\subsection{Dual space}
\subsection{Inner and outer product}
+\label{math_app:product}
\begin{definition}
The inner product ...
\end{definition}
\begin{definition}
-The outer product ...
+If $\vec{u}\in U$ and $\vec{v}\in V$ are vectors within the respective vector spaces and $V^{\dagger}$ is the dual space of $V$, the outer product of $\vec{u}$ and $\vec{v}$ is defined as the tensor product ...
\end{definition}
\section{Spherical coordinates}