FACTDOUBLE function

The FACTDOUBLE function in Excel is used to calculate the double factorial of a number. The double factorial of a number n is the product of all the integers from n down to 1 that have the same parity (odd or even) as n. In other words, it’s the product of every second integer.

Syntax

=FACTDOUBLE(number)

Parameters

• number: A positive integer or zero for which you want to find the double factorial. If the number is negative or non-integer, the function will return an error.

Return Value

The function returns the double factorial of the given number.

Example 1: Double Factorial of 6

To calculate 6!! (double factorial of 6):

=FACTDOUBLE(6)

Result: 48
Explanation: 6!! = 6 × 4 × 2 = 48.

Example 2: Double Factorial of 5

To calculate 5!! (double factorial of 5):

=FACTDOUBLE(5)

Result: 15
Explanation: 5!! = 5 × 3 × 1 = 15.

Example 3: Double Factorial of 0

To calculate 0!!:

=FACTDOUBLE(0)

Result: 1
Explanation: By definition, 0!! is equal to 1.

Important Notes

• The FACTDOUBLE function works with non-negative integers only. If you enter a negative number or a non-integer, Excel will return a #NUM! error.
• If the input is an even number, it calculates the product of all even integers from the number down to 2. If the input is an odd number, it calculates the product of all odd integers from the number down to 1.
• Double factorials are often used in combinatorics, such as in the calculation of permutations and combinations for specific constraints. They also appear in various areas of advanced mathematics and physics.