| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Lattice systems</title> |
| <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> |
| <link rel="home" href="../../index.html" title="Chapter 1. Boost.Numeric.Odeint"> |
| <link rel="up" href="../tutorial.html" title="Tutorial"> |
| <link rel="prev" href="complex_state_types.html" title="Complex state types"> |
| <link rel="next" href="ensembles_of_oscillators.html" title="Ensembles of oscillators"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../logo.jpg"></td> |
| <td align="center"><a href="../../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="complex_state_types.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="ensembles_of_oscillators.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_numeric_odeint.tutorial.lattice_systems"></a><a class="link" href="lattice_systems.html" title="Lattice systems">Lattice |
| systems</a> |
| </h3></div></div></div> |
| <p> |
| odeint can also be used to solve ordinary differential equations defined |
| on lattices. A prominent example is the Fermi-Pasta-Ulam system <a class="link" href="../literature.html#fpu_scholarpedia">[8] |
| </a>. It is a Hamiltonian system of nonlinear coupled harmonic oscillators. |
| The Hamiltonian is |
| </p> |
| <p> |
| <span class="emphasis"><em>H = Σ<sub>​i</sub> p<sub>​i</sub><sup>2</sup>/2 + 1/2 ( q<sub>​i+1</sub> - q<sub>​i</sub> )^2 + β / 4 ( q<sub>​i+1</sub> - q<sub>​i</sub> )^4 </em></span> |
| </p> |
| <p> |
| Remarkably, the Fermi-Pasta-Ulam system was the first numerical experiment |
| to be implemented on a computer. It was studied at Los Alamos in 1953 on |
| one of the first computers (a MANIAC I) and it triggered a whole new tree |
| of mathematical and physical science. |
| </p> |
| <p> |
| Like the <a class="link" href="solar_system.html" title="Solar system">Solar |
| System</a>, the FPU is solved again by a symplectic solver, but in this |
| case we can speed up the computation because the <span class="emphasis"><em>q</em></span> components |
| trivially reduce to <span class="emphasis"><em>dq<sub>​i</sub> / dt = p<sub>​i</sub></em></span>. odeint is capable |
| of doing this performance improvement. All you have to do is to call the |
| symplectic solver with an state function for the <span class="emphasis"><em>p</em></span> components. |
| Here is how this function looks like |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">vector</span><span class="special"><</span> <span class="keyword">double</span> <span class="special">></span> <span class="identifier">container_type</span><span class="special">;</span> |
| |
| <span class="keyword">struct</span> <span class="identifier">fpu</span> |
| <span class="special">{</span> |
| <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">m_beta</span><span class="special">;</span> |
| |
| <span class="identifier">fpu</span><span class="special">(</span> <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">beta</span> <span class="special">=</span> <span class="number">1.0</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_beta</span><span class="special">(</span> <span class="identifier">beta</span> <span class="special">)</span> <span class="special">{</span> <span class="special">}</span> |
| |
| <span class="comment">// system function defining the ODE</span> |
| <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">const</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">q</span> <span class="special">,</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">dpdt</span> <span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="identifier">size_t</span> <span class="identifier">n</span> <span class="special">=</span> <span class="identifier">q</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> |
| <span class="keyword">double</span> <span class="identifier">tmp</span> <span class="special">=</span> <span class="identifier">q</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">-</span> <span class="number">0.0</span><span class="special">;</span> |
| <span class="keyword">double</span> <span class="identifier">tmp2</span> <span class="special">=</span> <span class="identifier">tmp</span> <span class="special">+</span> <span class="identifier">m_beta</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">*</span> <span class="identifier">tmp</span><span class="special">;</span> |
| <span class="identifier">dpdt</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">tmp2</span><span class="special">;</span> |
| <span class="keyword">for</span><span class="special">(</span> <span class="identifier">size_t</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span> <span class="special">;</span> <span class="identifier">i</span><span class="special"><</span><span class="identifier">n</span><span class="special">-</span><span class="number">1</span> <span class="special">;</span> <span class="special">++</span><span class="identifier">i</span> <span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">tmp</span> <span class="special">=</span> <span class="identifier">q</span><span class="special">[</span><span class="identifier">i</span><span class="special">+</span><span class="number">1</span><span class="special">]</span> <span class="special">-</span> <span class="identifier">q</span><span class="special">[</span><span class="identifier">i</span><span class="special">];</span> |
| <span class="identifier">tmp2</span> <span class="special">=</span> <span class="identifier">tmp</span> <span class="special">+</span> <span class="identifier">m_beta</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">*</span> <span class="identifier">tmp</span><span class="special">;</span> |
| <span class="identifier">dpdt</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">tmp2</span><span class="special">;</span> |
| <span class="identifier">dpdt</span><span class="special">[</span><span class="identifier">i</span><span class="special">+</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">tmp2</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="identifier">tmp</span> <span class="special">=</span> <span class="special">-</span> <span class="identifier">q</span><span class="special">[</span><span class="identifier">n</span><span class="special">-</span><span class="number">1</span><span class="special">];</span> |
