How to use the FACTDOUBLE function
What is the FACTDOUBLE function?
The FACTDOUBLE returns the double factorial of a number.
How is the FACTDOUBLE function calculated?
The factorial double is the product of all positive integers up to n that have the same parity (odd or even) as n, in other words, it skips every other number.
The double factorial is calculates like this:
For even numbers: the double factorial of a number n is the product of all positive even integers less than or equal to n, and is represented by n!! Note the double exclamation marks.
n!! = n*(n-2)*(n-4)...(4)(2)
For odd numbers: the double factorial of a number n is the product of all positive odd integers less than or equal to n.
n!! = n*(n-2)*(n-4)...(3)(1)
Some examples:
1!! = 1
4!! = 4 * 2 * 1 = 8
5!! = 5 * 3 * 1 = 15
What is parity?
Parity is whether an integer is even or odd.
What is an even number?
The parity of a number determines if it is divisible by 2 or not without remainders. Even numbers alternate with odd numbers numerically.
What is an odd number?
Odd numbers are integers that are not divisible by 2. Some examples of odd numbers: 1, 3, 5, 7, 9, 11, 13, etc.
An number that is not divisible by two returns a remainder of one, in other words, it is not cleanly divided.
What is a remainder?
For example, if we divide 5 by 2:
5 / 2 = 2 with a remainder of 1.
What is the difference between the FACT function and the FACTDOUBLE function?
It may seem that the FACTDOUBLE returns a much larger number than the FACT function but that is not the case.
The factorial of a number n is the product of all positive integers less than or equal to n, whereas the factorial double only uses every other number.
Example of factorials:
5! = 5 * 4 * 3 * 2 * 1 = 120
6! = 6*5*4*3*2*1 = 720
Example of double factorials:
7!! = 7 * 5 * 3 * 1 = 105
6!! = 6 * 4 * 2 = 48
Formula in cell C3:
Excel Function Syntax
FACTDOUBLE(number)
Arguments
| number | Required. The number you want to calculate the double factorial of. |
Comments
The double factorial of 6 is 48, 6*4*2 = 48.
The double factorial of 9 is 945, 9*7*5*3 = 945.
Functions in 'Math and trigonometry' category
The FACTDOUBLE function function is one of many functions in the 'Math and trigonometry' category.
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