in\theta.$ As expected, we have fixed
      points at $\pm \pi, ...$ Now sketch the rest of the vector field.  Can you
      tell me which fixed points are stable? Some of them are stable i.s.L.,
      none are asymptotically stable.</p>

      <figure> <img width="80%" src="figures/pend_undamped_phase.svg"/>
      </figure>

      <subsubsection><h1>Orbit calculations</h1>

        <p> You might wonder how we drew the black contour lines in the figure
        above.  We could have obtained them by simulati