Status: New
Owner: ----
Labels: Type-Defect Priority-Medium
New issue 2452 by peterjef...@gmail.com: Problem with function evaluation
http://code.google.com/p/sympy/issues/detail?id=2452
Please forgive any poor technique in reporting issues, I'm new to this
process (if you know of any resources with bug reporting/due diligence
guidelines I'm always happy to read)
sympy VERSION 0.6.7
It looks to me like there is a problem evaluating the function for storage
a more organized form. The answer given by the following 2 entries should
be:
2 ⎛ 2⎞
(-1 + x) ⋅⎝1 + x ⎠
...but instead the negative one is not carried with the constant and
factored out of the variable x.
In [2]: ((x - 1)**2 * (x**2 + 1))
Out[2]:
2 ⎛ 2⎞
(1 - x) ⋅⎝1 + x ⎠
In [9]: ((-1+x)**2 * (x**2 + 1))
Out[9]:
2 ⎛ 2⎞
(1 - x) ⋅⎝1 + x ⎠
I came on this error trying to use apart to find partial fractions, the
exception raised displays a term that should not be in the polynomial I was
asking it to expand => (1 + 2*x**2 + x**4)**2, but it should (1 - 2*x**2 +
x**4). I thought the might be related because of the mishandling of the
negative. Im not at all sure why the power of 2 gets added, I imagined it
might be used in the decomposition technique, but I have not yet learned to
use a debugging tools to see if that might be true:
In [3]: f = (x**2 - 4*x -1)/((x - 1)**2 * (x**2 + 1))
In [4]: f
Out[4]:
⎛ 2⎞
-⎝1 + 4⋅x - x ⎠
─────────────────
⎛ 2⎞ 2
⎝1 + x ⎠⋅(1 - x)
In [5]:
In [6]: apart(f, x)
---------------------------------------------------------------------------
PolynomialError Traceback (most recent call last)
/home/mightymouse/<ipython console> in <module>()
/usr/lib/pymodules/python2.6/sympy/utilities/decorator.pyc in
threaded_decorator(expr, *args, **kwargs)
54 return Add(*[ func(f, *args, **kwargs) for f in
expr.args ])
55 else:
---> 56 return func(expr, *args, **kwargs)
57
58 threaded_decorator.__doc__ = func.__doc__
/usr/lib/pymodules/python2.6/sympy/simplify/rewrite.pyc in apart(f, z,
**flags)
278 Q = Q.subs(*subs[0])
279
--> 280 P, Q = Poly(P, z), Poly(Q, z)
281
282 G = poly_gcd(P, d)
/usr/lib/pymodules/python2.6/sympy/polys/polynomial.pyc in __new__(cls,
poly, *symbols, **flags)
452 terms = Poly._permute(poly, *symbols)
453 else:
--> 454 terms = Poly._decompose(poly, *symbols)
455
456 if terms:
/usr/lib/pymodules/python2.6/sympy/polys/polynomial.pyc in _decompose(poly,
*symbols)
596 continue
597
--> 598 raise PolynomialError("Can't
decompose %s" % factor)
599 else:
600 coeff *= factor
PolynomialError: Can't decompose (1 + 2*x**2 + x**4)**2
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