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<div class="titlepage"><div><div><h3 class="title">
<a name="math_toolkit.root_finding_examples.5th_root_eg"></a><a class="link" href="5th_root_eg.html" title="Computing the Fifth Root">Computing
the Fifth Root</a>
</h3></div></div></div>
<p>
Let's now suppose we want to find the <span class="bold"><strong>fifth root</strong></span>
of a number <span class="emphasis"><em>a</em></span>.
</p>
<p>
The equation we want to solve is :
</p>
<p>
  <span class="emphasis"><em>f</em></span>(x) = <span class="emphasis"><em>x<sup>5</sup> -a</em></span>
</p>
<p>
If your differentiation is a little rusty (or you are faced with an function
whose complexity makes differentiation daunting), then you can get help,
for example, from the invaluable <a href="http://www.wolframalpha.com/" target="_top">WolframAlpha
site.</a>
</p>
<p>
For example, entering the commmand: <code class="computeroutput"><span class="identifier">differentiate</span>
<span class="identifier">x</span> <span class="special">^</span> <span class="number">5</span></code>
</p>
<p>
or the Wolfram Language command: <code class="computeroutput"> <span class="identifier">D</span><span class="special">[</span><span class="identifier">x</span> <span class="special">^</span>
<span class="number">5</span><span class="special">,</span> <span class="identifier">x</span><span class="special">]</span></code>
</p>
<p>
gives the output: <code class="computeroutput"><span class="identifier">d</span><span class="special">/</span><span class="identifier">dx</span><span class="special">(</span><span class="identifier">x</span>
<span class="special">^</span> <span class="number">5</span><span class="special">)</span> <span class="special">=</span> <span class="number">5</span>
<span class="identifier">x</span> <span class="special">^</span> <span class="number">4</span></code>
</p>
<p>
and to get the second differential, enter: <code class="computeroutput"><span class="identifier">second</span>
<span class="identifier">differentiate</span> <span class="identifier">x</span>
<span class="special">^</span> <span class="number">5</span></code>
</p>
<p>
or the Wolfram Language command: <code class="computeroutput"><span class="identifier">D</span><span class="special">[</span><span class="identifier">x</span> <span class="special">^</span>
<span class="number">5</span><span class="special">,</span> <span class="special">{</span> <span class="identifier">x</span><span class="special">,</span>
<span class="number">2</span> <span class="special">}]</span></code>
</p>
<p>
to get the output: <code class="computeroutput"><span class="identifier">d</span> <span class="special">^</span>
<span class="number">2</span> <span class="special">/</span> <span class="identifier">dx</span> <span class="special">^</span> <span class="number">2</span><span class="special">(</span><span class="identifier">x</span> <span class="special">^</span>
<span class="number">5</span><span class="special">)</span> <span class="special">=</span> <span class="number">20</span> <span class="identifier">x</span>
<span class="special">^</span> <span class="number">3</span></code>
</p>
<p>
To get a reference value, we can enter: <code class="literal">fifth root 3126</code>
</p>
<p>
or: <code class="computeroutput"><span class="identifier">N</span><span class="special">[</span><span class="number">3126</span> <span class="special">^</span> <span class="special">(</span><span class="number">1</span> <span class="special">/</span> <span class="number">5</span><span class="special">),</span> <span class="number">50</span><span class="special">]</span></code>
</p>
<p>
to get a result with a precision of 50 decimal digits:
</p>
<p>
5.0003199590478625588206333405631053401128722314376
</p>
<p>
(We could also get a reference value using <a class="link" href="multiprecision_root.html" title="Root-finding using Boost.Multiprecision">multiprecision
root</a>).
