# Double factorial

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]