Issue 2814 in sympy: ValueError: can't raise polynomial to a negative power with solve()

[sy...@googlecode.com]

Status: Accepted
Owner: ----
Labels: Type-Defect Priority-Medium Solvers Polynomial

New issue 2814 by asme...@gmail.com: ValueError: can't raise polynomial to
a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

In [84]: solve(((x - 1.1)/(x + 0.1))**(0.83) - C*exp(t), x)
...
ValueError: can't raise polynomial to a negative power

[sy...@googlecode.com]

Comment #1 on issue 2814 by smi...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

>>> from sympy.solvers.solvers import _tsolve
>>> _tsolve(((x - 1.1)/(x + 0.1))**(0.83) - C*exp(t), x)
[0.1*(C*exp(t))**1.20481927710843/(-(C*exp(t))**1.20481927710843 + 1) +
1.1/(-(C
*exp(t))**1.20481927710843 + 1)]

[sy...@googlecode.com]

Comment #2 on issue 2814 by smi...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

Issue 2831 has been merged into this issue.

[sy...@googlecode.com]

Comment #3 on issue 2814 by asme...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

The expression from comment 1 now gives a recursion error, and the one from
comment 1 works.

But the one from issue 2831 still gives the problem. The problem is in
cancel().

Another expression that gives the problem, reported to me on IRC is

a = S("((-2*tanh(x - 3.0)**2 + 2)*(-tanh(x - 3.0)**2 + 1)*tanh(x +
3.0)**2/2 + (-2*tanh(x - 3.0)**2 + 2)*(-2*tanh(x + 3.0)**2 + 2)*tanh(x -
3.0)*tanh(x + 3.0) - (-2*tanh(x - 3.0)**2 + 2)*tanh(x - 3.0)**2*tanh(x +
3.0)**2 + (-2*tanh(x + 3.0)**2 + 2)*(-tanh(x + 3.0)**2 + 1)*tanh(x -
3.0)**2/2 - (-2*tanh(x + 3.0)**2 + 2)*tanh(x - 3.0)**2*tanh(x +
3.0)**2)/(2*sqrt(tanh(x - 3.0)**2*tanh(x + 3.0)**2 + 0.0001)) +
(-(-2*tanh(x - 3.0)**2 + 2)*tanh(x - 3.0)*tanh(x + 3.0)**2/2 - (-2*tanh(x +
3.0)**2 + 2)*tanh(x - 3.0)**2*tanh(x + 3.0)/2)*((-2*tanh(x - 3.0)**2 +
2)*tanh(x - 3.0)*tanh(x + 3.0)**2/2 + (-2*tanh(x + 3.0)**2 + 2)*tanh(x -
3.0)**2*tanh(x + 3.0)/2)/(2*(tanh(x - 3.0)**2*tanh(x + 3.0)**2 +
0.0001)**(3/2))")

[sy...@googlecode.com]

Updates:
Blockedon: 2935

Comment #4 on issue 2814 by asme...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

Well, I remember seeing this before, but I can't find the issue. It happens
because the while loop at line 1310 in densearith.py runs one too many
times, because the floating point coefficients do not cancel like the
algorithm expects them to. I don't know if this can be fixed without
implementing RR(eps). See issue 2935.

[sy...@googlecode.com]

Comment #5 on issue 2814 by smi...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

even if nsimplify is used to give Rationals instead of floats, infinite
recursion is generated.

[sy...@googlecode.com]

Comment #7 on issue 2814 by smi...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

Issue 3575 has been merged into this issue.

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Comment #8 on issue 2814 by patrick....@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

If I try
cancel((z/(x**2) + 0.1*z/sqrt(x**2))**2)
I get the
ValueError: can't raise polynomial to a negative power.
Is it the same issue?

[sy...@googlecode.com]

Comment #9 on issue 2814 by asme...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

https://github.com/sympy/sympy/pull/1921

And your example is fixed there too.

[sy...@googlecode.com]

Updates:
Cc: matt...@gmail.com

Comment #10 on issue 2814 by smi...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

Issue 2384 has been merged into this issue.

[sy...@googlecode.com]

Comment #11 on issue 2814 by smi...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

The OP expression still hangs and the expression in comment 8 still raises
an error.

[sy...@googlecode.com]

Updates:
Labels: NeedsReview

Comment #12 on issue 2814 by asme...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

It works in https://github.com/sympy/sympy/pull/2126

[sy...@googlecode.com]

Updates:
Status: Fixed
Labels: -NeedsReview

Comment #13 on issue 2814 by julien.r...@gmail.com: ValueError: can't raise

polynomial to a negative power with solve()
http://code.google.com/p/sympy/issues/detail?id=2814

This has been merged.