Updates:
Summary: ode_order failure with Derivative(x*f(x),x)
Status: Accepted
Labels: Solvers WrongResult EasyToFix
Comment #1 on issue 2736 by [email protected]: ode_order failure with
Derivative(x*f(x),x)
http://code.google.com/p/sympy/issues/detail?id=2736
The dsolve bug has fortunately already been fixed in master:
In [35]: dsolve(ode3, f(x))
Out[35]:
Cā
f(x) = āā + 1
x
However, the ode_order issue has not. If you look at the code for
ode_order, you can see that it assumes that any derivative must be a
derivative of f(x) only. This should be a straightforward fix if you want
to give it a try. I think it's better not to call doit() (as that can have
unintended side effects), but to just write the function recursively
correctly.
I also noticed a strange thing with that function. Why is the check
expr.match(a) (to check if expr doesn't have f(x)) inside the loop of
iterating through the args of expr?
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