\begin{definition}[Definition:Gamma Function/Hankel Form]
The '''{{AuthorRef|Hermann Hankel|Hankel}} form''' of the '''gamma function''' is:
:$\ds \frac 1 {\map \Gamma z} = \dfrac 1 {2 \pi i} \oint_\HH \frac {e^t \rd t} {t^z}$
where $\HH$ is the contour starting at $-\infty$, circling the origin in an anticlockwise direction, and returning to $-\infty$.
The '''Hankel form''' is valid for all $\C$.
\end{definition}