Status: Valid
Owner: ----
Labels: Type-Defect Priority-High WrongResult
New issue 3342 by [email protected]: subs coefficient error
http://code.google.com/p/sympy/issues/detail?id=3342
This was originally https://github.com/sympy/sympy/issues/1451
When using cse the resulting replacements and reduced_exprs did not give
the same result as the original expression. Digging into the differences, I
found the error came from the propagation of the replacements in
sympy.simplify.cse_main.cse. Looking further, the error appears to be in
the way that Mul handles substitution:
Input:
import sympy
x = sympy.Symbol('x')
y = sympy.Symbol('y')
expr = 4*x**2
print expr
print expr.subs([(2*x, y)])
Output:
4*x**2
2*y**2
The output should obviously be y**2. As a result of this bug, coefficients
are incorrectly calculated in cse. The reason for this seems to be the
incorrect handling of coefficients in sympy.core.mul._eval_subs.
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