\left( r^2 \frac{\partial f}{\partial r} \right) +
\frac{1}{r^2 \sin \theta} \frac{\partial f}{\partial \theta}
\left( \sin \theta \, \frac{\partial f}{\partial \theta} \right) +
\frac{1}{r^2 \sin^2\theta} \frac{\partial^2 f}{\partial \phi^2}\]</div>
<p>You can add a link to equations like the one above <a class="reference internal" href="#equation-this-is-a-label">(1)</a> by using <code class="docutils literal notranslate"><span class="pre">:eq:</span></code>.</p>
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<h2><a clas