Question Number 2931 by Filup last updated on 30/Nov/15
$$\mathrm{What}\:\mathrm{is}\:\mathrm{the}\:\mathrm{double}\:\mathrm{factorial}\:\mathrm{function}? \\ $$$${i}.{e}.\:\:\:\:\:{x}!! \\ $$
Answered by 123456 last updated on 30/Nov/15
$${x}!!={x}\left({x}−\mathrm{2}\right)!!,{x}\in\mathbb{N} \\ $$$$\mathrm{0}!!=\mathrm{1} \\ $$$$\mathrm{1}!!=\mathrm{1} \\ $$$$\mathrm{6}!!=\mathrm{6}×\mathrm{4}×\mathrm{2}×\mathrm{0}!!=\mathrm{6}×\mathrm{4}×\mathrm{2}=\mathrm{48} \\ $$$$\mathrm{5}!!=\mathrm{5}×\mathrm{3}×\mathrm{1}=\mathrm{15} \\ $$