def pde_1st_linear_constant_coeff(eq, func, order, match, solvefun): r""" Solves a first order linear partial differential equation with constant coefficients. The general form of this partial differential equation is .. math:: a \frac{df(x,y)}{dx} + b \frac{df(x,y)}{dy} + c f(x,y) = G(x,y) where `a`, `b` and `c` are constants and `G(x, y)` can be an arbitrary function in `x` and `y`. The general solution of the PDE is:: >>> from sympy.solvers import pdsolve >>> from sympy.abc import x, y, a, b, c >>> from sympy import Function, pprint >>> f = Function('f') >>> G = Function('G') >>> u = f(x,y) >>> ux = u.diff(x) >>> uy = u.diff(y) >>> genform = a*u + b*ux + c*uy - G(x,y) >>> pprint(genform) d d a*f(x, y) + b*--(f(x, y)) + c*--(f(x, y)) - G(x, y) dx dy >>> pprint(pdsolve(genform, hint='1st_linear_constant_coeff_Integral')) // b*x + c*y \ || / | || | | || | a*xi | || | ------- | || | 2 2 | || | /b*xi + c*eta -b*eta + c*xi\ b + c | || | G|------------, -------------|*e d(xi)| || | | 2 2 2 2 | | || | \ b + c b + c / | || | | || / | || | f(x, y) = ||F(eta) + -------------------------------------------------------|* || 2 2 | \\ b + c / <BLANKLINE> \| || || || || || || || || -a*xi || -------|| 2 2|| b + c || e || || /|eta=-b*y + c*x, xi=b*x + c*y Examples ======== >>> from sympy.solvers.pde import pdsolve >>> from sympy import Function, diff, pprint, exp >>> from sympy.abc import x,y >>> f = Function('f') >>> eq = -2*f(x,y).diff(x) + 4*f(x,y).diff(y) + 5*f(x,y) - exp(x + 3*y) >>> pdsolve(eq) f(x, y) == (F(4*x + 2*y) + exp(x/2 + 4*y)/15)*exp(x/2 - y) References ========== - Viktor Grigoryan, "Partial Differential Equations" Math 124A - Fall 2010, pp.7 """ # TODO : For now homogeneous first order linear PDE's having # two variables are implemented. Once there is support for # solving systems of ODE's, this can be extended to n variables. xi, eta = symbols("xi eta") f = func.func x = func.args[0] y = func.args[1] b = match[match['b']] c = match[match['c']] d = match[match['d']] e = -match[match['e']] expterm = exp(-S(d) / (b**2 + c**2) * xi) functerm = solvefun(eta) solvedict = solve((b * x + c * y - xi, c * x - b * y - eta), x, y) # Integral should remain as it is in terms of xi, # doit() should be done in _handle_Integral. genterm = (1 / S(b**2 + c**2)) * C.Integral( (1 / expterm * e).subs(solvedict), (xi, b * x + c * y)) return Eq( f(x, y), Subs(expterm * (functerm + genterm), (eta, xi), (c * x - b * y, b * x + c * y)))
def pde_1st_linear_constant_coeff(eq, func, order, match, solvefun): r""" Solves a first order linear partial differential equation with constant coefficients. The general form of this partial differential equation is .. math:: a \frac{\partial f(x,y)}{\partial x} + b \frac{\partial f(x,y)}{\partial y} + c f(x,y) = G(x,y) where `a`, `b` and `c` are constants and `G(x, y)` can be an arbitrary function in `x` and `y`. The general solution of the PDE is: .. math:: f(x, y) = \left. \left[F(\eta) + \frac{1}{a^2 + b^2} \int\limits^{a x + b y} G\left(\frac{a \xi + b \eta}{a^2 + b^2}, \frac{- a \eta + b \xi}{a^2 + b^2} \right) e^{\frac{c \xi}{a^2 + b^2}}\, d\xi\right] e^{- \frac{c \xi}{a^2 + b^2}} \right|_{\substack{\eta=- a y + b x\\ \xi=a x + b y }}\, , where `F(\eta)` is an arbitrary single-valued function. The solution can be found in SymPy with ``pdsolve``:: >>> from sympy.solvers import pdsolve >>> from sympy.abc import x, y, a, b, c >>> from sympy import Function, pprint >>> f = Function('f') >>> G = Function('G') >>> u = f(x,y) >>> ux = u.diff(x) >>> uy = u.diff(y) >>> genform = a*ux + b*uy + c*u - G(x,y) >>> pprint(genform) d d a*--(f(x, y)) + b*--(f(x, y)) + c*f(x, y) - G(x, y) dx dy >>> pprint(pdsolve(genform, hint='1st_linear_constant_coeff_Integral')) // a*x + b*y \ || / | || | | || | c*xi | || | ------- | || | 2 2 | || | /a*xi + b*eta -a*eta + b*xi\ a + b | || | G|------------, -------------|*e d(xi)| || | | 2 2 2 2 | | || | \ a + b a + b / | || | | || / | || | f(x, y) = ||F(eta) + -------------------------------------------------------|* || 2 2 | \\ a + b / <BLANKLINE> \| || || || || || || || || -c*xi || -------|| 2 2|| a + b || e || || /|eta=-a*y + b*x, xi=a*x + b*y Examples ======== >>> from sympy.solvers.pde import pdsolve >>> from sympy import Function, diff, pprint, exp >>> from sympy.abc import x,y >>> f = Function('f') >>> eq = -2*f(x,y).diff(x) + 4*f(x,y).diff(y) + 5*f(x,y) - exp(x + 3*y) >>> pdsolve(eq) Eq(f(x, y), (F(4*x + 2*y) + exp(x/2 + 4*y)/15)*exp(x/2 - y)) References ========== - Viktor Grigoryan, "Partial Differential Equations" Math 124A - Fall 2010, pp.7 """ # TODO : For now homogeneous first order linear PDE's having # two variables are implemented. Once there is support for # solving systems of ODE's, this can be extended to n variables. xi, eta = symbols("xi eta") f = func.func x = func.args[0] y = func.args[1] b = match[match['b']] c = match[match['c']] d = match[match['d']] e = -match[match['e']] expterm = exp(-S(d) / (b**2 + c**2) * xi) functerm = solvefun(eta) solvedict = solve((b * x + c * y - xi, c * x - b * y - eta), x, y) # Integral should remain as it is in terms of xi, # doit() should be done in _handle_Integral. genterm = (1 / S(b**2 + c**2)) * Integral( (1 / expterm * e).subs(solvedict), (xi, b * x + c * y)) return Eq( f(x, y), Subs(expterm * (functerm + genterm), (eta, xi), (c * x - b * y, b * x + c * y)))
def test_subs_in_derivative(): expr = sin(x*exp(y)) u = Function('u') v = Function('v') assert Derivative(expr, y).subs(expr, y) == Derivative(y, y) assert Derivative(expr, y).subs(y, x).doit() == \ Derivative(expr, y).doit().subs(y, x) assert Derivative(f(x, y), y).subs(y, x) == Subs(Derivative(f(x, y), y), y, x) assert Derivative(f(x, y), y).subs(x, y) == Subs(Derivative(f(x, y), y), x, y) assert Derivative(f(x, y), y).subs(y, g(x, y)) == Subs(Derivative(f(x, y), y), y, g(x, y)).doit() assert Derivative(f(x, y), y).subs(x, g(x, y)) == Subs(Derivative(f(x, y), y), x, g(x, y)) assert Derivative(f(x, y), g(y)).subs(x, g(x, y)) == Derivative(f(g(x, y), y), g(y)) assert Derivative(f(u(x), h(y)), h(y)).subs(h(y), g(x, y)) == \ Subs(Derivative(f(u(x), h(y)), h(y)), h(y), g(x, y)).doit() assert Derivative(f(x, y), y).subs(y, z) == Derivative(f(x, z), z) assert Derivative(f(x, y), y).subs(y, g(y)) == Derivative(f(x, g(y)), g(y)) assert