Definition:Double Factorial

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Definition

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


The double factorial of $n$ is defined inductively as:

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


Sources