diff --git "a/env-llmeval/lib/python3.10/site-packages/sympy/solvers/ode/ode.py" "b/env-llmeval/lib/python3.10/site-packages/sympy/solvers/ode/ode.py" new file mode 100644--- /dev/null +++ "b/env-llmeval/lib/python3.10/site-packages/sympy/solvers/ode/ode.py" @@ -0,0 +1,3563 @@ +r""" +This module contains :py:meth:`~sympy.solvers.ode.dsolve` and different helper +functions that it uses. + +:py:meth:`~sympy.solvers.ode.dsolve` solves ordinary differential equations. +See the docstring on the various functions for their uses. Note that partial +differential equations support is in ``pde.py``. Note that hint functions +have docstrings describing their various methods, but they are intended for +internal use. Use ``dsolve(ode, func, hint=hint)`` to solve an ODE using a +specific hint. See also the docstring on +:py:meth:`~sympy.solvers.ode.dsolve`. + +**Functions in this module** + + These are the user functions in this module: + + - :py:meth:`~sympy.solvers.ode.dsolve` - Solves ODEs. + - :py:meth:`~sympy.solvers.ode.classify_ode` - Classifies ODEs into + possible hints for :py:meth:`~sympy.solvers.ode.dsolve`. + - :py:meth:`~sympy.solvers.ode.checkodesol` - Checks if an equation is the + solution to an ODE. + - :py:meth:`~sympy.solvers.ode.homogeneous_order` - Returns the + homogeneous order of an expression. + - :py:meth:`~sympy.solvers.ode.infinitesimals` - Returns the infinitesimals + of the Lie group of point transformations of an ODE, such that it is + invariant. + - :py:meth:`~sympy.solvers.ode.checkinfsol` - Checks if the given infinitesimals + are the actual infinitesimals of a first order ODE. + + These are the non-solver helper functions that are for internal use. The + user should use the various options to + :py:meth:`~sympy.solvers.ode.dsolve` to obtain the functionality provided + by these functions: + + - :py:meth:`~sympy.solvers.ode.ode.odesimp` - Does all forms of ODE + simplification. + - :py:meth:`~sympy.solvers.ode.ode.ode_sol_simplicity` - A key function for + comparing solutions by simplicity. + - :py:meth:`~sympy.solvers.ode.constantsimp` - Simplifies arbitrary + constants. + - :py:meth:`~sympy.solvers.ode.ode.constant_renumber` - Renumber arbitrary + constants. + - :py:meth:`~sympy.solvers.ode.ode._handle_Integral` - Evaluate unevaluated + Integrals. + + See also the docstrings of these functions. + +**Currently implemented solver methods** + +The following methods are implemented for solving ordinary differential +equations. See the docstrings of the various hint functions for more +information on each (run ``help(ode)``): + + - 1st order separable differential equations. + - 1st order differential equations whose coefficients or `dx` and `dy` are + functions homogeneous of the same order. + - 1st order exact differential equations. + - 1st order linear differential equations. + - 1st order Bernoulli differential equations. + - Power series solutions for first order differential equations. + - Lie Group method of solving first order differential equations. + - 2nd order Liouville differential equations. + - Power series solutions for second order differential equations + at ordinary and regular singular points. + - `n`\th order differential equation that can be solved with algebraic + rearrangement and integration. + - `n`\th order linear homogeneous differential equation with constant + coefficients. + - `n`\th order linear inhomogeneous differential equation with constant + coefficients using the method of undetermined coefficients. + - `n`\th order linear inhomogeneous differential equation with constant + coefficients using the method of variation of parameters. + +**Philosophy behind this module** + +This module is designed to make it easy to add new ODE solving methods without +having to mess with the solving code for other methods. The idea is that +there is a :py:meth:`~sympy.solvers.ode.classify_ode` function, which takes in +an ODE and tells you what hints, if any, will solve the ODE. It does this +without attempting to solve the ODE, so it is fast. Each solving method is a +hint, and it has its own function, named ``ode_``. That function takes +in the ODE and any match expression gathered by +:py:meth:`~sympy.solvers.ode.classify_ode` and returns a solved result. If +this result has any integrals in it, the hint function will return an +unevaluated :py:class:`~sympy.integrals.integrals.Integral` class. +:py:meth:`~sympy.solvers.ode.dsolve`, which is the user wrapper function +around all of this, will then call :py:meth:`~sympy.solvers.ode.ode.odesimp` on +the result, which, among other things, will attempt to solve the equation for +the dependent variable (the function we are solving for), simplify the +arbitrary constants in the expression, and evaluate any integrals, if the hint +allows it. + +**How to add new solution methods** + +If you have an ODE that you want :py:meth:`~sympy.solvers.ode.dsolve` to be +able to solve, try to avoid adding special case code here. Instead, try +finding a general method that will solve your ODE, as well as others. This +way, the :py:mod:`~sympy.solvers.ode` module will become more robust, and +unhindered by special case hacks. WolphramAlpha and Maple's +DETools[odeadvisor] function are two resources you can use to classify a +specific ODE. It is also better for a method to work with an `n`\th order ODE +instead of only with specific orders, if possible. + +To add a new method, there are a few things that you need to do. First, you +need a hint name for your method. Try to name your hint so that it is +unambiguous with all other methods, including ones that may not be implemented +yet. If your method uses integrals, also include a ``hint_Integral`` hint. +If there is more than one way to solve ODEs with your method, include a hint +for each one, as well as a ``_best`` hint. Your ``ode__best()`` +function should choose the best using min with ``ode_sol_simplicity`` as the +key argument. See +:obj:`~sympy.solvers.ode.single.HomogeneousCoeffBest`, for example. +The function that uses your method will be called ``ode_()``, so the +hint must only use characters that are allowed in a Python function name +(alphanumeric characters and the underscore '``_``' character). Include a +function for every hint, except for ``_Integral`` hints +(:py:meth:`~sympy.solvers.ode.dsolve` takes care of those automatically). +Hint names should be all lowercase, unless a word is commonly capitalized +(such as Integral or Bernoulli). If you have a hint that you do not want to +run with ``all_Integral`` that does not have an ``_Integral`` counterpart (such +as a best hint that would defeat the purpose of ``all_Integral``), you will +need to remove it manually in the :py:meth:`~sympy.solvers.ode.dsolve` code. +See also the :py:meth:`~sympy.solvers.ode.classify_ode` docstring for +guidelines on writing a hint name. + +Determine *in general* how the solutions returned by your method compare with +other methods that can potentially solve the same ODEs. Then, put your hints +in the :py:data:`~sympy.solvers.ode.allhints` tuple in the order that they +should be called. The ordering of this tuple determines which hints are +default. Note that exceptions are ok, because it is easy for the user to +choose individual hints with :py:meth:`~sympy.solvers.ode.dsolve`. In +general, ``_Integral`` variants should go at the end of the list, and +``_best`` variants should go before the various hints they apply to. For +example, the ``undetermined_coefficients`` hint comes before the +``variation_of_parameters`` hint because, even though variation of parameters +is more general than undetermined coefficients, undetermined coefficients +generally returns cleaner results for the ODEs that it can solve than +variation of parameters does, and it does not require integration, so it is +much faster. + +Next, you need to have a match expression or a function that matches the type +of the ODE, which you should put in :py:meth:`~sympy.solvers.ode.classify_ode` +(if the match function is more than just a few lines. It should match the +ODE without solving for it as much as possible, so that +:py:meth:`~sympy.solvers.ode.classify_ode` remains fast and is not hindered by +bugs in solving code. Be sure to consider corner cases. For example, if your +solution method involves dividing by something, make sure you exclude the case +where that division will be 0. + +In most cases, the matching of the ODE will also give you the various parts +that you need to solve it. You should put that in a dictionary (``.match()`` +will do this for you), and add that as ``matching_hints['hint'] = matchdict`` +in the relevant part of :py:meth:`~sympy.solvers.ode.classify_ode`. +:py:meth:`~sympy.solvers.ode.classify_ode` will then send this to +:py:meth:`~sympy.solvers.ode.dsolve`, which will send it to your function as +the ``match`` argument. Your function should be named ``ode_(eq, func, +order, match)`. If you need to send more information, put it in the ``match`` +dictionary. For example, if you had to substitute in a dummy variable in +:py:meth:`~sympy.solvers.ode.classify_ode` to match the ODE, you will need to +pass it to your function using the `match` dict to access it. You can access +the independent variable using ``func.args[0]``, and the dependent variable +(the function you are trying to solve for) as ``func.func``. If, while trying +to solve the ODE, you find that you cannot, raise ``NotImplementedError``. +:py:meth:`~sympy.solvers.ode.dsolve` will catch this error with the ``all`` +meta-hint, rather than causing the whole routine to fail. + +Add a docstring to your function that describes the method employed. Like +with anything else in SymPy, you will need to add a doctest to the docstring, +in addition to real tests in ``test_ode.py``. Try to maintain consistency +with the other hint functions' docstrings. Add your method to the list at the +top of this docstring. Also, add your method to ``ode.rst`` in the +``docs/src`` directory, so that the Sphinx docs will pull its docstring into +the main SymPy documentation. Be sure to make the Sphinx documentation by +running ``make html`` from within the doc directory to verify that the +docstring formats correctly. + +If your solution method involves integrating, use :py:obj:`~.Integral` instead of +:py:meth:`~sympy.core.expr.Expr.integrate`. This allows the user to bypass +hard/slow integration by using the ``_Integral`` variant of your hint. In +most cases, calling :py:meth:`sympy.core.basic.Basic.doit` will integrate your +solution. If this is not the case, you will need to write special code in +:py:meth:`~sympy.solvers.ode.ode._handle_Integral`. Arbitrary constants should be +symbols named ``C1``, ``C2``, and so on. All solution methods should return +an equality instance. If you need an arbitrary number of arbitrary constants, +you can use ``constants = numbered_symbols(prefix='C', cls=Symbol, start=1)``. +If it is possible to solve for the dependent function in a general way, do so. +Otherwise, do as best as you can, but do not call solve in your +``ode_()`` function. :py:meth:`~sympy.solvers.ode.ode.odesimp` will attempt +to solve the solution for you, so you do not need to do that. Lastly, if your +ODE has a common simplification that can be applied to your solutions, you can +add a special case in :py:meth:`~sympy.solvers.ode.ode.odesimp` for it. For +example, solutions returned from the ``1st_homogeneous_coeff`` hints often +have many :obj:`~sympy.functions.elementary.exponential.log` terms, so +:py:meth:`~sympy.solvers.ode.ode.odesimp` calls +:py:meth:`~sympy.simplify.simplify.logcombine` on them (it also helps to write +the arbitrary constant as ``log(C1)`` instead of ``C1`` in this case). Also +consider common ways that you can rearrange your solution to have +:py:meth:`~sympy.solvers.ode.constantsimp` take better advantage of it. It is +better to put simplification in :py:meth:`~sympy.solvers.ode.ode.odesimp` than in +your method, because it can then be turned off with the simplify flag in +:py:meth:`~sympy.solvers.ode.dsolve`. If you have any extraneous +simplification in your function, be sure to only run it using ``if +match.get('simplify', True):``, especially if it can be slow or if it can +reduce the domain of the solution. + +Finally, as with every contribution to SymPy, your method will need to be +tested. Add a test for each method in ``test_ode.py``. Follow the +conventions there, i.e., test the solver using ``dsolve(eq, f(x), +hint=your_hint)``, and also test the solution using +:py:meth:`~sympy.solvers.ode.checkodesol` (you can put these in a separate +tests and skip/XFAIL if it runs too slow/does not work). Be sure to call your +hint specifically in :py:meth:`~sympy.solvers.ode.dsolve`, that way the test +will not be broken simply by the introduction of another matching hint. If your +method works for higher order (>1) ODEs, you will need to run ``sol = +constant_renumber(sol, 'C', 1, order)`` for each solution, where ``order`` is +the order of the ODE. This is because ``constant_renumber`` renumbers the +arbitrary constants by printing order, which is platform dependent. Try to +test every corner case of your solver, including a range of orders if it is a +`n`\th order solver, but if your solver is slow, such as if it involves hard +integration, try to keep the test run time down. + +Feel free to refactor existing hints to avoid duplicating code or creating +inconsistencies. If you can show that your method exactly duplicates an +existing method, including in the simplicity and speed of obtaining the +solutions, then you can remove the old, less general method. The existing +code is tested extensively in ``test_ode.py``, so if anything is broken, one +of those tests will surely fail. + +""" + +from sympy.core import Add, S, Mul, Pow, oo +from sympy.core.containers import Tuple +from sympy.core.expr import AtomicExpr, Expr +from sympy.core.function import (Function, Derivative, AppliedUndef, diff, + expand, expand_mul, Subs) +from sympy.core.multidimensional import vectorize +from sympy.core.numbers import nan, zoo, Number +from sympy.core.relational import Equality, Eq +from sympy.core.sorting import default_sort_key, ordered +from sympy.core.symbol import Symbol, Wild, Dummy, symbols +from sympy.core.sympify import sympify +from sympy.core.traversal import preorder_traversal + +from sympy.logic.boolalg import (BooleanAtom, BooleanTrue, + BooleanFalse) +from sympy.functions import exp, log, sqrt +from sympy.functions.combinatorial.factorials import factorial +from sympy.integrals.integrals import Integral +from sympy.polys import (Poly, terms_gcd, PolynomialError, lcm) +from sympy.polys.polytools import cancel +from sympy.series import Order +from sympy.series.series import series +from sympy.simplify import (collect, logcombine, powsimp, # type: ignore + separatevars, simplify, cse) +from sympy.simplify.radsimp import collect_const +from sympy.solvers import checksol, solve + +from sympy.utilities import numbered_symbols +from sympy.utilities.iterables import uniq, sift, iterable +from sympy.solvers.deutils import _preprocess, ode_order, _desolve + + +#: This is a list of hints in the order that they should be preferred by +#: :py:meth:`~sympy.solvers.ode.classify_ode`. In general, hints earlier in the +#: list should produce simpler solutions than those later in the list (for +#: ODEs that fit both). For now, the order of this list is based on empirical +#: observations by the developers of SymPy. +#: +#: The hint used by :py:meth:`~sympy.solvers.ode.dsolve` for a specific ODE +#: can be overridden (see the docstring). +#: +#: In general, ``_Integral`` hints are grouped at the end of the list, unless +#: there is a method that returns an unevaluable integral most of the time +#: (which go near the end of the list anyway). ``default``, ``all``, +#: ``best``, and ``all_Integral`` meta-hints should not be included in this +#: list, but ``_best`` and ``_Integral`` hints should be included. +allhints = ( + "factorable", + "nth_algebraic", + "separable", + "1st_exact", + "1st_linear", + "Bernoulli", + "1st_rational_riccati", + "Riccati_special_minus2", + "1st_homogeneous_coeff_best", + "1st_homogeneous_coeff_subs_indep_div_dep", + "1st_homogeneous_coeff_subs_dep_div_indep", + "almost_linear", + "linear_coefficients", + "separable_reduced", + "1st_power_series", + "lie_group", + "nth_linear_constant_coeff_homogeneous", + "nth_linear_euler_eq_homogeneous", + "nth_linear_constant_coeff_undetermined_coefficients", + "nth_linear_euler_eq_nonhomogeneous_undetermined_coefficients", + "nth_linear_constant_coeff_variation_of_parameters", + "nth_linear_euler_eq_nonhomogeneous_variation_of_parameters", + "Liouville", + "2nd_linear_airy", + "2nd_linear_bessel", + "2nd_hypergeometric", + "2nd_hypergeometric_Integral", + "nth_order_reducible", + "2nd_power_series_ordinary", + "2nd_power_series_regular", + "nth_algebraic_Integral", + "separable_Integral", + "1st_exact_Integral", + "1st_linear_Integral", + "Bernoulli_Integral", + "1st_homogeneous_coeff_subs_indep_div_dep_Integral", + "1st_homogeneous_coeff_subs_dep_div_indep_Integral", + "almost_linear_Integral", + "linear_coefficients_Integral", + "separable_reduced_Integral", + "nth_linear_constant_coeff_variation_of_parameters_Integral", + "nth_linear_euler_eq_nonhomogeneous_variation_of_parameters_Integral", + "Liouville_Integral", + "2nd_nonlinear_autonomous_conserved", + "2nd_nonlinear_autonomous_conserved_Integral", + ) + + + +def get_numbered_constants(eq, num=1, start=1, prefix='C'): + """ + Returns a list of constants that do not occur + in eq already. + """ + + ncs = iter_numbered_constants(eq, start, prefix) + Cs = [next(ncs) for i in range(num)] + return (Cs[0] if num == 1 else tuple(Cs)) + + +def iter_numbered_constants(eq, start=1, prefix='C'): + """ + Returns an iterator of constants that do not occur + in eq already. + """ + + if isinstance(eq, (Expr, Eq)): + eq = [eq] + elif not iterable(eq): + raise ValueError("Expected Expr or iterable but got %s" % eq) + + atom_set = set().union(*[i.free_symbols for i in eq]) + func_set = set().union(*[i.atoms(Function) for i in eq]) + if func_set: + atom_set |= {Symbol(str(f.func)) for f in func_set} + return numbered_symbols(start=start, prefix=prefix, exclude=atom_set) + + +def dsolve(eq, func=None, hint="default", simplify=True, + ics= None, xi=None, eta=None, x0=0, n=6, **kwargs): + r""" + Solves any (supported) kind of ordinary differential equation and + system of ordinary differential equations. + + For single ordinary differential equation + ========================================= + + It is classified under this when number of equation in ``eq`` is one. + **Usage** + + ``dsolve(eq, f(x), hint)`` -> Solve ordinary differential equation + ``eq`` for function ``f(x)``, using method ``hint``. + + **Details** + + ``eq`` can be any supported ordinary differential equation (see the + :py:mod:`~sympy.solvers.ode` docstring for supported methods). + This can either be an :py:class:`~sympy.core.relational.Equality`, + or an expression, which is assumed to be equal to ``0``. + + ``f(x)`` is a function of one variable whose derivatives in that + variable make up the ordinary differential equation ``eq``. In + many cases it is not necessary to provide this; it will be + autodetected (and an error raised if it could not be detected). + + ``hint`` is the solving method that you want dsolve to use. Use + ``classify_ode(eq, f(x))`` to get all of the possible hints for an + ODE. The default hint, ``default``, will use whatever hint is + returned first by :py:meth:`~sympy.solvers.ode.classify_ode`. See + Hints below for more options that you can use for hint. + + ``simplify`` enables simplification by + :py:meth:`~sympy.solvers.ode.ode.odesimp`. See its docstring for more + information. Turn this off, for example, to disable solving of + solutions for ``func`` or simplification of arbitrary constants. + It will still integrate with this hint. Note that the solution may + contain more arbitrary constants than the order of the ODE with + this option enabled. + + ``xi`` and ``eta`` are the infinitesimal functions of an ordinary + differential equation. They are the infinitesimals of the Lie group + of point transformations for which the differential equation is + invariant. The user can specify values for the infinitesimals. If + nothing is specified, ``xi`` and ``eta`` are calculated using + :py:meth:`~sympy.solvers.ode.infinitesimals` with the help of various + heuristics. + + ``ics`` is the set of initial/boundary conditions for the differential equation. + It should be given in the form of ``{f(x0): x1, f(x).diff(x).subs(x, x2): + x3}`` and so on. For power series solutions, if no initial + conditions are specified ``f(0)`` is assumed to be ``C0`` and the power + series solution is calculated about 0. + + ``x0`` is the point about which the power series solution of a differential + equation is to be evaluated. + + ``n`` gives the exponent of the dependent variable up to which the power series + solution of a differential equation is to be evaluated. + + **Hints** + + Aside from the various solving methods, there are also some meta-hints + that you can pass to :py:meth:`~sympy.solvers.ode.dsolve`: + + ``default``: + This uses whatever hint is returned first by + :py:meth:`~sympy.solvers.ode.classify_ode`. This is the + default argument to :py:meth:`~sympy.solvers.ode.dsolve`. + + ``all``: + To make :py:meth:`~sympy.solvers.ode.dsolve` apply all + relevant classification hints, use ``dsolve(ODE, func, + hint="all")``. This will return a dictionary of + ``hint:solution`` terms. If a hint causes dsolve to raise the + ``NotImplementedError``, value of that hint's key will be the + exception object raised. The dictionary will also include + some special keys: + + - ``order``: The order of the ODE. See also + :py:meth:`~sympy.solvers.deutils.ode_order` in + ``deutils.py``. + - ``best``: The simplest hint; what would be returned by + ``best`` below. + - ``best_hint``: The hint that would produce the solution + given by ``best``. If more than one hint produces the best + solution, the first one in the tuple returned by + :py:meth:`~sympy.solvers.ode.classify_ode` is chosen. + - ``default``: The solution that would be returned by default. + This is the one produced by the hint that appears first in + the tuple returned by + :py:meth:`~sympy.solvers.ode.classify_ode`. + + ``all_Integral``: + This is the same as ``all``, except if a hint also has a + corresponding ``_Integral`` hint, it only returns the + ``_Integral`` hint. This is useful if ``all`` causes + :py:meth:`~sympy.solvers.ode.dsolve` to hang because of a + difficult or impossible integral. This meta-hint will also be + much faster than ``all``, because + :py:meth:`~sympy.core.expr.Expr.integrate` is an expensive + routine. + + ``best``: + To have :py:meth:`~sympy.solvers.ode.dsolve` try all methods + and return the simplest one. This takes into account whether + the solution is solvable in the function, whether it contains + any Integral classes (i.e. unevaluatable integrals), and + which one is the shortest in size. + + See also the :py:meth:`~sympy.solvers.ode.classify_ode` docstring for + more info on hints, and the :py:mod:`~sympy.solvers.ode` docstring for + a list of all supported hints. + + **Tips** + + - You can declare the derivative of an unknown function this way: + + >>> from sympy import Function, Derivative + >>> from sympy.abc import x # x is the independent variable + >>> f = Function("f")(x) # f is a function of x + >>> # f_ will be the derivative of f with respect to x + >>> f_ = Derivative(f, x) + + - See ``test_ode.py`` for many tests, which serves also as a set of + examples for how to use :py:meth:`~sympy.solvers.ode.dsolve`. + - :py:meth:`~sympy.solvers.ode.dsolve` always returns an + :py:class:`~sympy.core.relational.Equality` class (except for the + case when the hint is ``all`` or ``all_Integral``). If possible, it + solves the solution explicitly for the function being solved for. + Otherwise, it returns an implicit solution. + - Arbitrary constants are symbols named ``C1``, ``C2``, and so on. + - Because all solutions should be mathematically equivalent, some + hints may return the exact same result for an ODE. Often, though, + two different hints will return the same solution formatted + differently. The two should be equivalent. Also note that sometimes + the values of the arbitrary constants in two different solutions may + not be the same, because one constant may have "absorbed" other + constants into it. + - Do ``help(ode.ode_)`` to get help more information on a + specific hint, where ```` is the name of a hint without + ``_Integral``. + + For system of ordinary differential equations + ============================================= + + **Usage** + ``dsolve(eq, func)`` -> Solve a system of ordinary differential + equations ``eq`` for ``func`` being list of functions including + `x(t)`, `y(t)`, `z(t)` where number of functions in the list depends + upon the number of equations provided in ``eq``. + + **Details** + + ``eq`` can be any supported system of ordinary differential equations + This can either be an :py:class:`~sympy.core.relational.Equality`, + or an expression, which is assumed to be equal to ``0``. + + ``func`` holds ``x(t)`` and ``y(t)`` being functions of one variable which + together with some of their derivatives make up the system of ordinary + differential equation ``eq``. It is not necessary to provide this; it + will be autodetected (and an error raised if it could not be detected). + + **Hints** + + The hints are formed by parameters returned by classify_sysode, combining + them give hints name used later for forming method name. + + Examples + ======== + + >>> from sympy import Function, dsolve, Eq, Derivative, sin, cos, symbols + >>> from sympy.abc import x + >>> f = Function('f') + >>> dsolve(Derivative(f(x), x, x) + 9*f(x), f(x)) + Eq(f(x), C1*sin(3*x) + C2*cos(3*x)) + + >>> eq = sin(x)*cos(f(x)) + cos(x)*sin(f(x))*f(x).diff(x) + >>> dsolve(eq, hint='1st_exact') + [Eq(f(x), -acos(C1/cos(x)) + 2*pi), Eq(f(x), acos(C1/cos(x)))] + >>> dsolve(eq, hint='almost_linear') + [Eq(f(x), -acos(C1/cos(x)) + 2*pi), Eq(f(x), acos(C1/cos(x)))] + >>> t = symbols('t') + >>> x, y = symbols('x, y', cls=Function) + >>> eq = (Eq(Derivative(x(t),t), 12*t*x(t) + 8*y(t)), Eq(Derivative(y(t),t), 21*x(t) + 7*t*y(t))) + >>> dsolve(eq) + [Eq(x(t), C1*x0(t) + C2*x0(t)*Integral(8*exp(Integral(7*t, t))*exp(Integral(12*t, t))/x0(t)**2, t)), + Eq(y(t), C1*y0(t) + C2*(y0(t)*Integral(8*exp(Integral(7*t, t))*exp(Integral(12*t, t))/x0(t)**2, t) + + exp(Integral(7*t, t))*exp(Integral(12*t, t))/x0(t)))] + >>> eq = (Eq(Derivative(x(t),t),x(t)*y(t)*sin(t)), Eq(Derivative(y(t),t),y(t)**2*sin(t))) + >>> dsolve(eq) + {Eq(x(t), -exp(C1)/(C2*exp(C1) - cos(t))), Eq(y(t), -1/(C1 - cos(t)))} + """ + if iterable(eq): + from sympy.solvers.ode.systems import dsolve_system + + # This may have to be changed in future + # when we have weakly and strongly + # connected components. This have to + # changed to show the systems that haven't + # been solved. + try: + sol = dsolve_system(eq, funcs=func, ics=ics, doit=True) + return sol[0] if len(sol) == 1 else sol + except NotImplementedError: + pass + + match = classify_sysode(eq, func) + + eq = match['eq'] + order = match['order'] + func = match['func'] + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + + # keep highest order term coefficient positive + for i in range(len(eq)): + for func_ in func: + if isinstance(func_, list): + pass + else: + if eq[i].coeff(diff(func[i],t,ode_order(eq[i], func[i]))).is_negative: + eq[i] = -eq[i] + match['eq'] = eq + if len(set(order.values()))!=1: + raise ValueError("It solves only those systems of equations whose orders are equal") + match['order'] = list(order.values())[0] + def recur_len(l): + return sum(recur_len(item) if isinstance(item,list) else 1 for item in l) + if recur_len(func) != len(eq): + raise ValueError("dsolve() and classify_sysode() work with " + "number of functions being equal to number of equations") + if match['type_of_equation'] is None: + raise NotImplementedError + else: + if match['is_linear'] == True: + solvefunc = globals()['sysode_linear_%(no_of_equation)seq_order%(order)s' % match] + else: + solvefunc = globals()['sysode_nonlinear_%(no_of_equation)seq_order%(order)s' % match] + sols = solvefunc(match) + if ics: + constants = Tuple(*sols).free_symbols - Tuple(*eq).free_symbols + solved_constants = solve_ics(sols, func, constants, ics) + return [sol.subs(solved_constants) for sol in sols] + return sols + else: + given_hint = hint # hint given by the user + + # See the docstring of _desolve for more details. + hints = _desolve(eq, func=func, + hint=hint, simplify=True, xi=xi, eta=eta, type='ode', ics=ics, + x0=x0, n=n, **kwargs) + eq = hints.pop('eq', eq) + all_ = hints.pop('all', False) + if all_: + retdict = {} + failed_hints = {} + gethints = classify_ode(eq, dict=True, hint='all') + orderedhints = gethints['ordered_hints'] + for hint in hints: + try: + rv = _helper_simplify(eq, hint, hints[hint], simplify) + except NotImplementedError as detail: + failed_hints[hint] = detail + else: + retdict[hint] = rv + func = hints[hint]['func'] + + retdict['best'] = min(list(retdict.values()), key=lambda x: + ode_sol_simplicity(x, func, trysolving=not simplify)) + if given_hint == 'best': + return retdict['best'] + for i in orderedhints: + if retdict['best'] == retdict.get(i, None): + retdict['best_hint'] = i + break + retdict['default'] = gethints['default'] + retdict['order'] = gethints['order'] + retdict.update(failed_hints) + return retdict + + else: + # The key 'hint' stores the hint needed to be solved for. + hint = hints['hint'] + return _helper_simplify(eq, hint, hints, simplify, ics=ics) + +def _helper_simplify(eq, hint, match, simplify=True, ics=None, **kwargs): + r""" + Helper function of dsolve that calls the respective + :py:mod:`~sympy.solvers.ode` functions to solve for the ordinary + differential equations. This minimizes the computation in calling + :py:meth:`~sympy.solvers.deutils._desolve` multiple times. + """ + r = match + func = r['func'] + order = r['order'] + match = r[hint] + + if isinstance(match, SingleODESolver): + solvefunc = match + elif hint.endswith('_Integral'): + solvefunc = globals()['ode_' + hint[:-len('_Integral')]] + else: + solvefunc = globals()['ode_' + hint] + + free = eq.free_symbols + cons = lambda s: s.free_symbols.difference(free) + + if simplify: + # odesimp() will attempt to integrate, if necessary, apply constantsimp(), + # attempt to solve for func, and apply any other hint specific + # simplifications + if isinstance(solvefunc, SingleODESolver): + sols = solvefunc.get_general_solution() + else: + sols = solvefunc(eq, func, order, match) + if iterable(sols): + rv = [odesimp(eq, s, func, hint) for s in sols] + else: + rv = odesimp(eq, sols, func, hint) + else: + # We still want to integrate (you can disable it separately with the hint) + if isinstance(solvefunc, SingleODESolver): + exprs = solvefunc.get_general_solution(simplify=False) + else: + match['simplify'] = False # Some hints can take advantage of this option + exprs = solvefunc(eq, func, order, match) + if isinstance(exprs, list): + rv = [_handle_Integral(expr, func, hint) for expr in exprs] + else: + rv = _handle_Integral(exprs, func, hint) + + if isinstance(rv, list): + if simplify: + rv = _remove_redundant_solutions(eq, rv, order, func.args[0]) + if len(rv) == 1: + rv = rv[0] + if ics and 'power_series' not in hint: + if isinstance(rv, (Expr, Eq)): + solved_constants = solve_ics([rv], [r['func']], cons(rv), ics) + rv = rv.subs(solved_constants) + else: + rv1 = [] + for s in rv: + try: + solved_constants = solve_ics([s], [r['func']], cons(s), ics) + except ValueError: + continue + rv1.append(s.subs(solved_constants)) + if len(rv1) == 1: + return rv1[0] + rv = rv1 + return rv + +def solve_ics(sols, funcs, constants, ics): + """ + Solve for the constants given initial conditions + + ``sols`` is a list of solutions. + + ``funcs`` is a list of functions. + + ``constants`` is a list of constants. + + ``ics`` is the set of initial/boundary conditions for