\ \textrm{error} &amp; \text{if } z \leq 0 \end{cases}
\]</span> where <span class="math display">\[ {K_v}(z) = \frac{\pi}{2}\cdot\frac{I_{-v}(z) -
I_{v}(z)}{\sin(v\pi)} \]</span></p>
<!-- real; falling_factorial; (real x, real n); -->
<p></p>
<p><code>real</code> <strong><code>falling_factorial</code></strong><code>(real x, real n)</code><br>
Return the falling factorial of x with power n defined for positive x
and real n. <span class="math display">\[ \mathrm{falling\_factorial}(x,n) = \begin{cases} (x)_