Derivative(f(g(x), h(y)), h(y)).subs(h(y), u(y)) == \ Derivative(f(g(x), u(y)), u(y)) assert Derivative(f(x, f(x, x)), f(x, x)).subs( f, Lambda((x, y), x + y)) == Subs( Derivative(z + x, z), z, 2*x) assert Subs(Derivative(f(f(x)), x), f, cos).doit() == sin(x)*sin(cos(x)) assert Subs(Derivative(f(f(x)), f(x)), f, cos).doit() == -sin(cos(x)) # Issue 13791. No comparison (it's a long formula) but this used to raise an exception. assert isinstance(v(x, y, u(x, y)).diff(y).diff(x).diff(y), Expr) # This is also related to issues 13791 and 13795; issue 15190 F = Lambda((x, y), exp(2*x + 3*y)) abstract = f(x, f(x, x)).diff(x, 2) concrete = F(x, F(x, x)).diff(x, 2) assert (abstract.subs(f, F).doit() - concrete).simplify() == 0 # don't introduce a new symbol if not necessary assert x in f(x).diff(x).subs(x, 0).atoms() # case (4) assert Derivative(f(x,f(x,y)), x, y).subs(x, g(y) ) == Subs(Derivative(f(x, f(x, y)), x, y), x, g(y)) assert Derivative(f(x, x), x).subs(x, 0 ) == Subs(Derivative(f(x, x), x), x, 0) # issue 15194 assert Derivative(f(y, g(x)), (x, z)).subs(z, x ) == Derivative(f(y, g(x)), (x, x)) df = f(x).diff(x) assert df.subs(df, 1) is S.One assert df.diff(df) is S.One dxy = Derivative(f(x, y), x, y) dyx = Derivative(f(x, y), y, x) assert dxy.subs(Derivative(f(x, y), y, x), 1) is S.One assert dxy.diff(dyx) is S.One assert Derivative(f(x, y), x, 2, y, 3).subs( dyx, g(x, y)) == Derivative(g(x, y), x, 1, y, 2) assert Derivative(f(x, x - y), y).subs(x, x + y) == Subs( Derivative(f(x, x - y), y), x, x + y)
def test_sympy__core__function__Subs(): from sympy.core.function import Subs assert _test_args(Subs(x + y, x, 2))
def test_Subs2(): # this reflects a limitation of subs(), probably won't fix assert Subs(f(x), x**2, x).doit() == f(sqrt(x))
def test_Subs(): assert Subs(1, (), ()) is S.One # check null subs influence on hashing assert Subs(x, y, z) != Subs(x, y, 1) # neutral subs works assert Subs(x, x, 1).subs(x, y).has(y) # self mapping var/point assert Subs(Derivative(f(x), (x, 2)), x, x).doit() == f(x).diff(x, x) assert Subs(x, x, 0).has(x) # it's a structural answer assert not Subs(x, x, 0).free_symbols assert Subs(Subs(x + y, x, 2), y, 1) == Subs(x + y, (x, y), (2, 1)) assert Subs(x, (x,), (0,)) == Subs(x, x, 0) assert Subs(x, x, 0) == Subs(y, y, 0) assert Subs(x, x, 0).subs(x, 1) == Subs(x, x, 0) assert Subs(y, x, 0).subs(y, 1) == Subs(1, x, 0) assert Subs(f(x), x, 0).doit() == f(0) assert Subs(f(x**2), x**2, 0).doit() == f(0) assert Subs(f(x, y, z), (x, y, z), (0, 1, 1)) != \ Subs(f(x, y, z), (x, y, z), (0, 0, 1)) assert Subs(x, y, 2).subs(x, y).doit() == 2 assert Subs(f(x, y), (x, y, z), (0, 1, 1)) != \ Subs(f(x, y) + z, (x, y, z), (0, 1, 0)) assert Subs(f(x, y), (x, y), (0, 1)).doit() == f(0, 1) assert Subs(Subs(f(x, y), x, 0), y, 1).doit() == f(0, 1) raises(ValueError, lambda: Subs(f(x, y), (x, y), (0, 0, 1))) raises(ValueError, lambda: Subs(f(x, y), (x, x, y), (0, 0, 1))) assert len(Subs(f(x, y), (x, y), (0, 1)).variables) == 2 assert Subs(f(x, y), (x, y), (0, 1)).point == Tuple(0, 1) assert Subs(f(x), x, 0) == Subs(f(y), y, 0) assert Subs(f(x, y), (x, y), (0, 