Status: Accepted
Owner: ----
Labels: Type-Defect Priority-Medium
New issue 3100 by [email protected]: power rules misapplied
http://code.google.com/p/sympy/issues/detail?id=3100
These can't all be right:
eq
(b**(11/3))**(5/7)
eq.subs(b,-1)
-(-1)**(4/21)
_.n()
-0.826238774315995 - 0.563320058063622*I # one val
powdenest(eq)
b**(55/21)
_.subs(b,-1).n()
-0.365341024366395 + 0.930873748644204*I #another val
((S(-1)**r(11,3)).n()**r(5,7)).n()
0.733051871829826 - 0.68017273777092*I #true val
eq.n(subs={b:-1})
0.733051871829826 - 0.680172737770919*I
Since subs uses powdenest and the powdenest value is wrong, I believe it is
the culprit. Perhaps the misconception is that although `b**(2*e/7)` is
`(b**(e/7))**2`, `(b**(2*e/7))**(3/7)` is `((b**(e/7))**2)**(3/7)` but this
is not `((b**(e/7))**(6/7)` (unless the base is positive).
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