Comment #2 on issue 4046 by [email protected]: solving equation with
multiple abs runs into problems if symbols are real
http://code.google.com/p/sympy/issues/detail?id=4046
So the problem is that the trick of replacing |x| with sqrt(x**2) fails if
x has real assumptions. The mul.flatten routine will end up rebuilding the
original |x|. The workaround is to not use symbols with assumptions. I
haven't figured out a good way to do this in solve that isn't hackish.
One thing that would help is if abs expressions could be re-written as a
Piecewise so that abs(x-2) - abs(x-4) -> Piecewise((-2,x<=2),(2*x - 6,
And(x>2,x<4)),(2, True)). Then that Piecewise could be solved.
Meanwhile, it would be nice to have an error message displayed to guide the
user to use vanilla symbols. There isn't a vanillify function but something
like this could be used:
def vanilla(eq):
s = [s for s in eq.free_symbols if s.assumptions0]
return eq.xreplace(dict([(s, Dummy(s.name)) for s in s]))
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