Definition:Double Factorial

Definition

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

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

$n!! = \begin {cases} 1 & : n \in \set {0, 1} \\ n \paren {n - 2}!! & : n > 1 \end {cases}$

Also see

• Results about double factorials can be found here.

Sources

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