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\begin{document} \huge

\begin{center}
{Mathematical Physics --- PHZ 3113}\\\vspace {2mm} 
{\bf Divergence,} (January 25, 2013) \\
\end{center}
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\centerline{------------------------------------------------------}

\bigskip

\begin{enumerate}

\item Calculate
\begin{eqnarray}
  \nabla\cdot\vec{r}
\end{eqnarray} 

\item Calculate
\begin{eqnarray} \label{vecrfr}
  \nabla\cdot\,\vec{r}\,f(r)
\end{eqnarray} 

\item Example: Calculate (\ref{vecrfr}) for $f(r)=r^{n-1}$.

\item Calculate the divergence of the electric field
\begin{eqnarray}
  \vec{E}=\frac{q\,\hat{r}}{4\pi\,\epsilon_0\,r^2}
\end{eqnarray} 
for $\vec{r}\ne \vec{0}$.

\end{enumerate}

\end{document}

