The factorial of $n\in\N$, written $n!$, is defined recursively:
- \[0! = 1.\]
For all $n\in\N^+$,
\[n! = (n-1)!\cdot n.\]For $n\in\N^+$ we have
\[n! = \prod_{k=1}^n k.\]
The factorial of $n\in\N$, written $n!$, is defined recursively:
- \[0! = 1.\]
For all $n\in\N^+$,
\[n! = (n-1)!\cdot n.\]For $n\in\N^+$ we have
\[n! = \prod_{k=1}^n k.\]