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