al">);</span>
<span class="comment">// evaluate second derivative at a point:</span>
<span class="keyword">double</span> <span class="identifier">d2vdt2</span> <span class="special">=</span> <span class="identifier">lanczos</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="number">18</span><span class="special">);</span>
<span class="comment">// evaluate second derivative of entire vector:</span>
<span class="identifier">std</span><span class="s