Double factorial

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 $n!!$. When n is a positive odd integer, $n!!$ is defined as $n\cdot (n-2)\cdot \,\cdots \,\cdot 3\cdot 1$. When n is an positive even integer, $n!!$ is defined as $n\cdot (n-2)\cdot \,\cdots \,\cdot 4\cdot 2$. By definition, $0!!=1$.

Related pages

• Factorial
• Permutation