Let $N \in \mathbb{N}$ be a D996: Natural number.
The factorial of $N$ is the D996: Natural number
\begin{equation}
N !
: =
\begin{cases}
\prod_{n = 1}^N n, \quad & N > 0 \\
1, \quad & N = 0
\end{cases}
\end{equation}
Let $N \in \mathbb{N}$ be a D996: Natural number.
The factorial of $N$ is the D996: Natural number
\begin{equation}
N !
: =
I_{N > 0} \left( \prod_{n = 1}^N n \right)
+ I_{N = 0}
\end{equation}
