\begin{definition}[Definition:Partial Derivative/Higher Derivative/Third Derivative]
Let $u = \map f {x, y, z}$ be a function of the $3$ independent variables $x$, $y$ and $z$.
The following is an example of one of the $3$rd derivatives of $f$:
:$\dfrac {\partial^3 u} {\partial z^2 \partial y} := \map {\dfrac \partial {\partial z} } {\dfrac {\partial^2 u} {\partial z \partial y} } =: \map {f_{2 3 3} } {x, y, z}$
\end{definition}