Excel FACTDOUBLE Function

The Excel FACTDOUBLE FUNCTION is used for calculating double factoring. A double factorial is calculated differently for even and odd numbers. The Double Factorial of a number, N, is given by the following:

If N is even: N * (N-2) * (N-4) * … * 4 * 2  If N is odd: N * (N-2) * (N-4) * … * 3 * 1

Purpose of Excel FACTDOUBLE Function

To calculate the double factorial.

Return value

FACTDOUBLE function returns the double factorial of a number. 

Syntax

= FACTDOUBLE(number)

Arguments

Usage notes

• If the number is in decimal then excel truncates it to integer and then return the value.
• If the supplied number is < 0,#NUM! error will occur.
• If the supplied number is non-numeric,#VALUE! error will occur.
• The FACTDOUBLE is a mathematical function and helps in finding the double factorial.
• The double factorial, symbolized by two exclamation marks (!!), is a quantity defined for all integers from -1 up to n that have the same parity (odd or even) as n.
• For an even integer n, the double factorial is the product of all even integers less than or equal to n but greater than or equal to 2. For an odd integer p, the double factorial is the product of all odd integers less than or equal to p and greater than or equal to 1.
• The double factorial values of 0 and -1 are defined as equal to 1. Double factorial values for integers less than -1 are not defined.
• Arithmetically the formulas for double factorial are as follows.
• If n is even, then

n !! = n ( n – 2)( n – 4)( n – 6) … (4)(2)

• If p is odd, then

p !! = p ( p – 2)( p – 4)( p – 6) … (3)(1)

• If q = 0 or q = -1, then q !! = 1
• The double factorial is used in multiple areas like statistics, calculus, and physics.

Availability

Since 2007 for Windows.
Since 2011 for Mac.