de>Double.NaN</code>, or  the limit inferior or limit superior of the
 difference quotient.
 <p>
 For scale <code>k</code>, and shape <code>α</code> of this distribution, the PDF
 is given by
 <ul>
 <li><code>0</code> if <code>x &lt; k</code>,</li>
 <li><code>α * k^α / x^(α + 1)</code> otherwise.</li>
 </ul></div>
<dl><dt><span class="strong">Parameters:</span></dt><dd><code>x</code> - the point at which the PDF is evaluated</dd>
<dt><span class="strong">Returns:</span></dt><dd>the value of the probabil