| <span class="identifier">tmp2</span> <span class="special">=</span> <span class="identifier">tmp</span> <span class="special">+</span> <span class="identifier">m_beta</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">*</span> <span class="identifier">tmp</span> <span class="special">*</span> <span class="identifier">tmp</span><span class="special">;</span> |
| <span class="identifier">dpdt</span><span class="special">[</span><span class="identifier">n</span><span class="special">-</span><span class="number">1</span><span class="special">]</span> <span class="special">+=</span> <span class="identifier">tmp2</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="comment">// calculates the energy of the system</span> |
| <span class="keyword">double</span> <span class="identifier">energy</span><span class="special">(</span> <span class="keyword">const</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">q</span> <span class="special">,</span> <span class="keyword">const</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">p</span> <span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="comment">// ...</span> |
| <span class="special">}</span> |
| |
| <span class="comment">// calculates the local energy of the system</span> |
| <span class="keyword">void</span> <span class="identifier">local_energy</span><span class="special">(</span> <span class="keyword">const</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">q</span> <span class="special">,</span> <span class="keyword">const</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">p</span> <span class="special">,</span> <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">e</span> <span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="comment">// ...</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| You can also use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span><span class="special"><</span> <span class="keyword">double</span> <span class="special">,</span> <span class="identifier">N</span> <span class="special">></span></code> |
| for the state type. |
| </p> |
| <p> |
| Now, you have to define your initial values and perform the integration: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">const</span> <span class="identifier">size_t</span> <span class="identifier">n</span> <span class="special">=</span> <span class="number">64</span><span class="special">;</span> |
| <span class="identifier">container_type</span> <span class="identifier">q</span><span class="special">(</span> <span class="identifier">n</span> <span class="special">,</span> <span class="number">0.0</span> <span class="special">)</span> <span class="special">,</span> <span class="identifier">p</span><span class="special">(</span> <span class="identifier">n</span> <span class="special">,</span> <span class="number">0.0</span> <span class="special">);</span> |
| |
| <span class="keyword">for</span><span class="special">(</span> <span class="identifier">size_t</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span> <span class="special">;</span> <span class="identifier">i</span><span class="special"><</span><span class="identifier">n</span> <span class="special">;</span> <span class="special">++</span><span class="identifier">i</span> <span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">p</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">;</span> |
| <span class="identifier">q</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="number">32.0</span> <span class="special">*</span> <span class="identifier">sin</span><span class="special">(</span> <span class="keyword">double</span><span class="special">(</span> <span class="identifier">i</span> <span class="special">+</span> <span class="number">1</span> <span class="special">)</span> <span class="special">/</span> <span class="keyword">double</span><span class="special">(</span> <span class="identifier">n</span> <span class="special">+</span> <span class="number">1</span> <span class="special">)</span> <span class="special">*</span> <span class="identifier">M_PI</span> <span class="special">);</span> |
| <span class="special">}</span> |
| |
| |
| <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">dt</span> <span class="special">=</span> <span class="number">0.1</span><span class="special">;</span> |
| |
| <span class="keyword">typedef</span> <span class="identifier">symplectic_rkn_sb3a_mclachlan</span><span class="special"><</span> <span class="identifier">container_type</span> <span class="special">></span> <span class="identifier">stepper_type</span><span class="special">;</span> |
| <span class="identifier">fpu</span> <span class="identifier">fpu_instance</span><span class="special">(</span> <span class="number">8.0</span> <span class="special">);</span> |
| |
| <span class="identifier">integrate_const</span><span class="special">(</span> <span class="identifier">stepper_type</span><span class="special">()</span> <span class="special">,</span> <span class="identifier">fpu_instance</span> <span class="special">,</span> |
| <span class="identifier">make_pair</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span> <span class="identifier">q</span> <span class="special">)</span> <span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">)</span> <span class="special">,</span> |
| <span class="number">0.0</span> <span class="special">,</span> <span class="number">1000.0</span> <span class="special">,</span> <span class="identifier">dt</span> <span class="special">,</span> <span class="identifier">streaming_observer</span><span class="special">(</span> <span class="identifier">cout</span> <span class="special">,</span> <span class="identifier">fpu_instance</span> <span class="special">,</span> <span class="number">10</span> <span class="special">)</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| The observer uses a reference to the system object to calculate the local |
| energies: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">streaming_observer</span> |