the differential + equation. It should be given in the form of ``{f(x0): x1, + f(x).diff(x).subs(x, x2): x3}`` and so on. + + Returns a dictionary mapping constants to values. + ``solution.subs(constants)`` will replace the constants in ``solution``. + + Example + ======= + >>> # From dsolve(f(x).diff(x) - f(x), f(x)) + >>> from sympy import symbols, Eq, exp, Function + >>> from sympy.solvers.ode.ode import solve_ics + >>> f = Function('f') + >>> x, C1 = symbols('x C1') + >>> sols = [Eq(f(x), C1*exp(x))] + >>> funcs = [f(x)] + >>> constants = [C1] + >>> ics = {f(0): 2} + >>> solved_constants = solve_ics(sols, funcs, constants, ics) + >>> solved_constants + {C1: 2} + >>> sols[0].subs(solved_constants) + Eq(f(x), 2*exp(x)) + + """ + # Assume ics are of the form f(x0): value or Subs(diff(f(x), x, n), (x, + # x0)): value (currently checked by classify_ode). To solve, replace x + # with x0, f(x0) with value, then solve for constants. For f^(n)(x0), + # differentiate the solution n times, so that f^(n)(x) appears. + x = funcs[0].args[0] + diff_sols = [] + subs_sols = [] + diff_variables = set() + for funcarg, value in ics.items(): + if isinstance(funcarg, AppliedUndef): + x0 = funcarg.args[0] + matching_func = [f for f in funcs if f.func == funcarg.func][0] + S = sols + elif isinstance(funcarg, (Subs, Derivative)): + if isinstance(funcarg, Subs): + # Make sure it stays a subs. Otherwise subs below will produce + # a different looking term. + funcarg = funcarg.doit() + if isinstance(funcarg, Subs): + deriv = funcarg.expr + x0 = funcarg.point[0] + variables = funcarg.expr.variables + matching_func = deriv + elif isinstance(funcarg, Derivative): + deriv = funcarg + x0 = funcarg.variables[0] + variables = (x,)*len(funcarg.variables) + matching_func = deriv.subs(x0, x) + for sol in sols: + if sol.has(deriv.expr.func): + diff_sols.append(Eq(sol.lhs.diff(*variables), sol.rhs.diff(*variables))) + diff_variables.add(variables) + S = diff_sols + else: + raise NotImplementedError("Unrecognized initial condition") + + for sol in S: + if sol.has(matching_func): + sol2 = sol + sol2 = sol2.subs(x, x0) + sol2 = sol2.subs(funcarg, value) + # This check is necessary because of issue #15724 + if not isinstance(sol2, BooleanAtom) or not subs_sols: + subs_sols = [s for s in subs_sols if not isinstance(s, BooleanAtom)] + subs_sols.append(sol2) + + # TODO: Use solveset here + try: + solved_constants = solve(subs_sols, constants, dict=True) + except NotImplementedError: + solved_constants = [] + + # XXX: We can't differentiate between the solution not existing because of + # invalid initial conditions, and not existing because solve is not smart + # enough. If we could use solveset, this might be improvable, but for now, + # we use NotImplementedError in this case. + if not solved_constants: + raise ValueError("Couldn't solve for initial conditions") + + if solved_constants == True: + raise ValueError("Initial conditions did not produce any solutions for constants. Perhaps they are degenerate.") + + if len(solved_constants) > 1: + raise NotImplementedError("Initial conditions produced too many solutions for constants") + + return solved_constants[0] + +def classify_ode(eq, func=None, dict=False, ics=None, *, prep=True, xi=None, eta=None, n=None, **kwargs): + r""" + Returns a tuple of possible :py:meth:`~sympy.solvers.ode.dsolve` + classifications for an ODE. + + The tuple is ordered so that first item is the classification that + :py:meth:`~sympy.solvers.ode.dsolve` uses to solve the ODE by default. In + general, classifications at the near the beginning of the list will + produce better solutions faster than those near the end, thought there are + always exceptions. To make :py:meth:`~sympy.solvers.ode.dsolve` use a + different classification, use ``dsolve(ODE, func, + hint=)``. See also the + :py:meth:`~sympy.solvers.ode.dsolve` docstring for different meta-hints + you can use. + + If ``dict`` is true, :py:meth:`~sympy.solvers.ode.classify_ode` will + return a dictionary of ``hint:match`` expression terms. This is intended + for internal use by :py:meth:`~sympy.solvers.ode.dsolve`. Note that + because dictionaries are ordered arbitrarily, this will most likely not be + in the same order as the tuple. + + You can get help on different hints by executing + ``help(ode.ode_hintname)``, where ``hintname`` is the name of the hint + without ``_Integral``. + + See :py:data:`~sympy.solvers.ode.allhints` or the + :py:mod:`~sympy.solvers.ode` docstring for a list of all supported hints + that can be returned from :py:meth:`~sympy.solvers.ode.classify_ode`. + + Notes + ===== + + These are remarks on hint names. + + ``_Integral`` + + If a classification has ``_Integral`` at the end, it will return the + expression with an unevaluated :py:class:`~.Integral` + class in it. Note that a hint may do this anyway if + :py:meth:`~sympy.core.expr.Expr.integrate` cannot do the integral, + though just using an ``_Integral`` will do so much faster. Indeed, an + ``_Integral`` hint will always be faster than its corresponding hint + without ``_Integral`` because + :py:meth:`~sympy.core.expr.Expr.integrate` is an expensive routine. + If :py:meth:`~sympy.solvers.ode.dsolve` hangs, it is probably because + :py:meth:`~sympy.core.expr.Expr.integrate` is hanging on a tough or + impossible integral. Try using an ``_Integral`` hint or + ``all_Integral`` to get it return something. + + Note that some hints do not have ``_Integral`` counterparts. This is + because :py:func:`~sympy.integrals.integrals.integrate` is not used in + solving the ODE for those method. For example, `n`\th order linear + homogeneous ODEs with constant coefficients do not require integration + to solve, so there is no + ``nth_linear_homogeneous_constant_coeff_Integrate`` hint. You can + easily evaluate any unevaluated + :py:class:`~sympy.integrals.integrals.Integral`\s in an expression by + doing ``expr.doit()``. + + Ordinals + + Some hints contain an ordinal such as ``1st_linear``. This is to help + differentiate them from other hints, as well as from other methods + that may not be implemented yet. If a hint has ``nth`` in it, such as + the ``nth_linear`` hints, this means that the method used to applies + to ODEs of any order. + + ``indep`` and ``dep`` + + Some hints contain the words ``indep`` or ``dep``. These reference + the independent variable and the dependent function, respectively. For + example, if an ODE is in terms of `f(x)`, then ``indep`` will refer to + `x` and ``dep`` will refer to `f`. + + ``subs`` + + If a hints has the word ``subs`` in it, it means that the ODE is solved + by substituting the expression given after the word ``subs`` for a + single dummy variable. This is usually in terms of ``indep`` and + ``dep`` as above. The substituted expression will be written only in + characters allowed for names of Python objects, meaning operators will + be spelled out. For example, ``indep``/``dep`` will be written as + ``indep_div_dep``. + + ``coeff`` + + The word ``coeff`` in a hint refers to the coefficients of something + in the ODE, usually of the derivative terms. See the docstring for + the individual methods for more info (``help(ode)``). This is + contrast to ``coefficients``, as in ``undetermined_coefficients``, + which refers to the common name of a method. + + ``_best`` + + Methods that have more than one fundamental way to solve will have a + hint for each sub-method and a ``_best`` meta-classification. This + will evaluate all hints and return the best, using the same + considerations as the normal ``best`` meta-hint. + + + Examples + ======== + + >>> from sympy import Function, classify_ode, Eq + >>> from sympy.abc import x + >>> f = Function('f') + >>> classify_ode(Eq(f(x).diff(x), 0), f(x)) + ('nth_algebraic', + 'separable', + '1st_exact', + '1st_linear', + 'Bernoulli', + '1st_homogeneous_coeff_best', + '1st_homogeneous_coeff_subs_indep_div_dep', + '1st_homogeneous_coeff_subs_dep_div_indep', + '1st_power_series', 'lie_group', 'nth_linear_constant_coeff_homogeneous', + 'nth_linear_euler_eq_homogeneous', + 'nth_algebraic_Integral', 'separable_Integral', '1st_exact_Integral', + '1st_linear_Integral', 'Bernoulli_Integral', + '1st_homogeneous_coeff_subs_indep_div_dep_Integral', + '1st_homogeneous_coeff_subs_dep_div_indep_Integral') + >>> classify_ode(f(x).diff(x, 2) + 3*f(x).diff(x) + 2*f(x) - 4) + ('factorable', 'nth_linear_constant_coeff_undetermined_coefficients', + 'nth_linear_constant_coeff_variation_of_parameters', + 'nth_linear_constant_coeff_variation_of_parameters_Integral') + + """ + ics = sympify(ics) + + if func and len(func.args) != 1: + raise ValueError("dsolve() and classify_ode() only " + "work with functions of one variable, not %s" % func) + + if isinstance(eq, Equality): + eq = eq.lhs - eq.rhs + + # Some methods want the unprocessed equation + eq_orig = eq + + if prep or func is None: + eq, func_ = _preprocess(eq, func) + if func is None: + func = func_ + x = func.args[0] + f = func.func + y = Dummy('y') + terms = 5 if n is None else n + + order = ode_order(eq, f(x)) + # hint:matchdict or hint:(tuple of matchdicts) + # Also will contain "default": and "order":order items. + matching_hints = {"order": order} + + df = f(x).diff(x) + a = Wild('a', exclude=[f(x)]) + d = Wild('d', exclude=[df, f(x).diff(x, 2)]) + e = Wild('e', exclude=[df]) + n = Wild('n', exclude=[x, f(x), df]) + c1 = Wild('c1', exclude=[x]) + a3 = Wild('a3', exclude=[f(x), df, f(x).diff(x, 2)]) + b3 = Wild('b3', exclude=[f(x), df, f(x).diff(x, 2)]) + c3 = Wild('c3', exclude=[f(x), df, f(x).diff(x, 2)]) + boundary = {} # Used to extract initial conditions + C1 = Symbol("C1") + + # Preprocessing to get the initial conditions out + if ics is not None: + for funcarg in ics: + # Separating derivatives + if isinstance(funcarg, (Subs, Derivative)): + # f(x).diff(x).subs(x, 0) is a Subs, but f(x).diff(x).subs(x, + # y) is a Derivative + if isinstance(funcarg, Subs): + deriv = funcarg.expr + old = funcarg.variables[0] + new = funcarg.point[0] + elif isinstance(funcarg, Derivative): + deriv = funcarg + # No information on this. Just assume it was x + old = x + new = funcarg.variables[0] + + if (isinstance(deriv, Derivative) and isinstance(deriv.args[0], + AppliedUndef) and deriv.args[0].func == f and + len(deriv.args[0].args) == 1 and old == x and not + new.has(x) and all(i == deriv.variables[0] for i in + deriv.variables) and x not in ics[funcarg].free_symbols): + + dorder = ode_order(deriv, x) + temp = 'f' + str(dorder) + boundary.update({temp: new, temp + 'val': ics[funcarg]}) + else: + raise ValueError("Invalid boundary conditions for Derivatives") + + + # Separating functions + elif isinstance(funcarg, AppliedUndef): + if (funcarg.func == f and len(funcarg.args) == 1 and + not funcarg.args[0].has(x) and x not in ics[funcarg].free_symbols): + boundary.update({'f0': funcarg.args[0], 'f0val': ics[funcarg]}) + else: + raise ValueError("Invalid boundary conditions for Function") + + else: + raise ValueError("Enter boundary conditions of the form ics={f(point): value, f(x).diff(x, order).subs(x, point): value}") + + ode = SingleODEProblem(eq_orig, func, x, prep=prep, xi=xi, eta=eta) + user_hint = kwargs.get('hint', 'default') + # Used when dsolve is called without an explicit hint. + # We exit early to return the first valid match + early_exit = (user_hint=='default') + if user_hint.endswith('_Integral'): + user_hint = user_hint[:-len('_Integral')] + user_map = solver_map + # An explicit hint has been given to dsolve + # Skip matching code for other hints + if user_hint not in ['default', 'all', 'all_Integral', 'best'] and user_hint in solver_map: + user_map = {user_hint: solver_map[user_hint]} + + for hint in user_map: + solver = user_map[hint](ode) + if solver.matches(): + matching_hints[hint] = solver + if user_map[hint].has_integral: + matching_hints[hint + "_Integral"] = solver + if dict and early_exit: + matching_hints["default"] = hint + return matching_hints + + eq = expand(eq) + # Precondition to try remove f(x) from highest order derivative + reduced_eq = None + if eq.is_Add: + deriv_coef = eq.coeff(f(x).diff(x, order)) + if deriv_coef not in (1, 0): + r = deriv_coef.match(a*f(x)**c1) + if r and r[c1]: + den = f(x)**r[c1] + reduced_eq = Add(*[arg/den for arg in eq.args]) + if not reduced_eq: + reduced_eq = eq + + if order == 1: + + # NON-REDUCED FORM OF EQUATION matches + r = collect(eq, df, exact=True).match(d + e * df) + if r: + r['d'] = d + r['e'] = e + r['y'] = y + r[d] = r[d].subs(f(x), y) + r[e] = r[e].subs(f(x), y) + + # FIRST ORDER POWER SERIES WHICH NEEDS INITIAL CONDITIONS + # TODO: Hint first order series should match only if d/e is analytic. + # For now, only d/e and (d/e).diff(arg) is checked for existence at + # at a given point. + # This is currently done internally in ode_1st_power_series. + point = boundary.get('f0', 0) + value = boundary.get('f0val', C1) + check = cancel(r[d]/r[e]) + check1 = check.subs({x: point, y: value}) + if not check1.has(oo) and not check1.has(zoo) and \ + not check1.has(nan) and not check1.has(-oo): + check2 = (check1.diff(x)).subs({x: point, y: value}) + if not check2.has(oo) and not check2.has(zoo) and \ + not check2.has(nan) and not check2.has(-oo): + rseries = r.copy() + rseries.update({'terms': terms, 'f0': point, 'f0val': value}) + matching_hints["1st_power_series"] = rseries + + elif order == 2: + # Homogeneous second order differential equation of the form + # a3*f(x).diff(x, 2) + b3*f(x).diff(x) + c3 + # It has a definite power series solution at point x0 if, b3/a3 and c3/a3 + # are analytic at x0. + deq = a3*(f(x).diff(x, 2)) + b3*df + c3*f(x) + r = collect(reduced_eq, + [f(x).diff(x, 2), f(x).diff(x), f(x)]).match(deq) + ordinary = False + if r: + if not all(r[key].is_polynomial() for key in r): + n, d = reduced_eq.as_numer_denom() + reduced_eq = expand(n) + r = collect(reduced_eq, + [f(x).diff(x, 2), f(x).diff(x), f(x)]).match(deq) + if r and r[a3] != 0: + p = cancel(r[b3]/r[a3]) # Used below + q = cancel(r[c3]/r[a3]) # Used below + point = kwargs.get('x0', 0) + check = p.subs(x, point) + if not check.has(oo, nan, zoo, -oo): + check = q.subs(x, point) + if not check.has(oo, nan, zoo, -oo): + ordinary = True + r.update({'a3': a3, 'b3': b3, 'c3': c3, 'x0': point, 'terms': terms}) + matching_hints["2nd_power_series_ordinary"] = r + + # Checking if the differential equation has a regular singular point + # at x0. It has a regular singular point at x0, if (b3/a3)*(x - x0) + # and (c3/a3)*((x - x0)**2) are analytic at x0. + if not ordinary: + p = cancel((x - point)*p) + check = p.subs(x, point) + if not check.has(oo, nan, zoo, -oo): + q = cancel(((x - point)**2)*q) + check = q.subs(x, point) + if not check.has(oo, nan, zoo, -oo): + coeff_dict = {'p': p, 'q': q, 'x0': point, 'terms': terms} + matching_hints["2nd_power_series_regular"] = coeff_dict + + + # Order keys based on allhints. + retlist = [i for i in allhints if i in matching_hints] + if dict: + # Dictionaries are ordered arbitrarily, so make note of which + # hint would come first for dsolve(). Use an ordered dict in Py 3. + matching_hints["default"] = retlist[0] if retlist else None + matching_hints["ordered_hints"] = tuple(retlist) + return matching_hints + else: + return tuple(retlist) + + +def classify_sysode(eq, funcs=None, **kwargs): + r""" + Returns a dictionary of parameter names and values that define the system + of ordinary differential equations in ``eq``. + The parameters are further used in + :py:meth:`~sympy.solvers.ode.dsolve` for solving that system. + + Some parameter names and values are: + + 'is_linear' (boolean), which tells whether the given system is linear. + Note that "linear" here refers to the operator: terms such as ``x*diff(x,t)`` are + nonlinear, whereas terms like ``sin(t)*diff(x,t)`` are still linear operators. + + 'func' (list) contains the :py:class:`~sympy.core.function.Function`s that + appear with a derivative in the ODE, i.e. those that we are trying to solve + the ODE for. + + 'order' (dict) with the maximum derivative for each element of the 'func' + parameter. + + 'func_coeff' (dict or Matrix) with the coefficient for each