| [/============================================================================ |
| Boost.odeint |
| |
| Copyright 2011 Mario Mulansky |
| Copyright 2011-2012 Karsten Ahnert |
| |
| Use, modification and distribution is subject to the Boost Software License, |
| Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
| http://www.boost.org/LICENSE_1_0.txt) |
| =============================================================================/] |
| |
| |
| [section Symplectic System] |
| |
| [heading Description] |
| |
| This concept describes how to define a symplectic system written with generalized coordinate `q` and generalized momentum `p`: |
| |
| [' q'(t) = f(p) ] |
| |
| [' p'(t) = g(q) ] |
| |
| Such a situation is typically found for Hamiltonian systems with a separable Hamiltonian: |
| |
| [' H(p,q) = H[sub kin](p) + V(q) ] |
| |
| which gives the equations of motion: |
| |
| [' q'(t) = dH[sub kin] / dp = f(p) ] |
| |
| [' p'(t) = dV / dq = g(q) ] |
| |
| |
| The algorithmic implementation of this situation is described by a pair of callable objects for /f/ and /g/ with a specific parameter signature. |
| Such a system should be implemented as a std::pair of functions or a functors. |
| Symplectic systems are used in symplectic steppers like `symplectic_rkn_sb3a_mclachlan`. |
| |
| [heading Notation] |
| |
| [variablelist |
| [[`System`] [A type that is a model of SymplecticSystem]] |
| [[`Coor`] [The type of the coordinate ['q]]] |
| [[`Momentum`] [The type of the momentum ['p]]] |
| [[`CoorDeriv`] [The type of the derivative of coordinate ['q']]] |
| [[`MomentumDeriv`] [The type of the derivative of momentum ['p']]] |
| [[`sys`] [An object of the type `System`]] |
| [[`q`] [Object of type Coor]] |
| [[`p`] [Object of type Momentum]] |
| [[`dqdt`] [Object of type CoorDeriv]] |
| [[`dpdt`] [Object of type MomentumDeriv]] |
| ] |
| |
| [heading Valid expressions] |
| |
| [table |
| [[Name] [Expression] [Type] [Semantics]] |
| [[Check for pair] [`boost::is_pair< System >::type`] [`boost::mpl::true_`] [Check if System is a pair]] |
| [[Calculate ['dq/dt = f(p)]] [`sys.first( p , dqdt )`] [`void`] [Calculates ['f(p)], the result is stored into `dqdt`] ] |
| [[Calculate ['dp/dt = g(q)]] [`sys.second( q , dpdt )`] [`void`] [Calculates ['g(q)], the result is stored into `dpdt`] ] |
| ] |
| |
| [endsect] |
| |
| |
| [section Simple Symplectic System] |
| |
| [heading Description] |
| |
| In most Hamiltonian systems the kinetic term is a quadratic term in the momentum ['H[sub kin] = p^2 / 2m] and in many cases it is possible to rescale coordinates and set /m=1/ which leads to a trivial equation of motion: |
| |
| [' q'(t) = f(p) = p. ] |
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| while for /p'/ we still have the general form |
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| [' p'(t) = g(q) ] |
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| As this case is very frequent we introduced a concept where only the nontrivial equation for /p'/ has to be provided to the symplectic stepper. |
| We call this concept ['SimpleSymplecticSystem] |
| |
| [heading Notation] |
| |
| [variablelist |
| [[System] [A type that is a model of SimpleSymplecticSystem]] |
| [[Coor] [The type of the coordinate ['q]]] |
| [[MomentumDeriv] [The type of the derivative of momentum ['p']]] |
| [[sys] [An object that models System]] |
| [[q] [Object of type Coor]] |
| [[dpdt] [Object of type MomentumDeriv]] |
| ] |
| |
| [heading Valid Expressions] |
| |
| [table |
| [[Name] [Expression] [Type] [Semantics]] |
| [[Check for pair] [`boost::is_pair< System >::type`] [`boost::mpl::false_`] [Check if System is a pair, should be evaluated to false in this case.]] |
| [[Calculate ['dp/dt = g(q)]] [`sys( q , dpdt )`] [`void`] [Calculates ['g(q)], the result is stored into `dpdt`] ] |
| ] |
| |
| [endsect] |