framalpha.com/input/?i=d^2%2Fda^2+[4+*+a+*+EllipticE%281+-+b^2%2Fa^2%29]</span>
      <span class="comment">// for the second derivative.</span>
      <span class="identifier">T</span> <span class="identifier">a</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max</span><span class="special">)(</span><span class="identifier">m_radius</span><span class="special">,</span> <span class="identifier">x</s