triple ``(equation number, + function, order)```. The coefficients are those subexpressions that do not + appear in 'func', and hence can be considered constant for purposes of ODE + solving. The value of this parameter can also be a Matrix if the system of ODEs are + linear first order of the form X' = AX where X is the vector of dependent variables. + Here, this function returns the coefficient matrix A. + + 'eq' (list) with the equations from ``eq``, sympified and transformed into + expressions (we are solving for these expressions to be zero). + + 'no_of_equations' (int) is the number of equations (same as ``len(eq)``). + + 'type_of_equation' (string) is an internal classification of the type of + ODE. + + 'is_constant' (boolean), which tells if the system of ODEs is constant coefficient + or not. This key is temporary addition for now and is in the match dict only when + the system of ODEs is linear first order constant coefficient homogeneous. So, this + key's value is True for now if it is available else it does not exist. + + 'is_homogeneous' (boolean), which tells if the system of ODEs is homogeneous. Like the + key 'is_constant', this key is a temporary addition and it is True since this key value + is available only when the system is linear first order constant coefficient homogeneous. + + References + ========== + -https://eqworld.ipmnet.ru/en/solutions/sysode/sode-toc1.htm + -A. D. Polyanin and A. V. Manzhirov, Handbook of Mathematics for Engineers and Scientists + + Examples + ======== + + >>> from sympy import Function, Eq, symbols, diff + >>> from sympy.solvers.ode.ode import classify_sysode + >>> from sympy.abc import t + >>> f, x, y = symbols('f, x, y', cls=Function) + >>> k, l, m, n = symbols('k, l, m, n', Integer=True) + >>> x1 = diff(x(t), t) ; y1 = diff(y(t), t) + >>> x2 = diff(x(t), t, t) ; y2 = diff(y(t), t, t) + >>> eq = (Eq(x1, 12*x(t) - 6*y(t)), Eq(y1, 11*x(t) + 3*y(t))) + >>> classify_sysode(eq) + {'eq': [-12*x(t) + 6*y(t) + Derivative(x(t), t), -11*x(t) - 3*y(t) + Derivative(y(t), t)], 'func': [x(t), y(t)], + 'func_coeff': {(0, x(t), 0): -12, (0, x(t), 1): 1, (0, y(t), 0): 6, (0, y(t), 1): 0, (1, x(t), 0): -11, (1, x(t), 1): 0, (1, y(t), 0): -3, (1, y(t), 1): 1}, 'is_linear': True, 'no_of_equation': 2, 'order': {x(t): 1, y(t): 1}, 'type_of_equation': None} + >>> eq = (Eq(diff(x(t),t), 5*t*x(t) + t**2*y(t) + 2), Eq(diff(y(t),t), -t**2*x(t) + 5*t*y(t))) + >>> classify_sysode(eq) + {'eq': [-t**2*y(t) - 5*t*x(t) + Derivative(x(t), t) - 2, t**2*x(t) - 5*t*y(t) + Derivative(y(t), t)], + 'func': [x(t), y(t)], 'func_coeff': {(0, x(t), 0): -5*t, (0, x(t), 1): 1, (0, y(t), 0): -t**2, (0, y(t), 1): 0, + (1, x(t), 0): t**2, (1, x(t), 1): 0, (1, y(t), 0): -5*t, (1, y(t), 1): 1}, 'is_linear': True, 'no_of_equation': 2, + 'order': {x(t): 1, y(t): 1}, 'type_of_equation': None} + + """ + + # Sympify equations and convert iterables of equations into + # a list of equations + def _sympify(eq): + return list(map(sympify, eq if iterable(eq) else [eq])) + + eq, funcs = (_sympify(w) for w in [eq, funcs]) + for i, fi in enumerate(eq): + if isinstance(fi, Equality): + eq[i] = fi.lhs - fi.rhs + + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + matching_hints = {"no_of_equation":i+1} + matching_hints['eq'] = eq + if i==0: + raise ValueError("classify_sysode() works for systems of ODEs. " + "For scalar ODEs, classify_ode should be used") + + # find all the functions if not given + order = {} + if funcs==[None]: + funcs = _extract_funcs(eq) + + funcs = list(set(funcs)) + if len(funcs) != len(eq): + raise ValueError("Number of functions given is not equal to the number of equations %s" % funcs) + + # This logic of list of lists in funcs to + # be replaced later. + func_dict = {} + for func in funcs: + if not order.get(func, False): + max_order = 0 + for i, eqs_ in enumerate(eq): + order_ = ode_order(eqs_,func) + if max_order < order_: + max_order = order_ + eq_no = i + if eq_no in func_dict: + func_dict[eq_no] = [func_dict[eq_no], func] + else: + func_dict[eq_no] = func + order[func] = max_order + + funcs = [func_dict[i] for i in range(len(func_dict))] + matching_hints['func'] = funcs + for func in funcs: + if isinstance(func, list): + for func_elem in func: + if len(func_elem.args) != 1: + raise ValueError("dsolve() and classify_sysode() work with " + "functions of one variable only, not %s" % func) + else: + if func and len(func.args) != 1: + raise ValueError("dsolve() and classify_sysode() work with " + "functions of one variable only, not %s" % func) + + # find the order of all equation in system of odes + matching_hints["order"] = order + + # find coefficients of terms f(t), diff(f(t),t) and higher derivatives + # and similarly for other functions g(t), diff(g(t),t) in all equations. + # Here j denotes the equation number, funcs[l] denotes the function about + # which we are talking about and k denotes the order of function funcs[l] + # whose coefficient we are calculating. + def linearity_check(eqs, j, func, is_linear_): + for k in range(order[func] + 1): + func_coef[j, func, k] = collect(eqs.expand(), [diff(func, t, k)]).coeff(diff(func, t, k)) + if is_linear_ == True: + if func_coef[j, func, k] == 0: + if k == 0: + coef = eqs.as_independent(func, as_Add=True)[1] + for xr in range(1, ode_order(eqs,func) + 1): + coef -= eqs.as_independent(diff(func, t, xr), as_Add=True)[1] + if coef != 0: + is_linear_ = False + else: + if eqs.as_independent(diff(func, t, k), as_Add=True)[1]: + is_linear_ = False + else: + for func_ in funcs: + if isinstance(func_, list): + for elem_func_ in func_: + dep = func_coef[j, func, k].as_independent(elem_func_, as_Add=True)[1] + if dep != 0: + is_linear_ = False + else: + dep = func_coef[j, func, k].as_independent(func_, as_Add=True)[1] + if dep != 0: + is_linear_ = False + return is_linear_ + + func_coef = {} + is_linear = True + for j, eqs in enumerate(eq): + for func in funcs: + if isinstance(func, list): + for func_elem in func: + is_linear = linearity_check(eqs, j, func_elem, is_linear) + else: + is_linear = linearity_check(eqs, j, func, is_linear) + matching_hints['func_coeff'] = func_coef + matching_hints['is_linear'] = is_linear + + + if len(set(order.values())) == 1: + order_eq = list(matching_hints['order'].values())[0] + if matching_hints['is_linear'] == True: + if matching_hints['no_of_equation'] == 2: + if order_eq == 1: + type_of_equation = check_linear_2eq_order1(eq, funcs, func_coef) + else: + type_of_equation = None + # If the equation does not match up with any of the + # general case solvers in systems.py and the number + # of equations is greater than 2, then NotImplementedError + # should be raised. + else: + type_of_equation = None + + else: + if matching_hints['no_of_equation'] == 2: + if order_eq == 1: + type_of_equation = check_nonlinear_2eq_order1(eq, funcs, func_coef) + else: + type_of_equation = None + elif matching_hints['no_of_equation'] == 3: + if order_eq == 1: + type_of_equation = check_nonlinear_3eq_order1(eq, funcs, func_coef) + else: + type_of_equation = None + else: + type_of_equation = None + else: + type_of_equation = None + + matching_hints['type_of_equation'] = type_of_equation + + return matching_hints + + +def check_linear_2eq_order1(eq, func, func_coef): + x = func[0].func + y = func[1].func + fc = func_coef + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + r = {} + # for equations Eq(a1*diff(x(t),t), b1*x(t) + c1*y(t) + d1) + # and Eq(a2*diff(y(t),t), b2*x(t) + c2*y(t) + d2) + r['a1'] = fc[0,x(t),1] ; r['a2'] = fc[1,y(t),1] + r['b1'] = -fc[0,x(t),0]/fc[0,x(t),1] ; r['b2'] = -fc[1,x(t),0]/fc[1,y(t),1] + r['c1'] = -fc[0,y(t),0]/fc[0,x(t),1] ; r['c2'] = -fc[1,y(t),0]/fc[1,y(t),1] + forcing = [S.Zero,S.Zero] + for i in range(2): + for j in Add.make_args(eq[i]): + if not j.has(x(t), y(t)): + forcing[i] += j + if not (forcing[0].has(t) or forcing[1].has(t)): + # We can handle homogeneous case and simple constant forcings + r['d1'] = forcing[0] + r['d2'] = forcing[1] + else: + # Issue #9244: nonhomogeneous linear systems are not supported + return None + + # Conditions to check for type 6 whose equations are Eq(diff(x(t),t), f(t)*x(t) + g(t)*y(t)) and + # Eq(diff(y(t),t), a*[f(t) + a*h(t)]x(t) + a*[g(t) - h(t)]*y(t)) + p = 0 + q = 0 + p1 = cancel(r['b2']/(cancel(r['b2']/r['c2']).as_numer_denom()[0])) + p2 = cancel(r['b1']/(cancel(r['b1']/r['c1']).as_numer_denom()[0])) + for n, i in enumerate([p1, p2]): + for j in Mul.make_args(collect_const(i)): + if not j.has(t): + q = j + if q and n==0: + if ((r['b2']/j - r['b1'])/(r['c1'] - r['c2']/j)) == j: + p = 1 + elif q and n==1: + if ((r['b1']/j - r['b2'])/(r['c2'] - r['c1']/j)) == j: + p = 2 + # End of condition for type 6 + + if r['d1']!=0 or r['d2']!=0: + return None + else: + if not any(r[k].has(t) for k in 'a1 a2 b1 b2 c1 c2'.split()): + return None + else: + r['b1'] = r['b1']/r['a1'] ; r['b2'] = r['b2']/r['a2'] + r['c1'] = r['c1']/r['a1'] ; r['c2'] = r['c2']/r['a2'] + if p: + return "type6" + else: + # Equations for type 7 are Eq(diff(x(t),t), f(t)*x(t) + g(t)*y(t)) and Eq(diff(y(t),t), h(t)*x(t) + p(t)*y(t)) + return "type7" +def check_nonlinear_2eq_order1(eq, func, func_coef): + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + f = Wild('f') + g = Wild('g') + u, v = symbols('u, v', cls=Dummy) + def check_type(x, y): + r1 = eq[0].match(t*diff(x(t),t) - x(t) + f) + r2 = eq[1].match(t*diff(y(t),t) - y(t) + g) + if not (r1 and r2): + r1 = eq[0].match(diff(x(t),t) - x(t)/t + f/t) + r2 = eq[1].match(diff(y(t),t) - y(t)/t + g/t) + if not (r1 and r2): + r1 = (-eq[0]).match(t*diff(x(t),t) - x(t) + f) + r2 = (-eq[1]).match(t*diff(y(t),t) - y(t) + g) + if not (r1 and r2): + r1 = (-eq[0]).match(diff(x(t),t) - x(t)/t + f/t) + r2 = (-eq[1]).match(diff(y(t),t) - y(t)/t + g/t) + if r1 and r2 and not (r1[f].subs(diff(x(t),t),u).subs(diff(y(t),t),v).has(t) \ + or r2[g].subs(diff(x(t),t),u).subs(diff(y(t),t),v).has(t)): + return 'type5' + else: + return None + for func_ in func: + if isinstance(func_, list): + x = func[0][0].func + y = func[0][1].func + eq_type = check_type(x, y) + if not eq_type: + eq_type = check_type(y, x) + return eq_type + x = func[0].func + y = func[1].func + fc = func_coef + n = Wild('n', exclude=[x(t),y(t)]) + f1 = Wild('f1', exclude=[v,t]) + f2 = Wild('f2', exclude=[v,t]) + g1 = Wild('g1', exclude=[u,t]) + g2 = Wild('g2', exclude=[u,t]) + for i in range(2): + eqs = 0 + for terms in Add.make_args(eq[i]): + eqs += terms/fc[i,func[i],1] + eq[i] = eqs + r = eq[0].match(diff(x(t),t) - x(t)**n*f) + if r: + g = (diff(y(t),t) - eq[1])/r[f] + if r and not (g.has(x(t)) or g.subs(y(t),v).has(t) or r[f].subs(x(t),u).subs(y(t),v).has(t)): + return 'type1' + r = eq[0].match(diff(x(t),t) - exp(n*x(t))*f) + if r: + g = (diff(y(t),t) - eq[1])/r[f] + if r and not (g.has(x(t)) or g.subs(y(t),v).has(t) or r[f].subs(x(t),u).subs(y(t),v).has(t)): + return 'type2' + g = Wild('g') + r1 = eq[0].match(diff(x(t),t) - f) + r2 = eq[1].match(diff(y(t),t) - g) + if r1 and r2 and not (r1[f].subs(x(t),u).subs(y(t),v).has(t) or \ + r2[g].subs(x(t),u).subs(y(t),v).has(t)): + return 'type3' + r1 = eq[0].match(diff(x(t),t) - f) + r2 = eq[1].match(diff(y(t),t) - g) + num, den = ( + (r1[f].subs(x(t),u).subs(y(t),v))/ + (r2[g].subs(x(t),u).subs(y(t),v))).as_numer_denom() + R1 = num.match(f1*g1) + R2 = den.match(f2*g2) + # phi = (r1[f].subs(x(t),u).subs(y(t),v))/num + if R1 and R2: + return 'type4' + return None + + +def check_nonlinear_2eq_order2(eq, func, func_coef): + return None + +def check_nonlinear_3eq_order1(eq, func, func_coef): + x = func[0].func + y = func[1].func + z = func[2].func + fc = func_coef + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + u, v, w = symbols('u, v, w', cls=Dummy) + a = Wild('a', exclude=[x(t), y(t), z(t), t]) + b = Wild('b', exclude=[x(t), y(t), z(t), t]) + c = Wild('c', exclude=[x(t), y(t), z(t), t]) + f = Wild('f') + F1 = Wild('F1') + F2 = Wild('F2') + F3 = Wild('F3') + for i in range(3): + eqs = 0 + for terms in Add.make_args(eq[i]): + eqs += terms/fc[i,func[i],1] + eq[i] = eqs + r1 = eq[0].match(diff(x(t),t) - a*y(t)*z(t)) + r2 = eq[1].match(diff(y(t),t) - b*z(t)*x(t)) + r3 = eq[2].match(diff(z(t),t) - c*x(t)*y(t)) + if r1 and r2 and r3: + num1, den1 = r1[a].as_numer_denom() + num2, den2 = r2[b].as_numer_denom() + num3, den3 = r3[c].as_numer_denom() + if solve([num1*u-den1*(v-w), num2*v-den2*(w-u), num3*w-den3*(u-v)],[u, v]): + return 'type1' + r = eq[0].match(diff(x(t),t) - y(t)*z(t)*f) + if r: + r1 = collect_const(r[f]).match(a*f) + r2 = ((diff(y(t),t) - eq[1])/r1[f]).match(b*z(t)*x(t)) + r3 = ((diff(z(t),t) - eq[2])/r1[f]).match(c*x(t)*y(t)) + if r1 and r2 and r3: + num1, den1 = r1[a].as_numer_denom() + num2, den2 = r2[b].as_numer_denom() + num3, den3 = r3[c].as_numer_denom() + if solve([num1*u-den1*(v-w), num2*v-den2*(w-u), num3*w-den3*(u-v)],[u, v]): + return 'type2' + r = eq[0].match(diff(x(t),t) - (F2-F3)) + if r: + r1 = collect_const(r[F2]).match(c*F2) + r1.update(collect_const(r[F3]).match(b*F3)) + if r1: + if eq[1].has(r1[F2]) and not eq[1].has(r1[F3]): + r1[F2], r1[F3] = r1[F3], r1[F2] + r1[c], r1[b] = -r1[b], -r1[c] + r2 = eq[1].match(diff(y(t),t) - a*r1[F3] + r1[c]*F1) + if r2: + r3 = (eq[2] == diff(z(t),t) - r1[b]*r2[F1] + r2[a]*r1[F2]) + if r1 and r2 and r3: + return 'type3' + r = eq[0].match(diff(x(t),t) - z(t)*F2 + y(t)*F3) + if r: + r1 = collect_const(r[F2]).match(c*F2) + r1.update(collect_const(r[F3]).match(b*F3)) + if r1: + if eq[1].has(r1[F2]) and not eq[1].has(r1[F3]): + r1[F2], r1[F3] = r1[F3], r1[F2] + r1[c], r1[b] = -r1[b], -r1[c] + r2 = (diff(y(t),t) - eq[1]).match(a*x(t)*r1[F3] - r1[c]*z(t)*F1) + if r2: + r3 = (diff(z(t),t) - eq[2] == r1[b]*y(t)*r2[F1] - r2[a]*x(t)*r1[F2]) + if r1 and r2 and r3: + return 'type4' + r = (diff(x(t),t) - eq[0]).match(x(t)*(F2 - F3)) + if r: + r1 = collect_const(r[F2]).match(c*F2) + r1.update(collect_const(r[F3]).match(b*F3)) + if r1: + if eq[1].has(r1[F2]) and not eq[1].has(r1[F3]): + r1[F2], r1[F3] = r1[F3], r1[F2] + r1[c], r1[b] = -r1[b], -r1[c] + r2 = (diff(y(t),t) - eq[1]).match(y(t)*(a*r1[F3] - r1[c]*F1)) + if r2: + r3 = (diff(z(t),t) - eq[2] == z(t)*(r1[b]*r2[F1] - r2[a]*r1[F2])) + if r1 and r2 and r3: + return 'type5' + return None + + +def check_nonlinear_3eq_order2(eq, func, func_coef): + return None + + +@vectorize(0) +def odesimp(ode, eq, func, hint): + r""" + Simplifies solutions of ODEs, including trying to solve for ``func`` and + running :py:meth:`~sympy.solvers.ode.constantsimp`. + + It may use knowledge of the type of solution that the hint returns to + apply additional simplifications. + + It also attempts to integrate any :py:class:`~sympy.integrals.integrals.Integral`\s + in the expression, if the hint is not an ``_Integral`` hint. + + This function should have no effect on expressions returned by + :py:meth:`~sympy.solvers.ode.dsolve`, as + :py:meth:`~sympy.solvers.ode.dsolve` already calls + :py:meth:`~sympy.solvers.ode.ode.odesimp`, but the individual hint functions + do not call :py:meth:`~sympy.solvers.ode.ode.odesimp` (because the + :py:meth:`~sympy.solvers.ode.dsolve` wrapper does). Therefore, this + function is designed for mainly internal use. + + Examples + ======== + + >>> from sympy import sin, symbols, dsolve, pprint, Function + >>> from sympy.solvers.ode.ode import odesimp + >>> x, u2, C1= symbols('x,u2,C1') + >>> f = Function('f') + + >>> eq = dsolve(x*f(x).diff(x) - f(x) - x*sin(f(x)/x), f(x), + ... hint='1st_homogeneous_coeff_subs_indep_div_dep_Integral', + ... simplify=False) + >>> pprint(eq, wrap_line=False) + x + ---- + f(x) + / + | + | / 1 \ + | -|u1 + -------| + | | /1 \| + | | sin|--|| + | \ \u1// + log(f(x)) = log(C1) + | ---------------- d(u1) + | 2 + | u1 + | + / + + >>> pprint(odesimp(eq, f(x), 1, {C1}, + ... hint='1st_homogeneous_coeff_subs_indep_div_dep' + ... )) #doctest: +SKIP + x + --------- = C1 + /f(x)\ + tan|----| + \2*x / + + """ + x = func.args[0] + f = func.func + C1 = get_numbered_constants(eq, num=1) + constants = eq.free_symbols - ode.free_symbols + + # First, integrate if the hint allows it. + eq = _handle_Integral(eq, func, hint) + if hint.startswith("nth_linear_euler_eq_nonhomogeneous"): + eq = simplify(eq) + if not