Cleaned up dsolve, added PDE support for deriv_degree, moved ODE handling
into its own function and add solver stub for PDEs.
Signed-off-by: Priit Laes <pl...@plaes.org>
---
diff --git a/sympy/solvers/solvers.py b/sympy/solvers/solvers.py
index 60a1ef5..30722c0 100644
--- a/sympy/solvers/solvers.py
+++ b/sympy/solvers/solvers.py
@@ -522,27 +522,29 @@ def dsolve(eq, funcs):
else:
f = funcs
- x = f.args[0]
- f = f.func
-
- #We first get the order of the equation, so that we can choose the
- #corresponding methods. Currently, only first and second
- #order odes can be handled.
- order = deriv_degree(eq, f(x))
-
- if order > 2 :
- raise NotImplementedError("dsolve: Cannot solve " + str(eq))
- elif order == 2:
- return solve_ODE_second_order(eq, f(x))
- elif order == 1:
- return solve_ODE_first_order(eq, f(x))
+ # Determine the order of the equation
+ order = deriv_degree(eq, f.func)
+
+ if len(f.args) == 1:
+ return dsolve_ODE(eq, f, order)
else:
- raise NotImplementedError("Not a differential equation!")
+ return dsolve_PDE(eq, f, order)
+
+ raise NotImplementedError("Cannot handle this differential equation!")
+
+def dsolve_PDE(eq, f, order):
+ raise NotImplementedError("dsolve_PDE: Cannot solve " + str(eq))
+
+def dsolve_ODE(eq, f, order):
+ if order == 1:
+ return solve_ODE_first_order(eq, f)
+ elif order == 2:
+ return solve_ODE_second_order(eq, f)
+ raise NotImplementedError("dsolve_ODE: Cannot solve " + str(eq))
def deriv_degree(expr, func):
""" get the order of a given ode, the function is implemented
recursively """
- a = Wild('a', exclude=[func])
order = 0
if isinstance(expr, Derivative):
@@ -551,12 +553,9 @@ def deriv_degree(expr, func):
for arg in expr.args:
if isinstance(arg, Derivative):
order = max(order, len(arg.symbols))
- elif expr.match(a):
- order = 0
else :
for arg1 in arg.args:
order = max(order, deriv_degree(arg1, func))
-
return order
def solve_ODE_first_order(eq, f):
diff --git a/sympy/solvers/tests/test_solvers.py
b/sympy/solvers/tests/test_solvers.py
index 389b661..d22b75d 100644
--- a/sympy/solvers/tests/test_solvers.py
+++ b/sympy/solvers/tests/test_solvers.py
@@ -165,12 +165,15 @@ def test_ODE_1():
def test_deriv_degree():
f = Function('f')
- x = Symbol('x')
+ x,y = symbols('xy')
+ # ODEs
assert deriv_degree(3*x*exp(f(x)), f(x)) == 0
assert deriv_degree(x*diff(f(x),x)+3*x*f(x)-sin(x)/x, f(x)) == 1
assert deriv_degree(x**2*f(x).diff(x,x)+x*diff(f(x),x)-f(x),f(x)) == 2
assert deriv_degree(diff(x*exp(f(x)),x,x), f(x)) == 2
assert deriv_degree(diff(x*diff(x*exp(f(x)), x,x), x), f(x)) == 3
+ # PDEs
+ assert deriv_degree(x**2*f(x,y).diff(x,x)-y**2*f(x,y).diff(y,y),f(x,y)) ==
2
# Note: multiple solutions exist for some of these equations, so the tests
# should be expected to break if the implementation of the solver changes
--
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