Status: Valid
Owner: ----
Labels: Type-Defect Priority-Medium NonCommutative Simplify WrongResult
New issue 3618 by [email protected]: Assumptions on replacement symbols
created by cse
http://code.google.com/p/sympy/issues/detail?id=3618
When cse creates replacement symbols for what it identifies as a common
subexpression, it does not preserve the assumptions on that subexpression.
from sympy import cse, symbols
A, B = symbols('A B', commutative=False)
cse(B*A*A + A*A)
([(x0, A**2)], [x0 + x0*B])
cse(B*A*A - A*A*B)
([], [0])
In the above result from cse, B*A*A has been replaced by A*A*B,
disregarding the noncommutativity. If you inspect the created x0 symbol, it
only has default assumptions on it:
cse(B*A*A + A*A)[0][0][0]
x0
cse(B*A*A + A*A)[0][0][0].assumptions0
{'commutative': True}
Probably that everything we know about the assumptions on the subexpression
should be passed on to the created replacement symbol. This is especially
important for is_commutative, since it leads to obvious wrong results.
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