Definition:Double Factorial

Definition
Let $n \in \Z_{\ge 0}$ be a positive integer.

The double factorial of $n$ is defined inductively as:
 * $n!! = \begin{cases}

1 & : n \in \left\{ {0, 1}\right\} \\ n \left({n - 2}\right)!! & : n > 1 \end{cases}$