Definition:Factorial

Let $$n \in \N$$.

Then the factorial of $$n$$ is defined as:
 * $$n! = \begin{cases}

1 & : n = 0 \\ n \left({n - 1}\right)! & : n > 0 \end{cases}$$

That is:
 * $$n! = \displaystyle \prod_{k=1}^n k = 1 \times 2 \times \cdots \times \left({n-1}\right) \times n$$.