</p>
<p>
The 1st and 2nd derivatives of x<sup>5</sup> are:
</p>
<p>
  <span class="emphasis"><em>f</em></span>'(x) = 5x<sup>4</sup>
</p>
<p>
  <span class="emphasis"><em>f</em></span>''(x) = 20x<sup>3</sup>
</p>
<p>
Using these expressions for the derivatives, the functor is:
</p>
<pre class="programlisting"><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">struct</span> <span class="identifier">fifth_functor_2deriv</span>
<span class="special">{</span>
<span class="comment">// Functor returning both 1st and 2nd derivatives.</span>
<span class="identifier">fifth_functor_2deriv</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">to_find_root_of</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">a</span><span class="special">(</span><span class="identifier">to_find_root_of</span><span class="special">)</span>
<span class="special">{</span> <span class="comment">/* Constructor stores value a to find root of, for example: */</span> <span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">T</span><span class="special">,</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// Return both f(x) and f'(x) and f''(x).</span>
<span class="identifier">T</span> <span class="identifier">fx</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">pow</span><span class="special"><</span><span class="number">5</span><span class="special">>(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">-</span> <span class="identifier">a</span><span class="special">;</span> <span class="comment">// Difference (estimate x^3 - value).</span>
<span class="identifier">T</span> <span class="identifier">dx</span> <span class="special">=</span> <span class="number">5</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">pow</span><span class="special"><</span><span class="number">4</span><span class="special">>(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// 1st derivative = 5x^4.</span>
<span class="identifier">T</span> <span class="identifier">d2x</span> <span class="special">=</span> <span class="number">20</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">pow</span><span class="special"><</span><span class="number">3</span><span class="special">>(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// 2nd derivative = 20 x^3</span>
<span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_tuple</span><span class="special">(</span><span class="identifier">fx</span><span class="special">,</span> <span class="identifier">dx</span><span class="special">,</span> <span class="identifier">d2x</span><span class="special">);</span> <span class="comment">// 'return' fx, dx and d2x.</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">T</span> <span class="identifier">a</span><span class="special">;</span> <span class="comment">// to be 'fifth_rooted'.</span>
<span class="special">};</span> <span class="comment">// struct fifth_functor_2deriv</span>
</pre>
<p>
Our fifth-root function is now:
</p>
<pre class="programlisting"><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="identifier">T</span> <span class="identifier">fifth_2deriv</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// return fifth root of x using 1st and 2nd derivatives and Halley.</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">std</span><span class="special">;</span> <span class="comment">// Help ADL of std functions.</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tools</span><span class="special">;</span> <span class="comment">// for halley_iterate.</span>
<span class="keyword">int</span> <span class="identifier">exponent</span><span class="special">;</span>
<span class="identifier">frexp</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="special">&</span><span class="identifier">exponent</span><span class="special">);</span> <span class="comment">// Get exponent of z (ignore mantissa).</span>
<span class="identifier">T</span> <span class="identifier">guess</span> <span class="special">=</span> <span class="identifier">ldexp</span><span class="special">(</span><span class="number">1.</span><span class="special">,</span> <span class="identifier">exponent</span> <span class="special">/</span> <span class="number">5</span><span class="special">);</span> <span class="comment">// Rough guess is to divide the exponent by five.</span>
<span class="identifier">T</span> <span class="identifier">min</span> <span class="special">=</span> <span class="identifier">ldexp</span><span class="special">(</span><span class="number">0.5</span><span class="special">,</span> <span class="identifier">exponent</span> <span class="special">/</span> <span class="number">5</span><span class="special">);</span> <span class="comment">// Minimum possible value is half our guess.</span>
<span class="identifier">T</span> <span class="identifier">max</span> <span class="special">=</span> <span class="identifier">ldexp</span><span class="special">(</span><span class="number">2.</span><span class="special">,</span> <span class="identifier">exponent</span> <span class="special">/</span> <span class="number">5</span><span class="special">);</span> <span class="comment">// Maximum possible value is twice our guess.</span>
<span class="comment">// Stop when slightly more than one of the digits are correct:</span>
<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">digits</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">digits</span> <span class="special">*</span> <span class="number">0.4</span><span class="special">);</span>
<span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uintmax_t</span> <span class="identifier">maxit</span> <span class="special">=</span> <span class="number">50</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">uintmax_t</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">maxit</span><span class="special">;</span>
<span class="identifier">T</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">halley_iterate</span><span class="special">(</span><span class="identifier">fifth_functor_2deriv</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">x</span><span class="special">),</span> <span class="identifier">guess</span><span class="special">,</span> <span class="identifier">min</span><span class="special">,</span> <span class="identifier">max</span><span class="special">,</span> <span class="identifier">digits</span><span class="special">,</span> <span class="identifier">it</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Full code of this example is at <a href="../../../../example/root_finding_example.cpp" target="_top">root_finding_example.cpp</a>
and <a href="../../../../example/root_finding_n_example.cpp" target="_top">root_finding_n_example.cpp</a>.
</p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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