an class="special">&gt;</span> <span class="identifier">dvdt</span><span class="special">(</span><span class="number">500</span><span class="special">);</span>
<span class="comment">// . . . define spacing, create and instance of discrete_lanczos_derivative</span>

<span class="comment">// populate dvdt, perhaps in a loop:</span>
<span class="identifier">lanczos</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">dvdt</span><span class