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<h1><a href="../index.html">
<img src="../..//docs/docs-src/Figures/Octagram184x184.png" width=72 height=72">
</a></h1>
<ul><li><a href="../inweb/index.html">inweb</a></li>
</ul><h2>Foundation Module</h2><ul>
<li><a href="../foundation-module/index.html">foundation</a></li>
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<li><a href="../foundation-test/index.html">foundation-test</a></li>
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</ul><h2>Example Webs</h2><ul>
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<li><a href="../goldbach/index.html">goldbach</a></li>
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<li><span class="unlink">twinprimes</span></li>
</ul><h2>External</h2><ul>
<li><a href="https://github.com/ganelson/inweb">github</a></li>
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Completed colonies
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</ul>
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<!--Weave of 'Complete Program' generated by 7-->
Clarified indexing engine
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><b>twinprimes</b></li></ul><p class="inwebparagraph"><a id="SP1"></a><b>&#167;1. The conjecture. </b>It is widely believed that there are an infinite number of twin primes, that
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is, prime numbers occurring in pairs different by 2. Twins are known to exist
at least as far out as \(10^{388,342}\) (as of 2016), and there are infinitely
many pairs of primes closer together than about 250 (Zhang, 2013; Tao, Maynard,
and many others, 2014).
</p>
<p class="inwebparagraph">This program finds a few small pairs of twins, by the simplest method possible,
and should print output like so:
</p>
<pre class="display">
<span class="plain">3 and 5</span>
<span class="plain">5 and 7</span>
<span class="plain">11 and 13</span>
<span class="plain">...</span>
</pre>
<p class="inwebparagraph"></p>
<pre class="definitions">
<span class="definitionkeyword">define</span> <span class="constant">RANGE</span><span class="plain"> </span><span class="constant">100</span><span class="plain"> </span><span class="comment"> the upper limit to the numbers we will consider</span>
</pre>
<pre class="display">
<span class="plain">#</span><span class="identifier">include</span><span class="plain"> &lt;</span><span class="identifier">stdio</span><span class="plain">.</span><span class="identifier">h</span><span class="plain">&gt;</span>
<span class="reserved">int</span><span class="plain"> </span><span class="functiontext">main</span><span class="plain">(</span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">argc</span><span class="plain">, </span><span class="reserved">char</span><span class="plain"> *</span><span class="identifier">argv</span><span class="plain">[]) {</span>
<span class="reserved">for</span><span class="plain"> (</span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">i</span><span class="plain">=1; </span><span class="identifier">i</span><span class="plain">&lt;</span><span class="constant">RANGE</span><span class="plain">; </span><span class="identifier">i</span><span class="plain">++)</span>
&lt;<span class="cwebmacro">Test for twin prime at i</span> <span class="cwebmacronumber">1.1</span>&gt;<span class="plain">;</span>
<span class="plain">}</span>
</pre>
<p class="inwebparagraph"></p>
<p class="inwebparagraph"><a id="SP1_1"></a><b>&#167;1.1. </b><code class="display">
&lt;<span class="cwebmacrodefn">Test for twin prime at i</span> <span class="cwebmacronumber">1.1</span>&gt; =
</code></p>
<pre class="displaydefn">
<span class="reserved">if</span><span class="plain"> ((</span><span class="functiontext"><a href="#SP2">isprime</a></span><span class="plain">(</span><span class="identifier">i</span><span class="plain">)) &amp;&amp; (</span><span class="functiontext"><a href="#SP2">isprime</a></span><span class="plain">(</span><span class="identifier">i</span><span class="plain">+2)))</span>
<span class="identifier">printf</span><span class="plain">(</span><span class="string">"%d and %d\n"</span><span class="plain">, </span><span class="identifier">i</span><span class="plain">, </span><span class="identifier">i</span><span class="plain">+2);</span>
</pre>
<p class="inwebparagraph"></p>
<p class="endnote">This code is used in <a href="#SP1">&#167;1</a>.</p>
<p class="inwebparagraph"><a id="SP2"></a><b>&#167;2. Primality. </b>This simple and slow test tries to divide by every whole number at least
2 and up to the square root: if none divide exactly, the number is prime.
A common error with this algorithm is to check where \(m^2 &lt; n\), rather
than \(m^2 \leq n\), thus wrongly considering 4, 9, 25, 49, ... as prime:
Cambridge folklore has it that this bug occurred on the first computation
of the EDSAC computer on 6 May 1949.
</p>
<pre class="definitions">
<span class="definitionkeyword">define</span> <span class="constant">TRUE</span><span class="plain"> </span><span class="constant">1</span>
<span class="definitionkeyword">define</span> <span class="constant">FALSE</span><span class="plain"> </span><span class="constant">0</span>
</pre>
<pre class="display">
<span class="reserved">int</span><span class="plain"> </span><span class="functiontext">isprime<button class="popup" onclick="togglePopup('usagePopup0')">...<span class="popuptext" id="usagePopup0">Usage of <b>isprime</b>:<br><a href="#SP1_1">&#167;1.1</a></span></button></span><span class="plain">(</span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">n</span><span class="plain">) {</span>
<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">n</span><span class="plain"> &lt;= </span><span class="constant">1</span><span class="plain">) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
<span class="reserved">for</span><span class="plain"> (</span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">m</span><span class="plain"> = </span><span class="constant">2</span><span class="plain">; </span><span class="identifier">m</span><span class="plain">*</span><span class="identifier">m</span><span class="plain"> &lt;= </span><span class="identifier">n</span><span class="plain">; </span><span class="identifier">m</span><span class="plain">++)</span>
<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">n</span><span class="plain"> % </span><span class="identifier">m</span><span class="plain"> == </span><span class="constant">0</span><span class="plain">)</span>
<span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
<span class="reserved">return</span><span class="plain"> </span><span class="constant">TRUE</span><span class="plain">;</span>
<span class="plain">}</span>
</pre>
<p class="inwebparagraph"></p>
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