Updates:
Cc: basti.kr
Labels: WrongResult
Comment #1 on issue 2005 by asmeurer: (x**(2*y)).subs(exp(3*y*log(x)), z)
gives z**(3/2) instead of z**(2/3)
http://code.google.com/p/sympy/issues/detail?id=2005
The solution is to use a hints engine to subs as discussed at the beginning
of the summer by Sebastian. That way, in Risch, you could just use
expr.subs(subsdict, rationalfunc=True), and it will only try to do the
substitution if it will make a rational function (integral powers). And
for the u-substitution, you could do expr.subs(subsdict, eliminate=x), and
it will try to eliminate x from the expression. And so on for the rest.
And then you just understand that if you leave subs to its own devices in
the most algebraic case, that it might not do what you expect.
Have you looked to see if any of Sebastian's branches at
http://github.com/bastikr/sympy can be worked off of?
Or Sebastian, do you still plan on working on this?
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