Status: New
Owner: ----
Labels: Type-Defect Priority-Medium
New issue 2081 by rosen.diankov: Pow substitution introduces wrong answers
http://code.google.com/p/sympy/issues/detail?id=2081
The most simplest example is:
x=Symbol('x')
(x**3).subs(x**2,Symbol('y'))
y**(3/2)
Regardless if x is Real or Complex, this is wrong because anything to the
power of 1.5 has two possible solutions, while anything to the power of 3
has exactly one solution. The clearest way to see this is to set x=-1, then:
(-1)**3 = -1
y = x**2 = 1
y**(3/2) = +- 1
The only way to make this substitution correct is to make sure all
exponents are integers. Go into core/power.py _eval_subs function and add
this condition:
coeff1.is_integer and coeff2.is_integer and (coeff1/coeff2).is_integer
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