Status: Accepted
Owner: asmeurer
Labels: Type-Defect Priority-Medium
New issue 1997 by asmeurer: Bug in dsolve() undetermined coefficients solver
http://code.google.com/p/sympy/issues/detail?id=1997
dsolve(f(x).diff(x, x) + f(x) - x*sin(x - 2), f(x),
hint='nth_linear_constant_coeff_undetermined_coefficients')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "sympy/solvers/ode.py", line 457, in dsolve
match=hints[hint]), func, hints['order'], hint)
File "sympy/solvers/ode.py", line 2370, in
ode_nth_linear_constant_coeff_undetermined_coefficients
return _solve_undetermined_coefficients(eq, func, order, match)
File "sympy/solvers/ode.py", line 2460, in
_solve_undetermined_coefficients
" method of undetermined coefficients (unable to solve for
coefficients).")
NotImplementedError: Could not solve -x*sin(-2 + x) + D(f(x), x, x) + f(x)
using the method of undetermined coefficients (unable to solve for
coefficients).
As I was explaining to Chris on IRC, any time you explicitly pass a hint to
dsolve() and it gives NotImplementedError, it means that there was a
problem in the particular solver, so it is a bug (which is another reason
why issue 1996 needs to be fixed).
I haven't looked into it yet, but if I had to guess, I would say it's
because of the fishy behavior of sin(2 - x) vs. sin(x - 2).
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