Multifactorial

The multifactorial is a generalization of the double factorial, defining:


 * $$n!! = n(n - 2)(n - 4)(n - 6)\ldots$$
 * $$n!!! = n(n - 3)(n - 6)(n - 9)\ldots$$
 * $$n!!!! = n(n - 4)(n - 8)(n - 12)\ldots$$

and so forth.

It is very important to note that multifactorials should not be interpreted as nested factorials, e.g. $$n!! \not= (n!)!$$ and $$n!!! \not= ((n!)!)!$$.