The multifactorial is a generalisation of the factorial. The double factorial is written `x`!! and is defined to be (`x`)*(`x`-2)*(`x`-4)*…. If x is even, then the last term is 2; if x is odd, then the last term is 1. The triple factorial is written `x`!!! and is defined to be (`x`)*(`x`-3)*(`x`-6)*… (again, until you run out of positve integers).

In general, the `k`-multifactorial of `x` can be written (with Eindhoven notation) as: (* : `i` is an integer and 0 ≤ `i` < ⌊`x`/`k`⌋ : `x` - `i``k`).

The product of `k` consecutive `k`-multifactorials is the factorial of the largest number.

Source: Wikipedia, http://www.wikipedia.org/wiki/Factorial