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<title>Modular Multiplicative Inverse</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_integer.mod_inverse"></a><a class="link" href="mod_inverse.html" title="Modular Multiplicative Inverse">Modular Multiplicative Inverse</a>
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<div class="toc"><dl class="toc">
<dt><span class="section"><a href="mod_inverse.html#boost_integer.mod_inverse.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="mod_inverse.html#boost_integer.mod_inverse.synopsis">Synopsis</a></span></dt>
<dt><span class="section"><a href="mod_inverse.html#boost_integer.mod_inverse.usage">Usage</a></span></dt>
<dt><span class="section"><a href="mod_inverse.html#boost_integer.mod_inverse.references">References</a></span></dt>
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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_integer.mod_inverse.introduction"></a><a class="link" href="mod_inverse.html#boost_integer.mod_inverse.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
        The modular multiplicative inverse of a number <span class="emphasis"><em>a</em></span> is
        that number <span class="emphasis"><em>x</em></span> which satisfies <span class="emphasis"><em>ax</em></span>
        = 1 mod <span class="emphasis"><em>p</em></span>. A fast algorithm for computing modular multiplicative
        inverses based on the extended Euclidean algorithm exists and is provided
        by Boost.
      </p>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_integer.mod_inverse.synopsis"></a><a class="link" href="mod_inverse.html#boost_integer.mod_inverse.synopsis" title="Synopsis">Synopsis</a>
</h3></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">integer</span><span class="special">/</span><span class="identifier">mod_inverse</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">integer</span> <span class="special">{</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Z</span><span class="special">&gt;</span>
<span class="identifier">Z</span> <span class="identifier">mod_inverse</span><span class="special">(</span><span class="identifier">Z</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">Z</span> <span class="identifier">m</span><span class="special">);</span>

<span class="special">}}</span>
</pre>
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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_integer.mod_inverse.usage"></a><a class="link" href="mod_inverse.html#boost_integer.mod_inverse.usage" title="Usage">Usage</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">mod_inverse</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="number">5</span><span class="special">);</span>
<span class="comment">// prints x = 3:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"x = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">x</span> <span class="special">&lt;&lt;</span> <span class="string">"\n"</span><span class="special">;</span>

<span class="keyword">int</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">mod_inverse</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="number">4</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">y</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"There is no inverse of 2 mod 4\n"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        Multiplicative modular inverses exist if and only if <span class="emphasis"><em>a</em></span>
        and <span class="emphasis"><em>m</em></span> are coprime. If <span class="emphasis"><em>a</em></span> and <span class="emphasis"><em>m</em></span>
        share a common factor, then <code class="computeroutput"><span class="identifier">mod_inverse</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span>
        <span class="identifier">m</span><span class="special">)</span></code>
        returns zero.
      </p>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_integer.mod_inverse.references"></a><a class="link" href="mod_inverse.html#boost_integer.mod_inverse.references" title="References">References</a>
</h3></div></div></div>
<p>
        Wagstaff, Samuel S., <span class="emphasis"><em>The Joy of Factoring</em></span>, Vol. 68.
        American Mathematical Soc., 2013.
      </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2001-2009 Beman
      Dawes, Daryle Walker, Gennaro Prota, John Maddock<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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