isinstance(eq, Equality): + raise TypeError("eq should be an instance of Equality") + + # Second, clean up the arbitrary constants. + # Right now, nth linear hints can put as many as 2*order constants in an + # expression. If that number grows with another hint, the third argument + # here should be raised accordingly, or constantsimp() rewritten to handle + # an arbitrary number of constants. + eq = constantsimp(eq, constants) + + # Lastly, now that we have cleaned up the expression, try solving for func. + # When CRootOf is implemented in solve(), we will want to return a CRootOf + # every time instead of an Equality. + + # Get the f(x) on the left if possible. + if eq.rhs == func and not eq.lhs.has(func): + eq = [Eq(eq.rhs, eq.lhs)] + + # make sure we are working with lists of solutions in simplified form. + if eq.lhs == func and not eq.rhs.has(func): + # The solution is already solved + eq = [eq] + + else: + # The solution is not solved, so try to solve it + try: + floats = any(i.is_Float for i in eq.atoms(Number)) + eqsol = solve(eq, func, force=True, rational=False if floats else None) + if not eqsol: + raise NotImplementedError + except (NotImplementedError, PolynomialError): + eq = [eq] + else: + def _expand(expr): + numer, denom = expr.as_numer_denom() + + if denom.is_Add: + return expr + else: + return powsimp(expr.expand(), combine='exp', deep=True) + + # XXX: the rest of odesimp() expects each ``t`` to be in a + # specific normal form: rational expression with numerator + # expanded, but with combined exponential functions (at + # least in this setup all tests pass). + eq = [Eq(f(x), _expand(t)) for t in eqsol] + + # special simplification of the lhs. + if hint.startswith("1st_homogeneous_coeff"): + for j, eqi in enumerate(eq): + newi = logcombine(eqi, force=True) + if isinstance(newi.lhs, log) and newi.rhs == 0: + newi = Eq(newi.lhs.args[0]/C1, C1) + eq[j] = newi + + # We cleaned up the constants before solving to help the solve engine with + # a simpler expression, but the solved expression could have introduced + # things like -C1, so rerun constantsimp() one last time before returning. + for i, eqi in enumerate(eq): + eq[i] = constantsimp(eqi, constants) + eq[i] = constant_renumber(eq[i], ode.free_symbols) + + # If there is only 1 solution, return it; + # otherwise return the list of solutions. + if len(eq) == 1: + eq = eq[0] + return eq + + +def ode_sol_simplicity(sol, func, trysolving=True): + r""" + Returns an extended integer representing how simple a solution to an ODE + is. + + The following things are considered, in order from most simple to least: + + - ``sol`` is solved for ``func``. + - ``sol`` is not solved for ``func``, but can be if passed to solve (e.g., + a solution returned by ``dsolve(ode, func, simplify=False``). + - If ``sol`` is not solved for ``func``, then base the result on the + length of ``sol``, as computed by ``len(str(sol))``. + - If ``sol`` has any unevaluated :py:class:`~sympy.integrals.integrals.Integral`\s, + this will automatically be considered less simple than any of the above. + + This function returns an integer such that if solution A is simpler than + solution B by above metric, then ``ode_sol_simplicity(sola, func) < + ode_sol_simplicity(solb, func)``. + + Currently, the following are the numbers returned, but if the heuristic is + ever improved, this may change. Only the ordering is guaranteed. + + +----------------------------------------------+-------------------+ + | Simplicity | Return | + +==============================================+===================+ + | ``sol`` solved for ``func`` | ``-2`` | + +----------------------------------------------+-------------------+ + | ``sol`` not solved for ``func`` but can be | ``-1`` | + +----------------------------------------------+-------------------+ + | ``sol`` is not solved nor solvable for | ``len(str(sol))`` | + | ``func`` | | + +----------------------------------------------+-------------------+ + | ``sol`` contains an | ``oo`` | + | :obj:`~sympy.integrals.integrals.Integral` | | + +----------------------------------------------+-------------------+ + + ``oo`` here means the SymPy infinity, which should compare greater than + any integer. + + If you already know :py:meth:`~sympy.solvers.solvers.solve` cannot solve + ``sol``, you can use ``trysolving=False`` to skip that step, which is the + only potentially slow step. For example, + :py:meth:`~sympy.solvers.ode.dsolve` with the ``simplify=False`` flag + should do this. + + If ``sol`` is a list of solutions, if the worst solution in the list + returns ``oo`` it returns that, otherwise it returns ``len(str(sol))``, + that is, the length of the string representation of the whole list. + + Examples + ======== + + This function is designed to be passed to ``min`` as the key argument, + such as ``min(listofsolutions, key=lambda i: ode_sol_simplicity(i, + f(x)))``. + + >>> from sympy import symbols, Function, Eq, tan, Integral + >>> from sympy.solvers.ode.ode import ode_sol_simplicity + >>> x, C1, C2 = symbols('x, C1, C2') + >>> f = Function('f') + + >>> ode_sol_simplicity(Eq(f(x), C1*x**2), f(x)) + -2 + >>> ode_sol_simplicity(Eq(x**2 + f(x), C1), f(x)) + -1 + >>> ode_sol_simplicity(Eq(f(x), C1*Integral(2*x, x)), f(x)) + oo + >>> eq1 = Eq(f(x)/tan(f(x)/(2*x)), C1) + >>> eq2 = Eq(f(x)/tan(f(x)/(2*x) + f(x)), C2) + >>> [ode_sol_simplicity(eq, f(x)) for eq in [eq1, eq2]] + [28, 35] + >>> min([eq1, eq2], key=lambda i: ode_sol_simplicity(i, f(x))) + Eq(f(x)/tan(f(x)/(2*x)), C1) + + """ + # TODO: if two solutions are solved for f(x), we still want to be + # able to get the simpler of the two + + # See the docstring for the coercion rules. We check easier (faster) + # things here first, to save time. + + if iterable(sol): + # See if there are Integrals + for i in sol: + if ode_sol_simplicity(i, func, trysolving=trysolving) == oo: + return oo + + return len(str(sol)) + + if sol.has(Integral): + return oo + + # Next, try to solve for func. This code will change slightly when CRootOf + # is implemented in solve(). Probably a CRootOf solution should fall + # somewhere between a normal solution and an unsolvable expression. + + # First, see if they are already solved + if sol.lhs == func and not sol.rhs.has(func) or \ + sol.rhs == func and not sol.lhs.has(func): + return -2 + # We are not so lucky, try solving manually + if trysolving: + try: + sols = solve(sol, func) + if not sols: + raise NotImplementedError + except NotImplementedError: + pass + else: + return -1 + + # Finally, a naive computation based on the length of the string version + # of the expression. This may favor combined fractions because they + # will not have duplicate denominators, and may slightly favor expressions + # with fewer additions and subtractions, as those are separated by spaces + # by the printer. + + # Additional ideas for simplicity heuristics are welcome, like maybe + # checking if a equation has a larger domain, or if constantsimp has + # introduced arbitrary constants numbered higher than the order of a + # given ODE that sol is a solution of. + return len(str(sol)) + + +def _extract_funcs(eqs): + funcs = [] + for eq in eqs: + derivs = [node for node in preorder_traversal(eq) if isinstance(node, Derivative)] + func = [] + for d in derivs: + func += list(d.atoms(AppliedUndef)) + for func_ in func: + funcs.append(func_) + funcs = list(uniq(funcs)) + + return funcs + + +def _get_constant_subexpressions(expr, Cs): + Cs = set(Cs) + Ces = [] + def _recursive_walk(expr): + expr_syms = expr.free_symbols + if expr_syms and expr_syms.issubset(Cs): + Ces.append(expr) + else: + if expr.func == exp: + expr = expr.expand(mul=True) + if expr.func in (Add, Mul): + d = sift(expr.args, lambda i : i.free_symbols.issubset(Cs)) + if len(d[True]) > 1: + x = expr.func(*d[True]) + if not x.is_number: + Ces.append(x) + elif isinstance(expr, Integral): + if expr.free_symbols.issubset(Cs) and \ + all(len(x) == 3 for x in expr.limits): + Ces.append(expr) + for i in expr.args: + _recursive_walk(i) + return + _recursive_walk(expr) + return Ces + +def __remove_linear_redundancies(expr, Cs): + cnts = {i: expr.count(i) for i in Cs} + Cs = [i for i in Cs if cnts[i] > 0] + + def _linear(expr): + if isinstance(expr, Add): + xs = [i for i in Cs if expr.count(i)==cnts[i] \ + and 0 == expr.diff(i, 2)] + d = {} + for x in xs: + y = expr.diff(x) + if y not in d: + d[y]=[] + d[y].append(x) + for y in d: + if len(d[y]) > 1: + d[y].sort(key=str) + for x in d[y][1:]: + expr = expr.subs(x, 0) + return expr + + def _recursive_walk(expr): + if len(expr.args) != 0: + expr = expr.func(*[_recursive_walk(i) for i in expr.args]) + expr = _linear(expr) + return expr + + if isinstance(expr, Equality): + lhs, rhs = [_recursive_walk(i) for i in expr.args] + f = lambda i: isinstance(i, Number) or i in Cs + if isinstance(lhs, Symbol) and lhs in Cs: + rhs, lhs = lhs, rhs + if lhs.func in (Add, Symbol) and rhs.func in (Add, Symbol): + dlhs = sift([lhs] if isinstance(lhs, AtomicExpr) else lhs.args, f) + drhs = sift([rhs] if isinstance(rhs, AtomicExpr) else rhs.args, f) + for i in [True, False]: + for hs in [dlhs, drhs]: + if i not in hs: + hs[i] = [0] + # this calculation can be simplified + lhs = Add(*dlhs[False]) - Add(*drhs[False]) + rhs = Add(*drhs[True]) - Add(*dlhs[True]) + elif lhs.func in (Mul, Symbol) and rhs.func in (Mul, Symbol): + dlhs = sift([lhs] if isinstance(lhs, AtomicExpr) else lhs.args, f) + if True in dlhs: + if False not in dlhs: + dlhs[False] = [1] + lhs = Mul(*dlhs[False]) + rhs = rhs/Mul(*dlhs[True]) + return Eq(lhs, rhs) + else: + return _recursive_walk(expr) + +@vectorize(0) +def constantsimp(expr, constants): + r""" + Simplifies an expression with arbitrary constants in it. + + This function is written specifically to work with + :py:meth:`~sympy.solvers.ode.dsolve`, and is not intended for general use. + + Simplification is done by "absorbing" the arbitrary constants into other + arbitrary constants, numbers, and symbols that they are not independent + of. + + The symbols must all have the same name with numbers after it, for + example, ``C1``, ``C2``, ``C3``. The ``symbolname`` here would be + '``C``', the ``startnumber`` would be 1, and the ``endnumber`` would be 3. + If the arbitrary constants are independent of the variable ``x``, then the + independent symbol would be ``x``. There is no need to specify the + dependent function, such as ``f(x)``, because it already has the + independent symbol, ``x``, in it. + + Because terms are "absorbed" into arbitrary constants and because + constants are renumbered after simplifying, the arbitrary constants in + expr are not necessarily equal to the ones of the same name in the + returned result. + + If two or more arbitrary constants are added, multiplied, or raised to the + power of each other, they are first absorbed together into a single + arbitrary constant. Then the new constant is combined into other terms if + necessary. + + Absorption of constants is done with limited assistance: + + 1. terms of :py:class:`~sympy.core.add.Add`\s are collected to try join + constants so `e^x (C_1 \cos(x) + C_2 \cos(x))` will simplify to `e^x + C_1 \cos(x)`; + + 2. powers with exponents that are :py:class:`~sympy.core.add.Add`\s are + expanded so `e^{C_1 + x}` will be simplified to `C_1 e^x`. + + Use :py:meth:`~sympy.solvers.ode.ode.constant_renumber` to renumber constants + after simplification or else arbitrary numbers on constants may appear, + e.g. `C_1 + C_3 x`. + + In rare cases, a single constant can be "simplified" into two constants. + Every differential equation solution should have as many arbitrary + constants as the order of the differential equation. The result here will + be technically correct, but it may, for example, have `C_1` and `C_2` in + an expression, when `C_1` is actually equal to `C_2`. Use your discretion + in such situations, and also take advantage of the ability to use hints in + :py:meth:`~sympy.solvers.ode.dsolve`. + + Examples + ======== + + >>> from sympy import symbols + >>> from sympy.solvers.ode.ode import constantsimp + >>> C1, C2, C3, x, y = symbols('C1, C2, C3, x, y') + >>> constantsimp(2*C1*x, {C1, C2, C3}) + C1*x + >>> constantsimp(C1 + 2 + x, {C1, C2, C3}) + C1 + x + >>> constantsimp(C1*C2 + 2 + C2 + C3*x, {C1, C2, C3}) + C1 + C3*x + + """ + # This function works recursively. The idea is that, for Mul, + # Add, Pow, and Function, if the class has a constant in it, then + # we can simplify it, which we do by recursing down and + # simplifying up. Otherwise, we can skip that part of the + # expression. + + Cs = constants + + orig_expr = expr + + constant_subexprs = _get_constant_subexpressions(expr, Cs) + for xe in constant_subexprs: + xes = list(xe.free_symbols) + if not xes: + continue + if all(expr.count(c) == xe.count(c) for c in xes): + xes.sort(key=str) + expr = expr.subs(xe, xes[0]) + + # try to perform common sub-expression elimination of constant terms + try: + commons, rexpr = cse(expr) + commons.reverse() + rexpr = rexpr[0] + for s in commons: + cs = list(s[1].atoms(Symbol)) + if len(cs) == 1 and cs[0] in Cs and \ + cs[0] not in rexpr.atoms(Symbol) and \ + not any(cs[0] in ex for ex in commons if ex != s): + rexpr = rexpr.subs(s[0], cs[0]) + else: + rexpr = rexpr.subs(*s) + expr = rexpr + except IndexError: + pass + expr = __remove_linear_redundancies(expr, Cs) + + def _conditional_term_factoring(expr): + new_expr = terms_gcd(expr, clear=False, deep=True, expand=False) + + # we do not want to factor exponentials, so handle this separately + if new_expr.is_Mul: + infac = False + asfac = False + for m in new_expr.args: + if isinstance(m, exp): + asfac = True + elif m.is_Add: + infac = any(isinstance(fi, exp) for t in m.args + for fi in Mul.make_args(t)) + if asfac and infac: + new_expr = expr + break + return new_expr + + expr = _conditional_term_factoring(expr) + + # call recursively if more simplification is possible + if orig_expr != expr: + return constantsimp(expr, Cs) + return expr + + +def constant_renumber(expr, variables=None, newconstants=None): + r""" + Renumber arbitrary constants in ``expr`` to use the symbol names as given + in ``newconstants``. In the process, this reorders expression terms in a + standard way. + + If ``newconstants`` is not provided then the new constant names will be + ``C1``, ``C2`` etc. Otherwise ``newconstants`` should be an iterable + giving the new symbols to use for the constants in order. + + The ``variables`` argument is a list of non-constant symbols. All other + free symbols found in ``expr`` are assumed to be constants and will be + renumbered. If ``variables`` is not given then any numbered symbol + beginning with ``C`` (e.g. ``C1``) is assumed to be a constant. + + Symbols are renumbered based on ``.sort_key()``, so they should be + numbered roughly in the order that they appear in the final, printed + expression. Note that this ordering is based in part on hashes, so it can + produce different results on different machines. + + The structure of this function is very similar to that of + :py:meth:`~sympy.solvers.ode.constantsimp`. + + Examples + ======== + + >>> from sympy import symbols + >>> from sympy.solvers.ode.ode import constant_renumber + >>> x, C1, C2, C3 = symbols('x,C1:4') + >>> expr = C3 + C2*x + C1*x**2 + >>> expr + C1*x**2 + C2*x + C3 + >>> constant_renumber(expr) + C1 + C2*x + C3*x**2 + + The ``variables`` argument specifies which are constants so that the + other symbols will not be renumbered: + + >>> constant_renumber(expr, [C1, x]) + C1*x**2 + C2 + C3*x + + The ``newconstants`` argument is used to specify what symbols to use when + replacing the constants: + + >>> constant_renumber(expr, [x], newconstants=symbols('E1:4')) + E1 + E2*x + E3*x**2 + + """ + + # System of expressions + if isinstance(expr, (set, list, tuple)): + return type(expr)(constant_renumber(Tuple(*expr), + variables=variables, newconstants=newconstants)) + + # Symbols in solution but not ODE are constants + if variables is not None: + variables = set(variables) + free_symbols = expr.free_symbols + constantsymbols = list(free_symbols - variables) + # Any Cn is a constant... + else: + variables = set() + isconstant = lambda s: s.startswith('C') and s[1:].isdigit() + constantsymbols = [sym for sym in expr.free_symbols if isconstant(sym.name)] + + # Find new constants checking that they aren't already in the ODE + if newconstants is None: + iter_constants = numbered_symbols(start=1, prefix='C', exclude=variables) + else: + iter_constants = (sym for sym in newconstants if sym not in variables) + + constants_found = [] + + # make a mapping to send all constantsymbols to S.One and use + # that to make sure that term ordering is not dependent on + # the indexed value of C + C_1 = [(ci, S.One) for ci in constantsymbols] + sort_key=lambda arg: default_sort_key(arg.subs(C_1)) + + def _constant_renumber(expr): + r""" + We need to have an internal recursive function + """ + + # For system of expressions + if isinstance(expr, Tuple): + renumbered = [_constant_renumber(e) for e in expr] + return Tuple(*renumbered) + + if isinstance(expr, Equality): + return Eq( + _constant_renumber(expr.lhs), + _constant_renumber(expr.rhs)) + + if type(expr) not in (Mul, Add, Pow) and not expr.is_Function and \ + not expr.has(*constantsymbols): + # Base case, as above. Hope there aren't constants inside + # of some other class, because they won't be renumbered. + return expr + elif expr.is_Piecewise: + return expr + elif expr in constantsymbols: + if expr not in constants_found: + constants_found.append(expr) + return expr + elif expr.is_Function or expr.is_Pow: + return expr.func( + *[_constant_renumber(x) for x in expr.args]) + else: + sortedargs = list(expr.args) + sortedargs.sort(key=sort_key) + return expr.func(*[_constant_renumber(x) for x in sortedargs]) + expr = _constant_renumber(expr) + + # Don't renumber symbols present in the ODE. + constants_found = [c for c in constants_found if c not in variables] + + # Renumbering happens here + subs_dict = {var: cons for var, cons in zip(constants_found, iter_constants)} + expr = expr.subs(subs_dict, simultaneous=True) + + return expr + + +def _handle_Integral(expr, func, hint): + r""" + Converts a solution with Integrals in it into an actual solution. + + For most hints, this simply runs ``expr.doit()``. + + """ + if hint == "nth_linear_constant_coeff_homogeneous": + sol = expr + elif not hint.endswith("_Integral"): + sol = expr.doit() + else: + sol = expr + return sol + + +# XXX: Should this function maybe go somewhere else? + + +def homogeneous_order(eq, *symbols): + r""" + Returns the order `n` if `g` is homogeneous and ``None`` if it is not + homogeneous. + + Determines if a function is homogeneous and if so of what order. A + function `f(x, y, \cdots)` is homogeneous of order `n` if `f(t x, t y, + \cdots) = t^n f(x, y, \cdots)`. + + If the function is of two variables, `F(x, y)`, then `f` being homogeneous + of any order is equivalent to being able to rewrite `F(x, y)` as `G(x/y)` + or `H(y/x)`. This fact is used to solve 1st order ordinary differential + equations whose coefficients are homogeneous of the same order (see the + docstrings of + :obj:`~sympy.solvers.ode.single.HomogeneousCoeffSubsDepDivIndep` and + :obj:`~sympy.solvers.ode.single.HomogeneousCoeffSubsIndepDivDep`). + + Symbols can be functions, but every argument of the function must be a + symbol, and the arguments of the function that appear in the expression + must match those given in the list of symbols. If a declared function + appears with different arguments than given in the list of symbols, + ``None`` is returned. + + Examples + ======== + + >>> from sympy import Function, homogeneous_order, sqrt + >>> from sympy.abc import x, y + >>> f = Function('f') + >>> homogeneous_order(f(x), f(x)) is None + True + >>> homogeneous_order(f(x,y), f(y, x), x, y) is None + True + >>> homogeneous_order(f(x), f(x), x) + 1 + >>> homogeneous_order(x**2*f(x)/sqrt(x**2+f(x)**2), x, f(x)) + 2 + >>> homogeneous_order(x**2+f(x), x, f(x)) is None + True + + """ + + if not symbols: + raise ValueError("homogeneous_order: no symbols were given.") + symset = set(symbols) + eq = sympify(eq) + + # The following are not supported + if eq.has(Order, Derivative): + return None + + # These are all constants + if (eq.is_Number or + eq.is_NumberSymbol or + eq.is_number + ): + return S.Zero + + # Replace all functions with dummy variables + dum = numbered_symbols(prefix='d', cls=Dummy) + newsyms = set() + for i in [j for j in symset if getattr(j, 'is_Function')]: + iargs = set(i.args) + if iargs.difference(symset): + return None + else: + dummyvar = next(dum) + eq = eq.subs(i, dummyvar) + symset.remove(i) + newsyms.add(dummyvar) + symset.update(newsyms) + + if not eq.free_symbols & symset: + return None + + # assuming order of a nested function can only be equal to zero + if isinstance(eq, Function): + return None if homogeneous_order( + eq.args[0], *tuple(symset)) != 0 else S.Zero + + # make the replacement of x with x*t and see if t can be factored out + t = Dummy('t', positive=True) # It is sufficient that t > 0 + eqs = separatevars(eq.subs([(i, t*i) for i in symset]), [t], dict=True)[t] + if eqs is S.One: + return S.Zero # there was no term with only t + i, d = eqs.as_independent(t, as_Add=False) + b, e = d.as_base_exp() + if b == t: + return e + + +def ode_2nd_power_series_ordinary(eq, func, order, match): + r""" + Gives a power series solution to a second order homogeneous differential + equation with polynomial coefficients at an ordinary point. A homogeneous + differential equation is of the form + + .. math :: P(x)\frac{d^2y}{dx^2} + Q(x)\frac{dy}{dx} + R(x) y(x) = 0 + + For simplicity it is assumed that `P(x)`, `Q(x)` and `R(x)` are polynomials, + it is sufficient that `\frac{Q(x)}{P(x)}` and `\frac{R(x)}{P(x)}` exists at + `x_{0}`. A recurrence relation is obtained by substituting `y` as `\sum_{n=0}^\infty a_{n}x^{n}`, + in the differential equation, and equating the nth term. Using this relation + various terms can be generated. + + + Examples + ======== + + >>> from sympy import dsolve, Function, pprint + >>> from sympy.abc import x + >>> f = Function("f") + >>> eq = f(x).diff(x, 2) + f(x) + >>> pprint(dsolve(eq, hint='2nd_power_series_ordinary')) + / 4 2 \ / 2\ + |x x | | x | / 6\ + f(x) = C2*|-- - -- + 1| + C1*x*|1 - --| + O\x / + \24 2 / \ 6 / + + + References + ========== + - https://tutorial.math.lamar.edu/Classes/DE/SeriesSolutions.aspx + - George E. Simmons, "Differential Equations with Applications and + Historical Notes", p.p 176 - 184 + + """ + x = func.args[0] + f = func.func + C0, C1 = get_numbered_constants(eq, num=2) + n = Dummy("n", integer=True) + s = Wild("s") + k = Wild("k", exclude=[x]) + x0 = match['x0'] + terms = match['terms'] + p = match[match['a3']] + q = match[match['b3']] + r = match[match['c3']] + seriesdict = {} + recurr = Function("r") + + # Generating the recurrence relation which works this way: + # for the second order term the summation begins at n = 2. The coefficients + # p is multiplied with an*(n - 1)*(n - 2)*x**n-2 and a substitution is made such that + # the exponent of x becomes n. + # For example, if p is x, then the second degree recurrence term is + # an*(n - 1)*(n - 2)*x**n-1, substituting (n - 1) as n, it transforms to + # an+1*n*(n - 1)*x**n. + # A similar process is done with the first order and zeroth order term. + + coefflist = [(recurr(n), r), (n*recurr(n), q), (n*(n - 1)*recurr(n), p)] + for index, coeff in enumerate(coefflist): + if coeff[1]: + f2 = powsimp(expand((coeff[1]*(x - x0)**(n - index)).subs(x, x + x0))) + if f2.is_Add: + addargs = f2.args + else: + addargs = [f2] + for arg in addargs: + powm = arg.match(s*x**k) + term = coeff[0]*powm[s] + if not powm[k].is_Symbol: + term = term.subs(n, n - powm[k].as_independent(n)[0]) + startind = powm[k].subs(n, index) + # Seeing if the startterm can be reduced further. + # If it vanishes for n lesser than startind, it is + # equal to summation from n. + if startind: + for i in reversed(range(startind)): + if not term.subs(n, i): + seriesdict[term] = i + else: + seriesdict[term] = i + 1 + break + else: + seriesdict[term] = S.Zero + + # Stripping of terms so that the sum starts with the same number. + teq = S.Zero + suminit = seriesdict.values() + rkeys = seriesdict.keys() + req = Add(*rkeys) + if any(suminit): + maxval = max(suminit) + for term in seriesdict: + val = seriesdict[term] + if val != maxval: + for i in range(val, maxval): + teq += term.subs(n, val) + + finaldict = {} + if teq: + fargs = teq.atoms(AppliedUndef) + if len(fargs) == 1: + finaldict[fargs.pop()] = 0 + else: + maxf = max(fargs, key = lambda x: x.args[0]) + sol = solve(teq, maxf) + if isinstance(sol, list): + sol = sol[0] + finaldict[maxf] = sol + + # Finding the recurrence relation in terms of the largest term. + fargs = req.atoms(AppliedUndef) + maxf = max(fargs, key = lambda x: x.args[0]) + minf = min(fargs, key = lambda x: x.args[0]) + if minf.args[0].is_Symbol: + startiter = 0 + else: + startiter = -minf.args[0].as_independent(n)[0] + lhs = maxf + rhs = solve(req, maxf) + if isinstance(rhs, list): + rhs = rhs[0] + + # Checking how many values are already present + tcounter = len([t for t in finaldict.values() if t]) + + for _ in range(tcounter, terms - 3): # Assuming c0 and c1 to be arbitrary + check = rhs.subs(n, startiter) + nlhs = lhs.subs(n, startiter) + nrhs = check.subs(finaldict) + finaldict[nlhs] = nrhs + startiter += 1 + + # Post processing + series = C0 + C1*(x - x0) + for term in finaldict: + if finaldict[term]: + fact = term.args[0] + series += (finaldict[term].subs([(recurr(0), C0), (recurr(1), C1)])*( + x - x0)**fact) + series = collect(expand_mul(series), [C0, C1]) + Order(x**terms) + return Eq(f(x), series) + + +def ode_2nd_power_series_regular(eq, func, order, match): + r""" + Gives a power series solution to a second order homogeneous differential + equation with polynomial coefficients at a regular point. A second order + homogeneous differential equation is of the form + + .. math :: P(x)\frac{d^2y}{dx^2} + Q(x)\frac{dy}{dx} + R(x) y(x) = 0 + + A point is said to regular singular at `x0` if `x - x0\frac{Q(x)}{P(x)}` + and `(x - x0)^{2}\frac{R(x)}{P(x)}` are analytic at `x0`. For simplicity + `P(x)`, `Q(x)` and `R(x)` are assumed to be polynomials. The algorithm for + finding the power series solutions is: + + 1. Try expressing `(x - x0)P(x)` and `((x - x0)^{2})Q(x)` as power series + solutions about x0. Find `p0` and `q0` which are the constants of the + power series expansions. + 2. Solve the indicial equation `f(m) = m(m - 1) + m*p0 + q0`, to obtain the + roots `m1` and `m2` of the indicial equation. + 3. If `m1 - m2` is a non integer there exists two series solutions. If + `m1 = m2`, there exists only one solution. If `m1 - m2` is an integer, + then the existence of one solution is confirmed. The other solution may + or may not exist. + + The power series solution is of the form `x^{m}\sum_{n=0}^\infty a_{n}x^{n}`. The + coefficients are determined by the following recurrence relation. + `a_{n} = -\frac{\sum_{k=0}^{n-1} q_{n-k} + (m + k)p_{n-k}}{f(m + n)}`. For the case + in which `m1 - m2` is an integer, it can be seen from the recurrence relation + that for the lower root `m`, when `n` equals the difference of both the + roots, the denominator becomes zero. So if the numerator is not equal to zero, + a second series solution exists. + + + Examples + ======== + + >>> from sympy import dsolve, Function, pprint + >>> from sympy.abc import x + >>> f = Function("f") + >>> eq = x*(f(x).diff(x, 2)) + 2*(f(x).diff(x)) + x*f(x) + >>> pprint(dsolve(eq, hint='2nd_power_series_regular')) + / 6 4 2 \ + | x x x | + / 4 2 \ C1*|- --- + -- - -- + 1| + | x x | \ 720 24 2 / / 6\ + f(x) = C2*|--- - -- + 1| + ------------------------ + O\x / + \120 6 / x + + + References + ========== + - George E. Simmons, "Differential Equations with Applications and + Historical Notes", p.p 176 - 184 + + """ + x = func.args[0] + f = func.func + C0, C1 = get_numbered_constants(eq, num=2) + m = Dummy("m") # for solving the indicial equation + x0 = match['x0'] + terms = match['terms'] + p = match['p'] + q = match['q'] + + # Generating the indicial equation + indicial = [] + for term in [p, q]: + if not term.has(x): + indicial.append(term) + else: + term = series(term, x=x, n=1, x0=x0) + if isinstance(term, Order): + indicial.append(S.Zero) + else: + for arg in term.args: + if not arg.has(x): + indicial.append(arg) + break + + p0, q0 = indicial + sollist = solve(m*(m - 1) + m*p0 + q0, m) + if sollist and isinstance(sollist, list) and all( + sol.is_real for sol in sollist): + serdict1 = {} + serdict2 = {} + if len(sollist) == 1: + # Only one series solution exists in this case. + m1 = m2 = sollist.pop() + if terms-m1-1 <= 0: + return Eq(f(x), Order(terms)) + serdict1 = _frobenius(terms-m1-1, m1, p0, q0, p, q, x0, x, C0) + + else: + m1 = sollist[0] + m2 = sollist[1] + if m1 < m2: + m1, m2 = m2, m1 + # Irrespective of whether m1 - m2 is an integer or not, one + # Frobenius series solution exists. + serdict1 = _frobenius(terms-m1-1, m1, p0, q0, p, q, x0, x, C0) + if not (m1 - m2).is_integer: + # Second frobenius series solution exists. + serdict2 = _frobenius(terms-m2-1, m2, p0, q0, p, q, x0, x, C1) + else: + # Check if second frobenius series solution exists. + serdict2 = _frobenius(terms-m2-1, m2, p0, q0, p, q, x0, x, C1, check=m1) + + if serdict1: + finalseries1 = C0 + for key in serdict1: + power = int(key.name[1:]) + finalseries1 += serdict1[key]*(x - x0)**power + finalseries1 = (x - x0)**m1*finalseries1 + finalseries2 = S.Zero + if serdict2: + for key in serdict2: + power = int(key.name[1:]) + finalseries2 += serdict2[key]*(x - x0)**power + finalseries2 += C1 + finalseries2 = (x - x0)**m2*finalseries2 + return Eq(f(x), collect(finalseries1 + finalseries2, + [C0, C1]) + Order(x**terms)) + + +def _frobenius(n, m, p0, q0, p, q, x0, x, c, check=None): + r""" + Returns a dict with keys as coefficients and values as their values in terms of C0 + """ + n = int(n) + # In cases where m1 - m2 is not an integer + m2 = check + + d = Dummy("d") + numsyms = numbered_symbols("C", start=0) + numsyms = [next(numsyms) for i in range(n + 1)] + serlist = [] + for ser in [p, q]: + # Order term not present + if ser.is_polynomial(x) and Poly(ser, x).degree() <= n: + if x0: + ser = ser.subs(x, x + x0) + dict_ = Poly(ser, x).as_dict() + # Order term present + else: + tseries = series(ser, x=x0, n=n+1) + # Removing order + dict_ = Poly(list(ordered(tseries.args))[: -1], x).as_dict() + # Fill in with zeros, if coefficients are zero. + for i in range(n + 1): + if (i,) not in dict_: + dict_[(i,)] = S.Zero + serlist.append(dict_) + + pseries = serlist[0] + qseries = serlist[1] + indicial = d*(d - 1) + d*p0 + q0 + frobdict = {} + for i in range(1, n + 1): + num = c*(m*pseries[(i,)] + qseries[(i,)]) + for j in range(1, i): + sym = Symbol("C" + str(j)) + num += frobdict[sym]*((m + j)*pseries[(i - j,)] + qseries[(i - j,)]) + + # Checking for cases when m1 - m2 is an integer. If num equals zero + # then a second Frobenius series solution cannot be found. If num is not zero + # then set constant as zero and proceed. + if m2 is not None and i == m2 - m: + if num: + return False + else: + frobdict[numsyms[i]] = S.Zero + else: + frobdict[numsyms[i]] = -num/(indicial.subs(d, m+i)) + + return frobdict + +def _remove_redundant_solutions(eq, solns, order, var): + r""" + Remove redundant solutions from the set of solutions. + + This function is needed because otherwise dsolve can return + redundant solutions. As an example consider: + + eq = Eq((f(x).diff(x, 2))*f(x).diff(x), 0) + + There are two ways to find solutions to eq. The first is to solve f(x).diff(x, 2) = 0 + leading to solution f(x)=C1 + C2*x. The second is to solve the equation f(x).diff(x) = 0 + leading to the solution f(x) = C1. In this particular case we then see + that the second solution is a special case of the first and we do not + want to return it. + + This does not always happen. If we have + + eq = Eq((f(x)**2-4)*(f(x).diff(x)-4), 0) + + then we get the algebraic solution f(x) = [-2, 2] and the integral solution + f(x) = x + C1 and in this case the two solutions are not equivalent wrt + initial