1)) == Subs(f(x, y), (y, x), (1, 0)) assert Subs(f(x)*y, (x, y), (0, 1)) == Subs(f(y)*x, (y, x), (0, 1)) assert Subs(f(x)*y, (x, y), (1, 1)) == Subs(f(y)*x, (x, y), (1, 1)) assert Subs(f(x), x, 0).subs(x, 1).doit() == f(0) assert Subs(f(x), x, y).subs(y, 0) == Subs(f(x), x, 0) assert Subs(y*f(x), x, y).subs(y, 2) == Subs(2*f(x), x, 2) assert (2 * Subs(f(x), x, 0)).subs(Subs(f(x), x, 0), y) == 2*y assert Subs(f(x), x, 0).free_symbols == set() assert Subs(f(x, y), x, z).free_symbols == {y, z} assert Subs(f(x).diff(x), x, 0).doit(), Subs(f(x).diff(x), x, 0) assert Subs(1 + f(x).diff(x), x, 0).doit(), 1 + Subs(f(x).diff(x), x, 0) assert Subs(y*f(x, y).diff(x), (x, y), (0, 2)).doit() == \ 2*Subs(Derivative(f(x, 2), x), x, 0) assert Subs(y**2*f(x), x, 0).diff(y) == 2*y*f(0) e = Subs(y**2*f(x), x, y) assert e.diff(y) == e.doit().diff(y) == y**2*Derivative(f(y), y) + 2*y*f(y) assert Subs(f(x), x, 0) + Subs(f(x), x, 0) == 2*Subs(f(x), x, 0) e1 = Subs(z*f(x), x, 1) e2 = Subs(z*f(y), y, 1) assert e1 + e2 == 2*e1 assert e1.__hash__() == e2.__hash__() assert Subs(z*f(x + 1), x, 1) not in [ e1, e2 ] assert Derivative(f(x), x).subs(x, g(x)) == Derivative(f(g(x)), g(x)) assert Derivative(f(x), x).subs(x, x + y) == Subs(Derivative(f(x), x), x, x + y) assert Subs(f(x)*cos(y) + z, (x, y), (0, pi/3)).n(2) == \ Subs(f(x)*cos(y) + z, (x, y), (0, pi/3)).evalf(2) == \ z + Rational('1/2').n(2)*f(0) assert f(x).diff(x).subs(x, 0).subs(x, y) == f(x).diff(x).subs(x, 0) assert (x*f(x).diff(x).subs(x, 0)).subs(x, y) == y*f(x).diff(x).subs(x, 0) assert Subs(Derivative(g(x)**2, g(x), x), g(x), exp(x) ).doit() == 2*exp(x) assert Subs(Derivative(g(x)**2, g(x), x), g(x), exp(x) ).doit(deep=False) == 2*Derivative(exp(x), x) assert Derivative(f(x, g(x)), x).doit() == Derivative( f(x, g(x)), g(x))*Derivative(g(x), x) + Subs(Derivative( f(y, g(x)), y), y, x)
def test_issue_15226(): assert Subs(Derivative(f(y), x, y), y, g(x)).doit() != 0
def test_issue_12005(): e1 = Subs(Derivative(f(x), x), x, x) assert e1.diff(x) == Derivative(f(x), x, x) e2 = Subs(Derivative(f(x), x), x, x**2 + 1) assert e2.diff(x) == 2*x*Subs(Derivative(f(x), x, x), x, x**2 + 1) e3 = Subs(Derivative(f(x) + y**2 - y, y), y, y**2) assert e3.diff(y) == 4*y e4 = Subs(Derivative(f(x + y), y), y, (x**2)) assert e4.diff(y) is S.Zero e5 = Subs(Derivative(f(x), x), (y, z), (y, z)) assert e5.diff(x) == Derivative(f(x), x, x) assert f(g(x)).diff(g(x), g(x)) == Derivative(f(g(x)), g(x), g(x))
def test_series_of_Subs(): from sympy.abc import z subs1 = Subs(sin(x), x, y) subs2 = Subs(sin(x) * cos(z), x, y) subs3 = Subs(sin(x * z), (x, z), (y, x)) assert subs1.series(x) == subs1 subs1_series = (Subs(x, x, y) + Subs(-x**3/6, x, y) + Subs(x**5/120, x, y) + O(y**6)) assert subs1.series() == subs1_series assert subs1.series(y) == subs1_series assert subs1.series(z) == subs1 assert subs2.series(z) == (Subs(z**4*sin(x)/24, x, y) + Subs(-z**2*sin(x)/2, x, y) + Subs(sin(x), x, y) + O(z**6)) assert subs3.series(x).doit() == subs3.doit().series(x) assert subs3.series(z).doit() == sin(x*y) raises(ValueError, lambda: Subs(x + 2*y, y, z).series()) assert Subs(x + y, y, z).series(x).doit() == x + z