Status: Accepted
Owner: smichr
Labels: Type-Defect Priority-Medium
New issue 2039 by smichr: Mul.eval_subs problems
http://code.google.com/p/sympy/issues/detail?id=2039
Issue 2022 identified a problem with handling of non-commutative terms in a
Mul substitution. There are a couple of other problems it has:
1) it is protecting against removal of ele from comms_final but then
removes the terms from comm_self; this can cause it to fail when trying to
remove an element that is not there:
{lower case commutative, upper case non-commutative}
(a*A*B).subs(a**2*A,1)
.
.
.
ValueError: list.remove(x): x not in list
2) It doesn't handle rational powers:
var('x y')
(x, y)
(x**2*y**(3*x/2)).subs(x*y**(x/2),2)
x**2*y**(3*x/2)
It should have returned 4*y**(x/2).
3) It doesn't handle exp(); it should treat it like a power:
(x*exp(x*2)).subs(x*exp(x),2)
x*exp(2*x)
Should have been 2*exp(x).
Point 2 and 3 have working parallels when the power is an integer:
(x*y**2).subs(x*y,2)
2*y
And the current behavior would allow exp(3*x/2).subs(exp(x/2), y) -> y**3
as long as the exp(3*x/2) isn't in a Mul.
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