| <span class="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span><span class="special">&</span> <span class="identifier">m_out</span><span class="special">;</span> |
| <span class="keyword">const</span> <span class="identifier">fpu</span> <span class="special">&</span><span class="identifier">m_fpu</span><span class="special">;</span> |
| <span class="identifier">size_t</span> <span class="identifier">m_write_every</span><span class="special">;</span> |
| <span class="identifier">size_t</span> <span class="identifier">m_count</span><span class="special">;</span> |
| |
| <span class="identifier">streaming_observer</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span> <span class="special">&</span><span class="identifier">out</span> <span class="special">,</span> <span class="keyword">const</span> <span class="identifier">fpu</span> <span class="special">&</span><span class="identifier">f</span> <span class="special">,</span> <span class="identifier">size_t</span> <span class="identifier">write_every</span> <span class="special">=</span> <span class="number">100</span> <span class="special">)</span> |
| <span class="special">:</span> <span class="identifier">m_out</span><span class="special">(</span> <span class="identifier">out</span> <span class="special">)</span> <span class="special">,</span> <span class="identifier">m_fpu</span><span class="special">(</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">,</span> <span class="identifier">m_write_every</span><span class="special">(</span> <span class="identifier">write_every</span> <span class="special">)</span> <span class="special">,</span> <span class="identifier">m_count</span><span class="special">(</span> <span class="number">0</span> <span class="special">)</span> <span class="special">{</span> <span class="special">}</span> |
| |
| <span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">State</span> <span class="special">></span> |
| <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span> <span class="keyword">const</span> <span class="identifier">State</span> <span class="special">&</span><span class="identifier">x</span> <span class="special">,</span> <span class="keyword">double</span> <span class="identifier">t</span> <span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span><span class="special">(</span> <span class="special">(</span> <span class="identifier">m_count</span> <span class="special">%</span> <span class="identifier">m_write_every</span> <span class="special">)</span> <span class="special">==</span> <span class="number">0</span> <span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">q</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">first</span><span class="special">;</span> |
| <span class="identifier">container_type</span> <span class="special">&</span><span class="identifier">p</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">second</span><span class="special">;</span> |
| <span class="identifier">container_type</span> <span class="identifier">energy</span><span class="special">(</span> <span class="identifier">q</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">);</span> |
| <span class="identifier">m_fpu</span><span class="special">.</span><span class="identifier">local_energy</span><span class="special">(</span> <span class="identifier">q</span> <span class="special">,</span> <span class="identifier">p</span> <span class="special">,</span> <span class="identifier">energy</span> <span class="special">);</span> |
| <span class="keyword">for</span><span class="special">(</span> <span class="identifier">size_t</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span> <span class="special">;</span> <span class="identifier">i</span><span class="special"><</span><span class="identifier">q</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special">;</span> <span class="special">++</span><span class="identifier">i</span> <span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">m_out</span> <span class="special"><<</span> <span class="identifier">t</span> <span class="special"><<</span> <span class="string">"\t"</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="string">"\t"</span> <span class="special"><<</span> <span class="identifier">q</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special"><<</span> <span class="string">"\t"</span> <span class="special"><<</span> <span class="identifier">p</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special"><<</span> <span class="string">"\t"</span> <span class="special"><<</span> <span class="identifier">energy</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special"><<</span> <span class="string">"\n"</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="identifier">m_out</span> <span class="special"><<</span> <span class="string">"\n"</span><span class="special">;</span> |
| <span class="identifier">clog</span> <span class="special"><<</span> <span class="identifier">t</span> <span class="special"><<</span> <span class="string">"\t"</span> <span class="special"><<</span> <span class="identifier">accumulate</span><span class="special">(</span> <span class="identifier">energy</span><span class="special">.</span><span class="identifier">begin</span><span class="special">()</span> <span class="special">,</span> <span class="identifier">energy</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span> <span class="special">,</span> <span class="number">0.0</span> <span class="special">)</span> <span class="special"><<</span> <span class="string">"\n"</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">++</span><span class="identifier">m_count</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| The full cpp file for this FPU example can be found here <a href="https://github.com/headmyshoulder/odeint-v2/blob/master/examples/fpu.cpp" target="_top">fpu.cpp</a> |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2009-2012 Karsten |
| Ahnert and Mario Mulansky<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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="complex_state_types.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="ensembles_of_oscillators.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
