time derivatives</p>
<p><span class="math display">\[
\begin{array}{rcl}
\frac{d \theta}{d t} &amp; = &amp; +\frac{\partial T}{\partial \rho}
\\[2pt]
\frac{d \rho}{d t} &amp; = &amp; -\frac{\partial V}{\partial \theta}.
\end{array}
\]</span></p>
</div>
<div id="leapfrog-integrator" class="section level3 unnumbered">
<h3>Leapfrog Integrator</h3>
<p>The last section leaves a two-state differential equation to solve.
Stan, like most other HMC implementations, uses the leapfrog
integrator, which is a numerical 