Double factorial

The double factorial of a non-negative integer $n$ , denoted by $n factorial factorial$ , is defined as the product of all integers between 1 and $n$ that have the same parity as $n$ :

$n factorial factorial space equals open curly brackets table attributes columnalign left end attributes row cell n times left parenthesis n minus 2 right parenthesis times left parenthesis n minus 4 right parenthesis space midline horizontal ellipsis space 4 space times space 2 space text if n is even end text space end cell row cell n times left parenthesis n minus 2 right parenthesis times left parenthesis n minus 4 right parenthesis midline horizontal ellipsis 3 times 1 text if n is odd end text end cell end table close$

$0 factorial factorial equals 1$ by convention.

Integer!!

Description

Integer!!

Given an integer between 1 and 65535, returns $n factorial factorial$ .

Factorial

Double factorial

Syntax

Description

Integer!!

Related functions