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
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $518$