# Double factorial

product of all the integers from 1 up to the integral input of the function that have the same parity as this input

Double factorial is a method of calculating how many times a number is repeated in a geometric equation. This way, we can calculate the number of times a product is used in its life-time.

The double factorial of n is written as ${\displaystyle n!!}$.[1] When n is a positive odd integer, ${\displaystyle n!!}$ is defined as ${\displaystyle n\cdot (n-2)\cdot \,\cdots \,\cdot 3\cdot 1}$. When n is an positive even integer, ${\displaystyle n!!}$ is defined as ${\displaystyle n\cdot (n-2)\cdot \,\cdots \,\cdot 4\cdot 2}$. By definition, ${\displaystyle 0!!=1}$.[2][3]

## References

1. "List of Probability and Statistics Symbols". Math Vault. 2020-04-26. Retrieved 2020-09-10.
2. Weisstein, Eric W. "Double Factorial". mathworld.wolfram.com. Retrieved 2020-09-10.
3. "Double Factorials and Multifactorials | Brilliant Math & Science Wiki". brilliant.org. Retrieved 2020-09-10.