 1, 2, 3, ... are 1, 1, 2, 3, 8, 15, 48, 105, 384, 945, 3840, 10395, 46080, 135135, and 645120.