conditions so both should be returned. + """ + def is_special_case_of(soln1, soln2): + return _is_special_case_of(soln1, soln2, eq, order, var) + + unique_solns = [] + for soln1 in solns: + for soln2 in unique_solns[:]: + if is_special_case_of(soln1, soln2): + break + elif is_special_case_of(soln2, soln1): + unique_solns.remove(soln2) + else: + unique_solns.append(soln1) + + return unique_solns + +def _is_special_case_of(soln1, soln2, eq, order, var): + r""" + True if soln1 is found to be a special case of soln2 wrt some value of the + constants that appear in soln2. False otherwise. + """ + # The solutions returned by dsolve may be given explicitly or implicitly. + # We will equate the sol1=(soln1.rhs - soln1.lhs), sol2=(soln2.rhs - soln2.lhs) + # of the two solutions. + # + # Since this is supposed to hold for all x it also holds for derivatives. + # For an order n ode we should be able to differentiate + # each solution n times to get n+1 equations. + # + # We then try to solve those n+1 equations for the integrations constants + # in sol2. If we can find a solution that does not depend on x then it + # means that some value of the constants in sol1 is a special case of + # sol2 corresponding to a particular choice of the integration constants. + + # In case the solution is in implicit form we subtract the sides + soln1 = soln1.rhs - soln1.lhs + soln2 = soln2.rhs - soln2.lhs + + # Work for the series solution + if soln1.has(Order) and soln2.has(Order): + if soln1.getO() == soln2.getO(): + soln1 = soln1.removeO() + soln2 = soln2.removeO() + else: + return False + elif soln1.has(Order) or soln2.has(Order): + return False + + constants1 = soln1.free_symbols.difference(eq.free_symbols) + constants2 = soln2.free_symbols.difference(eq.free_symbols) + + constants1_new = get_numbered_constants(Tuple(soln1, soln2), len(constants1)) + if len(constants1) == 1: + constants1_new = {constants1_new} + for c_old, c_new in zip(constants1, constants1_new): + soln1 = soln1.subs(c_old, c_new) + + # n equations for sol1 = sol2, sol1'=sol2', ... + lhs = soln1 + rhs = soln2 + eqns = [Eq(lhs, rhs)] + for n in range(1, order): + lhs = lhs.diff(var) + rhs = rhs.diff(var) + eq = Eq(lhs, rhs) + eqns.append(eq) + + # BooleanTrue/False awkwardly show up for trivial equations + if any(isinstance(eq, BooleanFalse) for eq in eqns): + return False + eqns = [eq for eq in eqns if not isinstance(eq, BooleanTrue)] + + try: + constant_solns = solve(eqns, constants2) + except NotImplementedError: + return False + + # Sometimes returns a dict and sometimes a list of dicts + if isinstance(constant_solns, dict): + constant_solns = [constant_solns] + + # after solving the issue 17418, maybe we don't need the following checksol code. + for constant_soln in constant_solns: + for eq in eqns: + eq=eq.rhs-eq.lhs + if checksol(eq, constant_soln) is not True: + return False + + # If any solution gives all constants as expressions that don't depend on + # x then there exists constants for soln2 that give soln1 + for constant_soln in constant_solns: + if not any(c.has(var) for c in constant_soln.values()): + return True + + return False + + +def ode_1st_power_series(eq, func, order, match): + r""" + The power series solution is a method which gives the Taylor series expansion + to the solution of a differential equation. + + For a first order differential equation `\frac{dy}{dx} = h(x, y)`, a power + series solution exists at a point `x = x_{0}` if `h(x, y)` is analytic at `x_{0}`. + The solution is given by + + .. math:: y(x) = y(x_{0}) + \sum_{n = 1}^{\infty} \frac{F_{n}(x_{0},b)(x - x_{0})^n}{n!}, + + where `y(x_{0}) = b` is the value of y at the initial value of `x_{0}`. + To compute the values of the `F_{n}(x_{0},b)` the following algorithm is + followed, until the required number of terms are generated. + + 1. `F_1 = h(x_{0}, b)` + 2. `F_{n+1} = \frac{\partial F_{n}}{\partial x} + \frac{\partial F_{n}}{\partial y}F_{1}` + + Examples + ======== + + >>> from sympy import Function, pprint, exp, dsolve + >>> from sympy.abc import x + >>> f = Function('f') + >>> eq = exp(x)*(f(x).diff(x)) - f(x) + >>> pprint(dsolve(eq, hint='1st_power_series')) + 3 4 5 + C1*x C1*x C1*x / 6\ + f(x) = C1 + C1*x - ----- + ----- + ----- + O\x / + 6 24 60 + + + References + ========== + + - Travis W. Walker, Analytic power series technique for solving first-order + differential equations, p.p 17, 18 + + """ + x = func.args[0] + y = match['y'] + f = func.func + h = -match[match['d']]/match[match['e']] + point = match['f0'] + value = match['f0val'] + terms = match['terms'] + + # First term + F = h + if not h: + return Eq(f(x), value) + + # Initialization + series = value + if terms > 1: + hc = h.subs({x: point, y: value}) + if hc.has(oo) or hc.has(nan) or hc.has(zoo): + # Derivative does not exist, not analytic + return Eq(f(x), oo) + elif hc: + series += hc*(x - point) + + for factcount in range(2, terms): + Fnew = F.diff(x) + F.diff(y)*h + Fnewc = Fnew.subs({x: point, y: value}) + # Same logic as above + if Fnewc.has(oo) or Fnewc.has(nan) or Fnewc.has(-oo) or Fnewc.has(zoo): + return Eq(f(x), oo) + series += Fnewc*((x - point)**factcount)/factorial(factcount) + F = Fnew + series += Order(x**terms) + return Eq(f(x), series) + + +def checkinfsol(eq, infinitesimals, func=None, order=None): + r""" + This function is used to check if the given infinitesimals are the + actual infinitesimals of the given first order differential equation. + This method is specific to the Lie Group Solver of ODEs. + + As of now, it simply checks, by substituting the infinitesimals in the + partial differential equation. + + + .. math:: \frac{\partial \eta}{\partial x} + \left(\frac{\partial \eta}{\partial y} + - \frac{\partial \xi}{\partial x}\right)*h + - \frac{\partial \xi}{\partial y}*h^{2} + - \xi\frac{\partial h}{\partial x} - \eta\frac{\partial h}{\partial y} = 0 + + + where `\eta`, and `\xi` are the infinitesimals and `h(x,y) = \frac{dy}{dx}` + + The infinitesimals should be given in the form of a list of dicts + ``[{xi(x, y): inf, eta(x, y): inf}]``, corresponding to the + output of the function infinitesimals. It returns a list + of values of the form ``[(True/False, sol)]`` where ``sol`` is the value + obtained after substituting the infinitesimals in the PDE. If it + is ``True``, then ``sol`` would be 0. + + """ + if isinstance(eq, Equality): + eq = eq.lhs - eq.rhs + if not func: + eq, func = _preprocess(eq) + variables = func.args + if len(variables) != 1: + raise ValueError("ODE's have only one independent variable") + else: + x = variables[0] + if not order: + order = ode_order(eq, func) + if order != 1: + raise NotImplementedError("Lie groups solver has been implemented " + "only for first order differential equations") + else: + df = func.diff(x) + a = Wild('a', exclude = [df]) + b = Wild('b', exclude = [df]) + match = collect(expand(eq), df).match(a*df + b) + + if match: + h = -simplify(match[b]/match[a]) + else: + try: + sol = solve(eq, df) + except NotImplementedError: + raise NotImplementedError("Infinitesimals for the " + "first order ODE could not be found") + else: + h = sol[0] # Find infinitesimals for one solution + + y = Dummy('y') + h = h.subs(func, y) + xi = Function('xi')(x, y) + eta = Function('eta')(x, y) + dxi = Function('xi')(x, func) + deta = Function('eta')(x, func) + pde = (eta.diff(x) + (eta.diff(y) - xi.diff(x))*h - + (xi.diff(y))*h**2 - xi*(h.diff(x)) - eta*(h.diff(y))) + soltup = [] + for sol in infinitesimals: + tsol = {xi: S(sol[dxi]).subs(func, y), + eta: S(sol[deta]).subs(func, y)} + sol = simplify(pde.subs(tsol).doit()) + if sol: + soltup.append((False, sol.subs(y, func))) + else: + soltup.append((True, 0)) + return soltup + + +def sysode_linear_2eq_order1(match_): + x = match_['func'][0].func + y = match_['func'][1].func + func = match_['func'] + fc = match_['func_coeff'] + eq = match_['eq'] + r = {} + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + for i in range(2): + eq[i] = Add(*[terms/fc[i,func[i],1] for terms in Add.make_args(eq[i])]) + + # for equations Eq(a1*diff(x(t),t), a*x(t) + b*y(t) + k1) + # and Eq(a2*diff(x(t),t), c*x(t) + d*y(t) + k2) + r['a'] = -fc[0,x(t),0]/fc[0,x(t),1] + r['c'] = -fc[1,x(t),0]/fc[1,y(t),1] + r['b'] = -fc[0,y(t),0]/fc[0,x(t),1] + r['d'] = -fc[1,y(t),0]/fc[1,y(t),1] + forcing = [S.Zero,S.Zero] + for i in range(2): + for j in Add.make_args(eq[i]): + if not j.has(x(t), y(t)): + forcing[i] += j + if not (forcing[0].has(t) or forcing[1].has(t)): + r['k1'] = forcing[0] + r['k2'] = forcing[1] + else: + raise NotImplementedError("Only homogeneous problems are supported" + + " (and constant inhomogeneity)") + + if match_['type_of_equation'] == 'type6': + sol = _linear_2eq_order1_type6(x, y, t, r, eq) + if match_['type_of_equation'] == 'type7': + sol = _linear_2eq_order1_type7(x, y, t, r, eq) + return sol + +def _linear_2eq_order1_type6(x, y, t, r, eq): + r""" + The equations of this type of ode are . + + .. math:: x' = f(t) x + g(t) y + + .. math:: y' = a [f(t) + a h(t)] x + a [g(t) - h(t)] y + + This is solved by first multiplying the first equation by `-a` and adding + it to the second equation to obtain + + .. math:: y' - a x' = -a h(t) (y - a x) + + Setting `U = y - ax` and integrating the equation we arrive at + + .. math:: y - ax = C_1 e^{-a \int h(t) \,dt} + + and on substituting the value of y in first equation give rise to first order ODEs. After solving for + `x`, we can obtain `y` by substituting the value of `x` in second equation. + + """ + C1, C2, C3, C4 = get_numbered_constants(eq, num=4) + p = 0 + q = 0 + p1 = cancel(r['c']/cancel(r['c']/r['d']).as_numer_denom()[0]) + p2 = cancel(r['a']/cancel(r['a']/r['b']).as_numer_denom()[0]) + for n, i in enumerate([p1, p2]): + for j in Mul.make_args(collect_const(i)): + if not j.has(t): + q = j + if q!=0 and n==0: + if ((r['c']/j - r['a'])/(r['b'] - r['d']/j)) == j: + p = 1 + s = j + break + if q!=0 and n==1: + if ((r['a']/j - r['c'])/(r['d'] - r['b']/j)) == j: + p = 2 + s = j + break + + if p == 1: + equ = diff(x(t),t) - r['a']*x(t) - r['b']*(s*x(t) + C1*exp(-s*Integral(r['b'] - r['d']/s, t))) + hint1 = classify_ode(equ)[1] + sol1 = dsolve(equ, hint=hint1+'_Integral').rhs + sol2 = s*sol1 + C1*exp(-s*Integral(r['b'] - r['d']/s, t)) + elif p ==2: + equ = diff(y(t),t) - r['c']*y(t) - r['d']*s*y(t) + C1*exp(-s*Integral(r['d'] - r['b']/s, t)) + hint1 = classify_ode(equ)[1] + sol2 = dsolve(equ, hint=hint1+'_Integral').rhs + sol1 = s*sol2 + C1*exp(-s*Integral(r['d'] - r['b']/s, t)) + return [Eq(x(t), sol1), Eq(y(t), sol2)] + +def _linear_2eq_order1_type7(x, y, t, r, eq): + r""" + The equations of this type of ode are . + + .. math:: x' = f(t) x + g(t) y + + .. math:: y' = h(t) x + p(t) y + + Differentiating the first equation and substituting the value of `y` + from second equation will give a second-order linear equation + + .. math:: g x'' - (fg + gp + g') x' + (fgp - g^{2} h + f g' - f' g) x = 0 + + This above equation can be easily integrated if following conditions are satisfied. + + 1. `fgp - g^{2} h + f g' - f' g = 0` + + 2. `fgp - g^{2} h + f g' - f' g = ag, fg + gp + g' = bg` + + If first condition is satisfied then it is solved by current dsolve solver and in second case it becomes + a constant coefficient differential equation which is also solved by current solver. + + Otherwise if the above condition fails then, + a particular solution is assumed as `x = x_0(t)` and `y = y_0(t)` + Then the general solution is expressed as + + .. math:: x = C_1 x_0(t) + C_2 x_0(t) \int \frac{g(t) F(t) P(t)}{x_0^{2}(t)} \,dt + + .. math:: y = C_1 y_0(t) + C_2 [\frac{F(t) P(t)}{x_0(t)} + y_0(t) \int \frac{g(t) F(t) P(t)}{x_0^{2}(t)} \,dt] + + where C1 and C2 are arbitrary constants and + + .. math:: F(t) = e^{\int f(t) \,dt}, P(t) = e^{\int p(t) \,dt} + + """ + C1, C2, C3, C4 = get_numbered_constants(eq, num=4) + e1 = r['a']*r['b']*r['c'] - r['b']**2*r['c'] + r['a']*diff(r['b'],t) - diff(r['a'],t)*r['b'] + e2 = r['a']*r['c']*r['d'] - r['b']*r['c']**2 + diff(r['c'],t)*r['d'] - r['c']*diff(r['d'],t) + m1 = r['a']*r['b'] + r['b']*r['d'] + diff(r['b'],t) + m2 = r['a']*r['c'] + r['c']*r['d'] + diff(r['c'],t) + if e1 == 0: + sol1 = dsolve(r['b']*diff(x(t),t,t) - m1*diff(x(t),t)).rhs + sol2 = dsolve(diff(y(t),t) - r['c']*sol1 - r['d']*y(t)).rhs + elif e2 == 0: + sol2 = dsolve(r['c']*diff(y(t),t,t) - m2*diff(y(t),t)).rhs + sol1 = dsolve(diff(x(t),t) - r['a']*x(t) - r['b']*sol2).rhs + elif not (e1/r['b']).has(t) and not (m1/r['b']).has(t): + sol1 = dsolve(diff(x(t),t,t) - (m1/r['b'])*diff(x(t),t) - (e1/r['b'])*x(t)).rhs + sol2 = dsolve(diff(y(t),t) - r['c']*sol1 - r['d']*y(t)).rhs + elif not (e2/r['c']).has(t) and not (m2/r['c']).has(t): + sol2 = dsolve(diff(y(t),t,t) - (m2/r['c'])*diff(y(t),t) - (e2/r['c'])*y(t)).rhs + sol1 = dsolve(diff(x(t),t) - r['a']*x(t) - r['b']*sol2).rhs + else: + x0 = Function('x0')(t) # x0 and y0 being particular solutions + y0 = Function('y0')(t) + F = exp(Integral(r['a'],t)) + P = exp(Integral(r['d'],t)) + sol1 = C1*x0 + C2*x0*Integral(r['b']*F*P/x0**2, t) + sol2 = C1*y0 + C2*(F*P/x0 + y0*Integral(r['b']*F*P/x0**2, t)) + return [Eq(x(t), sol1), Eq(y(t), sol2)] + + +def sysode_nonlinear_2eq_order1(match_): + func = match_['func'] + eq = match_['eq'] + fc = match_['func_coeff'] + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + if match_['type_of_equation'] == 'type5': + sol = _nonlinear_2eq_order1_type5(func, t, eq) + return sol + x = func[0].func + y = func[1].func + for i in range(2): + eqs = 0 + for terms in Add.make_args(eq[i]): + eqs += terms/fc[i,func[i],1] + eq[i] = eqs + if match_['type_of_equation'] == 'type1': + sol = _nonlinear_2eq_order1_type1(x, y, t, eq) + elif match_['type_of_equation'] == 'type2': + sol = _nonlinear_2eq_order1_type2(x, y, t, eq) + elif match_['type_of_equation'] == 'type3': + sol = _nonlinear_2eq_order1_type3(x, y, t, eq) + elif match_['type_of_equation'] == 'type4': + sol = _nonlinear_2eq_order1_type4(x, y, t, eq) + return sol + + +def _nonlinear_2eq_order1_type1(x, y, t, eq): + r""" + Equations: + + .. math:: x' = x^n F(x,y) + + .. math:: y' = g(y) F(x,y) + + Solution: + + .. math:: x = \varphi(y), \int \frac{1}{g(y) F(\varphi(y),y)} \,dy = t + C_2 + + where + + if `n \neq 1` + + .. math:: \varphi = [C_1 + (1-n) \int \frac{1}{g(y)} \,dy]^{\frac{1}{1-n}} + + if `n = 1` + + .. math:: \varphi = C_1 e^{\int \frac{1}{g(y)} \,dy} + + where `C_1` and `C_2` are arbitrary constants. + + """ + C1, C2 = get_numbered_constants(eq, num=2) + n = Wild('n', exclude=[x(t),y(t)]) + f = Wild('f') + u, v = symbols('u, v') + r = eq[0].match(diff(x(t),t) - x(t)**n*f) + g = ((diff(y(t),t) - eq[1])/r[f]).subs(y(t),v) + F = r[f].subs(x(t),u).subs(y(t),v) + n = r[n] + if n!