Double factorial
= Double factorial
{title2=$n!!$}
{wiki}
The double factorial multiplies the positive integers of the same parity as $n$ up to $n$: $(2m-1)!!=1\cdot3\cdots(2m-1)$ and $(2m)!!=2\cdot4\cdots2m$. Set $(-1)!!=0!!=1$. In particular, $(2m-1)!!=(2m)!/(2^m m!)$, the number of pairings of $2m$ labelled objects.