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<title>Falling Factorial</title>
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<div class="section math_toolkit_special_factorials_sf_falling_factorial">
<div class="titlepage"><div><div><h4 class="title">
<a name="math_toolkit.special.factorials.sf_falling_factorial"></a><a class="link" href="sf_falling_factorial.html" title="Falling Factorial">Falling
Factorial</a>
</h4></div></div></div>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">special_functions</span><span class="special">/</span><span class="identifier">factorials</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<a class="link" href="../../main_overview/result_type.html" title="Calculation of the Type of the Result"><span class="emphasis"><em>calculated-result-type</em></span></a> <span class="identifier">falling_factorial</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">unsigned</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <a class="link" href="../../policy.html" title="Policies">Policy</a><span class="special">></span>
<a class="link" href="../../main_overview/result_type.html" title="Calculation of the Type of the Result"><span class="emphasis"><em>calculated-result-type</em></span></a> <span class="identifier">falling_factorial</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">unsigned</span> <span class="identifier">i</span><span class="special">,</span> <span class="keyword">const</span> <a class="link" href="../../policy.html" title="Policies">Policy</a><span class="special">&);</span>
<span class="special">}}</span> <span class="comment">// namespaces</span>
</pre>
<p>
Returns the falling factorial of <span class="emphasis"><em>x</em></span> and <span class="emphasis"><em>i</em></span>:
</p>
<p>
falling_factorial(x, i) = x(x-1)(x-2)(x-3)...(x-i+1)
</p>
<p>
Note that this function is only defined for positive <span class="emphasis"><em>i</em></span>,
hence the <code class="computeroutput"><span class="keyword">unsigned</span></code> second
argument. Argument <span class="emphasis"><em>x</em></span> can be either positive or negative
however.
</p>
<p>
The final <a class="link" href="../../policy.html" title="Policies">Policy</a> argument is
optional and can be used to control the behaviour of the function: how
it handles errors, what level of precision to use etc. Refer to the <a class="link" href="../../policy.html" title="Policies">policy documentation for more details</a>.
</p>
<p>
May return the result of <a class="link" href="../../main_overview/error_handling.html#overflow_error">overflow_error</a>
if the result is too large to represent in type T.
</p>
<p>
The return type of these functions is computed using the <a class="link" href="../../main_overview/result_type.html" title="Calculation of the Type of the Result"><span class="emphasis"><em>result
type calculation rules</em></span></a>: the type of the result is <code class="computeroutput"><span class="keyword">double</span></code> if T is an integer type, otherwise
the type of the result is T.
</p>
<h5>
<a name="math_toolkit.special.factorials.sf_falling_factorial.h0"></a>
<span><a name="math_toolkit.special.factorials.sf_falling_factorial.accuracy"></a></span><a class="link" href="sf_falling_factorial.html#math_toolkit.special.factorials.sf_falling_factorial.accuracy">Accuracy</a>
</h5>
<p>
The accuracy will be the same as the <a class="link" href="../sf_gamma/gamma_ratios.html" title="Ratios of Gamma Functions">tgamma_delta_ratio</a>
function.
</p>
<h5>
<a name="math_toolkit.special.factorials.sf_falling_factorial.h1"></a>
<span><a name="math_toolkit.special.factorials.sf_falling_factorial.testing"></a></span><a class="link" href="sf_falling_factorial.html#math_toolkit.special.factorials.sf_falling_factorial.testing">Testing</a>
</h5>
<p>
The spot tests for the falling factorials use data generated by functions.wolfram.com.
</p>
<h5>
<a name="math_toolkit.special.factorials.sf_falling_factorial.h2"></a>
<span><a name="math_toolkit.special.factorials.sf_falling_factorial.implementation"></a></span><a class="link" href="sf_falling_factorial.html#math_toolkit.special.factorials.sf_falling_factorial.implementation">Implementation</a>
</h5>
<p>
Rising and falling factorials are implemented as ratios of gamma functions
using <a class="link" href="../sf_gamma/gamma_ratios.html" title="Ratios of Gamma Functions">tgamma_delta_ratio</a>.
Optimisations for small integer arguments are handled internally by that
function.
</p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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