FactDouble

1 Description
2 Syntax
3 Parameters
4 Return
5 Example
6 See Also

Description

This function is used to calculate the double factorial of a non-negative integer n, denoted by:

$n!! = \begin{cases} n*(n-2)...5*3*1, & \mbox{if }n \mbox{ is odd} \\ n*(n-2)...6*4*2, & \mbox{if }n \mbox{ is even} \\ 1, & n=0 \end{cases}$

Please note that, to avoid overflow, the biggest integer can be specified is 299, if bigger than 299, this function will return missing value directly.

Syntax

double FactDouble(int n)

Parameters

The non-negative number for calculating double factorial.

Return

Return the double factorial of the specified number.

Example

factdouble(5) = ;  // 5*3*1=15
factdouble(6) = ;  // 6*4*2=48
factdouble(0) = ;  // 1
factdouble(1) = ;  // 1