=1: + phi = (C1 + (1-n)*Integral(1/g, v))**(1/(1-n)) + else: + phi = C1*exp(Integral(1/g, v)) + phi = phi.doit() + sol2 = solve(Integral(1/(g*F.subs(u,phi)), v).doit() - t - C2, v) + sol = [] + for sols in sol2: + sol.append(Eq(x(t),phi.subs(v, sols))) + sol.append(Eq(y(t), sols)) + return sol + +def _nonlinear_2eq_order1_type2(x, y, t, eq): + r""" + Equations: + + .. math:: x' = e^{\lambda x} F(x,y) + + .. math:: y' = g(y) F(x,y) + + Solution: + + .. math:: x = \varphi(y), \int \frac{1}{g(y) F(\varphi(y),y)} \,dy = t + C_2 + + where + + if `\lambda \neq 0` + + .. math:: \varphi = -\frac{1}{\lambda} log(C_1 - \lambda \int \frac{1}{g(y)} \,dy) + + if `\lambda = 0` + + .. math:: \varphi = C_1 + \int \frac{1}{g(y)} \,dy + + where `C_1` and `C_2` are arbitrary constants. + + """ + C1, C2 = get_numbered_constants(eq, num=2) + n = Wild('n', exclude=[x(t),y(t)]) + f = Wild('f') + u, v = symbols('u, v') + r = eq[0].match(diff(x(t),t) - exp(n*x(t))*f) + g = ((diff(y(t),t) - eq[1])/r[f]).subs(y(t),v) + F = r[f].subs(x(t),u).subs(y(t),v) + n = r[n] + if n: + phi = -1/n*log(C1 - n*Integral(1/g, v)) + else: + phi = C1 + Integral(1/g, v) + phi = phi.doit() + sol2 = solve(Integral(1/(g*F.subs(u,phi)), v).doit() - t - C2, v) + sol = [] + for sols in sol2: + sol.append(Eq(x(t),phi.subs(v, sols))) + sol.append(Eq(y(t), sols)) + return sol + +def _nonlinear_2eq_order1_type3(x, y, t, eq): + r""" + Autonomous system of general form + + .. math:: x' = F(x,y) + + .. math:: y' = G(x,y) + + Assuming `y = y(x, C_1)` where `C_1` is an arbitrary constant is the general + solution of the first-order equation + + .. math:: F(x,y) y'_x = G(x,y) + + Then the general solution of the original system of equations has the form + + .. math:: \int \frac{1}{F(x,y(x,C_1))} \,dx = t + C_1 + + """ + C1, C2, C3, C4 = get_numbered_constants(eq, num=4) + v = Function('v') + u = Symbol('u') + f = Wild('f') + g = Wild('g') + r1 = eq[0].match(diff(x(t),t) - f) + r2 = eq[1].match(diff(y(t),t) - g) + F = r1[f].subs(x(t), u).subs(y(t), v(u)) + G = r2[g].subs(x(t), u).subs(y(t), v(u)) + sol2r = dsolve(Eq(diff(v(u), u), G/F)) + if isinstance(sol2r, Equality): + sol2r = [sol2r] + for sol2s in sol2r: + sol1 = solve(Integral(1/F.subs(v(u), sol2s.rhs), u).doit() - t - C2, u) + sol = [] + for sols in sol1: + sol.append(Eq(x(t), sols)) + sol.append(Eq(y(t), (sol2s.rhs).subs(u, sols))) + return sol + +def _nonlinear_2eq_order1_type4(x, y, t, eq): + r""" + Equation: + + .. math:: x' = f_1(x) g_1(y) \phi(x,y,t) + + .. math:: y' = f_2(x) g_2(y) \phi(x,y,t) + + First integral: + + .. math:: \int \frac{f_2(x)}{f_1(x)} \,dx - \int \frac{g_1(y)}{g_2(y)} \,dy = C + + where `C` is an arbitrary constant. + + On solving the first integral for `x` (resp., `y` ) and on substituting the + resulting expression into either equation of the original solution, one + arrives at a first-order equation for determining `y` (resp., `x` ). + + """ + C1, C2 = get_numbered_constants(eq, num=2) + u, v = symbols('u, v') + U, V = symbols('U, V', cls=Function) + f = Wild('f') + g = Wild('g') + f1 = Wild('f1', exclude=[v,t]) + f2 = Wild('f2', exclude=[v,t]) + g1 = Wild('g1', exclude=[u,t]) + g2 = Wild('g2', exclude=[u,t]) + r1 = eq[0].match(diff(x(t),t) - f) + r2 = eq[1].match(diff(y(t),t) - g) + num, den = ( + (r1[f].subs(x(t),u).subs(y(t),v))/ + (r2[g].subs(x(t),u).subs(y(t),v))).as_numer_denom() + R1 = num.match(f1*g1) + R2 = den.match(f2*g2) + phi = (r1[f].subs(x(t),u).subs(y(t),v))/num + F1 = R1[f1]; F2 = R2[f2] + G1 = R1[g1]; G2 = R2[g2] + sol1r = solve(Integral(F2/F1, u).doit() - Integral(G1/G2,v).doit() - C1, u) + sol2r = solve(Integral(F2/F1, u).doit() - Integral(G1/G2,v).doit() - C1, v) + sol = [] + for sols in sol1r: + sol.append(Eq(y(t), dsolve(diff(V(t),t) - F2.subs(u,sols).subs(v,V(t))*G2.subs(v,V(t))*phi.subs(u,sols).subs(v,V(t))).rhs)) + for sols in sol2r: + sol.append(Eq(x(t), dsolve(diff(U(t),t) - F1.subs(u,U(t))*G1.subs(v,sols).subs(u,U(t))*phi.subs(v,sols).subs(u,U(t))).rhs)) + return set(sol) + +def _nonlinear_2eq_order1_type5(func, t, eq): + r""" + Clairaut system of ODEs + + .. math:: x = t x' + F(x',y') + + .. math:: y = t y' + G(x',y') + + The following are solutions of the system + + `(i)` straight lines: + + .. math:: x = C_1 t + F(C_1, C_2), y = C_2 t + G(C_1, C_2) + + where `C_1` and `C_2` are arbitrary constants; + + `(ii)` envelopes of the above lines; + + `(iii)` continuously differentiable lines made up from segments of the lines + `(i)` and `(ii)`. + + """ + C1, C2 = get_numbered_constants(eq, num=2) + f = Wild('f') + g = Wild('g') + def check_type(x, y): + r1 = eq[0].match(t*diff(x(t),t) - x(t) + f) + r2 = eq[1].match(t*diff(y(t),t) - y(t) + g) + if not (r1 and r2): + r1 = eq[0].match(diff(x(t),t) - x(t)/t + f/t) + r2 = eq[1].match(diff(y(t),t) - y(t)/t + g/t) + if not (r1 and r2): + r1 = (-eq[0]).match(t*diff(x(t),t) - x(t) + f) + r2 = (-eq[1]).match(t*diff(y(t),t) - y(t) + g) + if not (r1 and r2): + r1 = (-eq[0]).match(diff(x(t),t) - x(t)/t + f/t) + r2 = (-eq[1]).match(diff(y(t),t) - y(t)/t + g/t) + return [r1, r2] + for func_ in func: + if isinstance(func_, list): + x = func[0][0].func + y = func[0][1].func + [r1, r2] = check_type(x, y) + if not (r1 and r2): + [r1, r2] = check_type(y, x) + x, y = y, x + x1 = diff(x(t),t); y1 = diff(y(t),t) + return {Eq(x(t), C1*t + r1[f].subs(x1,C1).subs(y1,C2)), Eq(y(t), C2*t + r2[g].subs(x1,C1).subs(y1,C2))} + +def sysode_nonlinear_3eq_order1(match_): + x = match_['func'][0].func + y = match_['func'][1].func + z = match_['func'][2].func + eq = match_['eq'] + t = list(list(eq[0].atoms(Derivative))[0].atoms(Symbol))[0] + if match_['type_of_equation'] == 'type1': + sol = _nonlinear_3eq_order1_type1(x, y, z, t, eq) + if match_['type_of_equation'] == 'type2': + sol = _nonlinear_3eq_order1_type2(x, y, z, t, eq) + if match_['type_of_equation'] == 'type3': + sol = _nonlinear_3eq_order1_type3(x, y, z, t, eq) + if match_['type_of_equation'] == 'type4': + sol = _nonlinear_3eq_order1_type4(x, y, z, t, eq) + if match_['type_of_equation'] == 'type5': + sol = _nonlinear_3eq_order1_type5(x, y, z, t, eq) + return sol + +def _nonlinear_3eq_order1_type1(x, y, z, t, eq): + r""" + Equations: + + .. math:: a x' = (b - c) y z, \enspace b y' = (c - a) z x, \enspace c z' = (a - b) x y + + First Integrals: + + .. math:: a x^{2} + b y^{2} + c z^{2} = C_1 + + .. math:: a^{2} x^{2} + b^{2} y^{2} + c^{2} z^{2} = C_2 + + where `C_1` and `C_2` are arbitrary constants. On solving the integrals for `y` and + `z` and on substituting the resulting expressions into the first equation of the + system, we arrives at a separable first-order equation on `x`. Similarly doing that + for other two equations, we will arrive at first order equation on `y` and `z` too. + + References + ========== + -https://eqworld.ipmnet.ru/en/solutions/sysode/sode0401.pdf + + """ + C1, C2 = get_numbered_constants(eq, num=2) + u, v, w = symbols('u, v, w') + p = Wild('p', exclude=[x(t), y(t), z(t), t]) + q = Wild('q', exclude=[x(t), y(t), z(t), t]) + s = Wild('s', exclude=[x(t), y(t), z(t), t]) + r = (diff(x(t),t) - eq[0]).match(p*y(t)*z(t)) + r.update((diff(y(t),t) - eq[1]).match(q*z(t)*x(t))) + r.update((diff(z(t),t) - eq[2]).match(s*x(t)*y(t))) + n1, d1 = r[p].as_numer_denom() + n2, d2 = r[q].as_numer_denom() + n3, d3 = r[s].as_numer_denom() + val = solve([n1*u-d1*v+d1*w, d2*u+n2*v-d2*w, d3*u-d3*v-n3*w],[u,v]) + vals = [val[v], val[u]] + c = lcm(vals[0].as_numer_denom()[1], vals[1].as_numer_denom()[1]) + b = vals[0].subs(w, c) + a = vals[1].subs(w, c) + y_x = sqrt(((c*C1-C2) - a*(c-a)*x(t)**2)/(b*(c-b))) + z_x = sqrt(((b*C1-C2) - a*(b-a)*x(t)**2)/(c*(b-c))) + z_y = sqrt(((a*C1-C2) - b*(a-b)*y(t)**2)/(c*(a-c))) + x_y = sqrt(((c*C1-C2) - b*(c-b)*y(t)**2)/(a*(c-a))) + x_z = sqrt(((b*C1-C2) - c*(b-c)*z(t)**2)/(a*(b-a))) + y_z = sqrt(((a*C1-C2) - c*(a-c)*z(t)**2)/(b*(a-b))) + sol1 = dsolve(a*diff(x(t),t) - (b-c)*y_x*z_x) + sol2 = dsolve(b*diff(y(t),t) - (c-a)*z_y*x_y) + sol3 = dsolve(c*diff(z(t),t) - (a-b)*x_z*y_z) + return [sol1, sol2, sol3] + + +def _nonlinear_3eq_order1_type2(x, y, z, t, eq): + r""" + Equations: + + .. math:: a x' = (b - c) y z f(x, y, z, t) + + .. math:: b y' = (c - a) z x f(x, y, z, t) + + .. math:: c z' = (a - b) x y f(x, y, z, t) + + First Integrals: + + .. math:: a x^{2} + b y^{2} + c z^{2} = C_1 + + .. math:: a^{2} x^{2} + b^{2} y^{2} + c^{2} z^{2} = C_2 + + where `C_1` and `C_2` are arbitrary constants. On solving the integrals for `y` and + `z` and on substituting the resulting expressions into the first equation of the + system, we arrives at a first-order differential equations on `x`. Similarly doing + that for other two equations we will arrive at first order equation on `y` and `z`. + + References + ========== + -https://eqworld.ipmnet.ru/en/solutions/sysode/sode0402.pdf + + """ + C1, C2 = get_numbered_constants(eq, num=2) + u, v, w = symbols('u, v, w') + p = Wild('p', exclude=[x(t), y(t), z(t), t]) + q = Wild('q', exclude=[x(t), y(t), z(t), t]) + s = Wild('s', exclude=[x(t), y(t), z(t), t]) + f = Wild('f') + r1 = (diff(x(t),t) - eq[0]).match(y(t)*z(t)*f) + r = collect_const(r1[f]).match(p*f) + r.update(((diff(y(t),t) - eq[1])/r[f]).match(q*z(t)*x(t))) + r.update(((diff(z(t),t) - eq[2])/r[f]).match(s*x(t)*y(t))) + n1, d1 = r[p].as_numer_denom() + n2, d2 = r[q].as_numer_denom() + n3, d3 = r[s].as_numer_denom() + val = solve([n1*u-d1*v+d1*w, d2*u+n2*v-d2*w, -d3*u+d3*v+n3*w],[u,v]) + vals = [val[v], val[u]] + c = lcm(vals[0].as_numer_denom()[1], vals[1].as_numer_denom()[1]) + a = vals[0].subs(w, c) + b = vals[1].subs(w, c) + y_x = sqrt(((c*C1-C2) - a*(c-a)*x(t)**2)/(b*(c-b))) + z_x = sqrt(((b*C1-C2) - a*(b-a)*x(t)**2)/(c*(b-c))) + z_y = sqrt(((a*C1-C2) - b*(a-b)*y(t)**2)/(c*(a-c))) + x_y = sqrt(((c*C1-C2) - b*(c-b)*y(t)**2)/(a*(c-a))) + x_z = sqrt(((b*C1-C2) - c*(b-c)*z(t)**2)/(a*(b-a))) + y_z = sqrt(((a*C1-C2) - c*(a-c)*z(t)**2)/(b*(a-b))) + sol1 = dsolve(a*diff(x(t),t) - (b-c)*y_x*z_x*r[f]) + sol2 = dsolve(b*diff(y(t),t) - (c-a)*z_y*x_y*r[f]) + sol3 = dsolve(c*diff(z(t),t) - (a-b)*x_z*y_z*r[f]) + return [sol1, sol2, sol3] + +def _nonlinear_3eq_order1_type3(x, y, z, t, eq): + r""" + Equations: + + .. math:: x' = c F_2 - b F_3, \enspace y' = a F_3 - c F_1, \enspace z' = b F_1 - a F_2 + + where `F_n = F_n(x, y, z, t)`. + + 1. First Integral: + + .. math:: a x + b y + c z = C_1, + + where C is an arbitrary constant. + + 2. If we assume function `F_n` to be independent of `t`,i.e, `F_n` = `F_n (x, y, z)` + Then, on eliminating `t` and `z` from the first two equation of the system, one + arrives at the first-order equation + + .. math:: \frac{dy}{dx} = \frac{a F_3 (x, y, z) - c F_1 (x, y, z)}{c F_2 (x, y, z) - + b F_3 (x, y, z)} + + where `z = \frac{1}{c} (C_1 - a x - b y)` + + References + ========== + -https://eqworld.ipmnet.ru/en/solutions/sysode/sode0404.pdf + + """ + C1 = get_numbered_constants(eq, num=1) + u, v, w = symbols('u, v, w') + fu, fv, fw = symbols('u, v, w', cls=Function) + p = Wild('p', exclude=[x(t), y(t), z(t), t]) + q = Wild('q', exclude=[x(t), y(t), z(t), t]) + s = Wild('s', exclude=[x(t), y(t), z(t), t]) + F1, F2, F3 = symbols('F1, F2, F3', cls=Wild) + r1 = (diff(x(t), t) - eq[0]).match(F2-F3) + r = collect_const(r1[F2]).match(s*F2) + r.update(collect_const(r1[F3]).match(q*F3)) + if eq[1].has(r[F2]) and not eq[1].has(r[F3]): + r[F2], r[F3] = r[F3], r[F2] + r[s], r[q] = -r[q], -r[s] + r.update((diff(y(t), t) - eq[1]).match(p*r[F3] - r[s]*F1)) + a = r[p]; b = r[q]; c = r[s] + F1 = r[F1].subs(x(t), u).subs(y(t),v).subs(z(t), w) + F2 = r[F2].subs(x(t), u).subs(y(t),v).subs(z(t), w) + F3 = r[F3].subs(x(t), u).subs(y(t),v).subs(z(t), w) + z_xy = (C1-a*u-b*v)/c + y_zx = (C1-a*u-c*w)/b + x_yz = (C1-b*v-c*w)/a + y_x = dsolve(diff(fv(u),u) - ((a*F3-c*F1)/(c*F2-b*F3)).subs(w,z_xy).subs(v,fv(u))).rhs + z_x = dsolve(diff(fw(u),u) - ((b*F1-a*F2)/(c*F2-b*F3)).subs(v,y_zx).subs(w,fw(u))).rhs + z_y = dsolve(diff(fw(v),v) - ((b*F1-a*F2)/(a*F3-c*F1)).subs(u,x_yz).subs(w,fw(v))).rhs + x_y = dsolve(diff(fu(v),v) - ((c*F2-b*F3)/(a*F3-c*F1)).subs(w,z_xy).subs(u,fu(v))).rhs + y_z = dsolve(diff(fv(w),w) - ((a*F3-c*F1)/(b*F1-a*F2)).subs(u,x_yz).subs(v,fv(w))).rhs + x_z = dsolve(diff(fu(w),w) - ((c*F2-b*F3)/(b*F1-a*F2)).subs(v,y_zx).subs(u,fu(w))).rhs + sol1 = dsolve(diff(fu(t),t) - (c*F2 - b*F3).subs(v,y_x).subs(w,z_x).subs(u,fu(t))).rhs + sol2 = dsolve(diff(fv(t),t) - (a*F3 - c*F1).subs(u,x_y).subs(w,z_y).subs(v,fv(t))).rhs + sol3 = dsolve(diff(fw(t),t) - (b*F1 - a*F2).subs(u,x_z).subs(v,y_z).subs(w,fw(t))).rhs + return [sol1, sol2, sol3] + +def _nonlinear_3eq_order1_type4(x, y, z, t, eq): + r""" + Equations: + + .. math:: x' = c z F_2 - b y F_3, \enspace y' = a x F_3 - c z F_1, \enspace z' = b y F_1 - a x F_2 + + where `F_n = F_n (x, y, z, t)` + + 1. First integral: + + .. math:: a x^{2} + b y^{2} + c z^{2} = C_1 + + where `C` is an arbitrary constant. + + 2. Assuming the function `F_n` is independent of `t`: `F_n = F_n (x, y, z)`. Then on + eliminating `t` and `z` from the first two equations of the system, one arrives at + the first-order equation + + .. math:: \frac{dy}{dx} = \frac{a x F_3 (x, y, z) - c z F_1 (x, y, z)} + {c z F_2 (x, y, z) - b y F_3 (x, y, z)} + + where `z = \pm \sqrt{\frac{1}{c} (C_1 - a x^{2} - b y^{2})}` + + References + ========== + -https://eqworld.ipmnet.ru/en/solutions/sysode/sode0405.pdf + + """ + C1 = get_numbered_constants(eq, num=1) + u, v, w = symbols('u, v, w') + p = Wild('p', exclude=[x(t), y(t), z(t), t]) + q = Wild('q', exclude=[x(t), y(t), z(t), t]) + s = Wild('s', exclude=[x(t), y(t), z(t), t]) + F1, F2, F3 = symbols('F1, F2, F3', cls=Wild) + r1 = eq[0].match(diff(x(t),t) - z(t)*F2 + y(t)*F3) + r = collect_const(r1[F2]).match(s*F2) + r.update(collect_const(r1[F3]).match(q*F3)) + if eq[1].has(r[F2]) and not eq[1].has(r[F3]): + r[F2], r[F3] = r[F3], r[F2] + r[s], r[q] = -r[q], -r[s] + r.update((diff(y(t),t) - eq[1]).match(p*x(t)*r[F3] - r[s]*z(t)*F1)) + a = r[p]; b = r[q]; c = r[s] + F1 = r[F1].subs(x(t),u).subs(y(t),v).subs(z(t),w) + F2 = r[F2].subs(x(t),u).subs(y(t),v).subs(z(t),w) + F3 = r[F3].subs(x(t),u).subs(y(t),v).subs(z(t),w) + x_yz = sqrt((C1 - b*v**2 - c*w**2)/a) + y_zx = sqrt((C1 - c*w**2 - a*u**2)/b) + z_xy = sqrt((C1 - a*u**2 - b*v**2)/c) + y_x = dsolve(diff(v(u),u) - ((a*u*F3-c*w*F1)/(c*w*F2-b*v*F3)).subs(w,z_xy).subs(v,v(u))).rhs + z_x = dsolve(diff(w(u),u) - ((b*v*F1-a*u*F2)/(c*w*F2-b*v*F3)).subs(v,y_zx).subs(w,w(u))).rhs + z_y = dsolve(diff(w(v),v) - ((b*v*F1-a*u*F2)/(a*u*F3-c*w*F1)).subs(u,x_yz).subs(w,w(v))).rhs + x_y = dsolve(diff(u(v),v) - ((c*w*F2-b*v*F3)/(a*u*F3-c*w*F1)).subs(w,z_xy).subs(u,u(v))).rhs + y_z = dsolve(diff(v(w),w) - ((a*u*F3-c*w*F1)/(b*v*F1-a*u*F2)).subs(u,x_yz).subs(v,v(w))).rhs + x_z = dsolve(diff(u(w),w) - ((c*w*F2-b*v*F3)/(b*v*F1-a*u*F2)).subs(v,y_zx).subs(u,u(w))).rhs + sol1 = dsolve(diff(u(t),t) - (c*w*F2 - b*v*F3).subs(v,y_x).subs(w,z_x).subs(u,u(t))).rhs + sol2 = dsolve(diff(v(t),t) - (a*u*F3 - c*w*F1).subs(u,x_y).subs(w,z_y).subs(v,v(t))).rhs + sol3 = dsolve(diff(w(t),t) - (b*v*F1 - a*u*F2).subs(u,x_z).subs(v,y_z).subs(w,w(t))).rhs + return [sol1, sol2, sol3] + +def _nonlinear_3eq_order1_type5(x, y, z, t, eq): + r""" + .. math:: x' = x (c F_2 - b F_3), \enspace y' = y (a F_3 - c F_1), \enspace z' = z (b F_1 - a F_2) + + where `F_n = F_n (x, y, z, t)` and are arbitrary functions. + + First Integral: + + .. math:: \left|x\right|^{a} \left|y\right|^{b} \left|z\right|^{c} = C_1 + + where `C` is an arbitrary constant. If the function `F_n` is independent of `t`, + then, by eliminating `t` and `z` from the first two equations of the system, one + arrives at a first-order equation. + + References + ========== + -https://eqworld.ipmnet.ru/en/solutions/sysode/sode0406.pdf + + """ + C1 = get_numbered_constants(eq, num=1) + u, v, w = symbols('u, v, w') + fu, fv, fw = symbols('u, v, w', cls=Function) + p = Wild('p', exclude=[x(t), y(t), z(t), t]) + q = Wild('q', exclude=[x(t), y(t), z(t), t]) + s = Wild('s', exclude=[x(t), y(t), z(t), t]) + F1, F2, F3 = symbols('F1, F2, F3', cls=Wild) + r1 = eq[0].match(diff(x(t), t) - x(t)*F2 + x(t)*F3) + r = collect_const(r1[F2]).match(s*F2) + r.update(collect_const(r1[F3]).match(q*F3)) + if eq[1].has(r[F2]) and not eq[1].has(r[F3]): + r[F2], r[F3] = r[F3], r[F2] + r[s], r[q] = -r[q], -r[s] + r.update((diff(y(t), t) - eq[1]).match(y(t)*(p*r[F3] - r[s]*F1))) + a = r[p]; b = r[q]; c = r[s] + F1 = r[F1].subs(x(t), u).subs(y(t), v).subs(z(t), w) + F2 = r[F2].subs(x(t), u).subs(y(t), v).subs(z(t), w) + F3 = r[F3].subs(x(t), u).subs(y(t), v).subs(z(t), w) + x_yz = (C1*v**-b*w**-c)**-a + y_zx = (C1*w**-c*u**-a)**-b + z_xy = (C1*u**-a*v**-b)**-c + y_x = dsolve(diff(fv(u), u) - ((v*(a*F3 - c*F1))/(u*(c*F2 - b*F3))).subs(w, z_xy).subs(v, fv(u))).rhs + z_x = dsolve(diff(fw(u), u) - ((w*(b*F1 - a*F2))/(u*(c*F2 - b*F3))).subs(v, y_zx).subs(w, fw(u))).rhs + z_y = dsolve(diff(fw(v), v) - ((w*(b*F1 - a*F2))/(v*(a*F3 - c*F1))).subs(u, x_yz).subs(w, fw(v))).rhs + x_y = dsolve(diff(fu(v), v) - ((u*(c*F2 - b*F3))/(v*(a*F3 - c*F1))).subs(w, z_xy).subs(u, fu(v))).rhs + y_z = dsolve(diff(fv(w), w) - ((v*(a*F3 - c*F1))/(w*(b*F1 - a*F2))).subs(u, x_yz).subs(v, fv(w))).rhs + x_z = dsolve(diff(fu(w), w) - ((u*(c*F2 - b*F3))/(w*(b*F1 - a*F2))).subs(v, y_zx).subs(u, fu(w))).rhs + sol1 = dsolve(diff(fu(t), t) - (u*(c*F2 - b*F3)).subs(v, y_x).subs(w, z_x).subs(u, fu(t))).rhs + sol2 = dsolve(diff(fv(t), t) - (v*(a*F3 - c*F1)).subs(u, x_y).subs(w, z_y).subs(v, fv(t))).rhs + sol3 = dsolve(diff(fw(t), t) - (w*(b*F1 - a*F2)).subs(u, x_z).subs(v, y_z).subs(w, fw(t))).rhs + return [sol1, sol2, sol3] + + +#This import is written at the bottom to avoid circular imports. +from .single import SingleODEProblem, SingleODESolver, solver_map