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Series/limit fails unless expression is simplified first. #9173

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mforbes opened this Issue Mar 20, 2015 · 2 comments

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mforbes commented Mar 20, 2015

Consider the following:

>>> var('p_0 p_1 p_2 p_3 b_0 b_1 b_2');
>>> Q = (p_0 + (p_1 + (p_2 + p_3/y)/y)/y)/(1 + ((p_3/(b_0*y) + (b_0*p_2 - b_1*p_3)/b_0**2)/y + (b_0**2*p_1 - b_0*b_1*p_2 - p_3*(b_0*b_2 - b_1**2))/b_0**3)/y)
>>> Q.series(y, n=3)
y**2*(-b_0*p_2**2/p_3**2 + 2*b_1*p_2/p_3 + b_2 - b_1**2/b_0) + b_1*y + b_0 + O(y**3)
>>> Q.simplify().series(y, n=3)
b_2*y**2 + b_1*y + b_0 + O(y**3)

The y**2 coefficient should be b_2, but is computed to be something else unless the expression is simplified first. The issues seems to be with limit, and so might be related to #9075:

>>> Q.diff(y, y).limit(y, 0).simplify()
-72*b_0*p_2**2/p_3**2 + 144*b_1*p_2/p_3 + 2*b_2 - 72*b_1**2/b_0
>>> expr.diff(y, y).simplify().limit(y, 0)
2*b_2

@mforbes mforbes changed the title Series fails unless expression is simplified first. Series/limit fails unless expression is simplified first. Mar 20, 2015

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mforbes commented Nov 20, 2015

Any update on why this might be happening? It still seems to be an issue.

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jksuom commented Nov 20, 2015

It seems that there is a bug in the computation of the series expansion of 1/f when the order of f is negative (and that of 1/f is positive).

>>> f = 1/x**2 - 1/x
>>> (1/f).series(x, n=6)
x**2 + x**3 + x**4 + O(x**6)
>>> (1/f).series(x, n=7)
x**2 + x**3 + x**4 + x**5 + O(x**7)
>>> (1/f).series(x, n=8)
x**2 + x**3 + x**4 + x**5 + O(x**8)

It is caused by an incorrect loop count in Pow._eval_nseries (on this line).

@asmeurer asmeurer added the series label Nov 20, 2015

skirpichev added a commit to diofant/diofant that referenced this issue Feb 20, 2016

skirpichev added a commit to diofant/diofant that referenced this issue Feb 21, 2016

skirpichev added a commit to skirpichev/diofant that referenced this issue Nov 2, 2016

Add regression tests & mention closed issues
    close sympy/sympy#3112 (MrvAsympt was added in diofant#6)
    close sympy/sympy#9173 (test was added in 5a510ac)
    close sympy/sympy#9808 (fixed in 09e539b)
    close sympy/sympy#9341 (fixed in af98a00)
    close sympy/sympy#9908 (fixed in cc3fa8d)
    close sympy/sympy#6171 (test added in d278031)
    close sympy/sympy#9276 (diagnose_imports.py removed in ab8c535)
    close sympy/sympy#10201 (fixed in 0d0fc5f)
    close sympy/sympy#9057 (test was added in 8290a0c)
    close sympy/sympy#11159 (test was added in ffb76cb)
    close sympy/sympy#2839 (new AST transformers are used, see diofant#278 and diofant#167)
    close sympy/sympy#11081 (see ed01e16 and bb92329)
    close sympy/sympy#10974 (see 73fc425)
    close sympy/sympy#10806 (test in 539929a)
    close sympy/sympy#10801 (test in 2fe3da5)
    close sympy/sympy#9549 (test in 88bdefa)
    close sympy/sympy#4231 (test was added in fb411d5)
    close sympy/sympy#8634 (see 2fcbb58)
    close sympy/sympy#8481 (see 1ef20d3)
    close sympy/sympy#9956 (fixed in a34735f)
    close sympy/sympy#9747 (see e117c60)
    close sympy/sympy#7853 (see 3e4fbed)
    close sympy/sympy#9634 (see 2be03f5)
    close sympy/sympy#8500 (fixed in diofant#104 and finally in diofant#316)
    close sympy/sympy#9192 (see 9bf622f)
    close sympy/sympy#7130 (see e068fa3)
    close sympy/sympy#8514 (see b2d543b)
    close sympy/sympy#9334 (see 90de625)
    close sympy/sympy#8229 (see 9755b89)
    close sympy/sympy#8061 (see 7054f06)
    close sympy/sympy#7872 (tested in diofant#6)
    close sympy/sympy#3496 (tested in test_log_symbolic)
    close sympy/sympy#2929 (see da7db7a)
    close sympy/sympy#8203 (oo is not a real, see diofant#36)
    close sympy/sympy#7649 (0 is imaginary since diofant#8)
    close sympy/sympy#7256 (fixed in c0a4549)
    close sympy/sympy#6783 (see cb28d63)
    close sympy/sympy#5662 (is_integer issue fixed in 6bfa9f8, there is no is_bounded anymore)
    close sympy/sympy#5295 (fixed with diofant#354)
    close sympy/sympy#4856 (we now have flake/pep tests)
    close sympy/sympy#4555 (flake8 enabled after diofant#214)
    close sympy/sympy#5773 (cmp_to_key removed after diofant#164 and c9acbf0)
    close sympy/sympy#5484 (see above)

    Added regression tests:
    from https://groups.google.com/forum/#!topic/sympy/LkTMQKC_BOw
    fixes sympy/sympy#8825 (probably via diofant#209)
    fixes sympy/sympy#8635
    fixes sympy/sympy#8157
    fixes sympy/sympy#7872
    fixes sympy/sympy#7599
    fixes sympy/sympy#6179
    fixes sympy/sympy#5415
    fixes sympy/sympy#2865
    fixes sympy/sympy#5907
    fixes sympy/sympy#11722

    Closes diofant#347

skirpichev added a commit to skirpichev/diofant that referenced this issue Nov 2, 2016

Add regression tests & mention closed issues
    close sympy/sympy#3112 (MrvAsympt was added in diofant#6)
    close sympy/sympy#9173 (test was added in 5a510ac)
    close sympy/sympy#9808 (fixed in 09e539b)
    close sympy/sympy#9341 (fixed in af98a00)
    close sympy/sympy#9908 (fixed in cc3fa8d)
    close sympy/sympy#6171 (test added in d278031)
    close sympy/sympy#9276 (diagnose_imports.py removed in ab8c535)
    close sympy/sympy#10201 (fixed in 0d0fc5f)
    close sympy/sympy#9057 (test was added in 8290a0c)
    close sympy/sympy#11159 (test was added in ffb76cb)
    close sympy/sympy#2839 (new AST transformers are used, see diofant#278 and diofant#167)
    close sympy/sympy#11081 (see ed01e16 and bb92329)
    close sympy/sympy#10974 (see 73fc425)
    close sympy/sympy#10806 (test in 539929a)
    close sympy/sympy#10801 (test in 2fe3da5)
    close sympy/sympy#9549 (test in 88bdefa)
    close sympy/sympy#4231 (test was added in fb411d5)
    close sympy/sympy#8634 (see 2fcbb58)
    close sympy/sympy#8481 (see 1ef20d3)
    close sympy/sympy#9956 (fixed in a34735f)
    close sympy/sympy#9747 (see e117c60)
    close sympy/sympy#7853 (see 3e4fbed)
    close sympy/sympy#9634 (see 2be03f5)
    close sympy/sympy#8500 (fixed in diofant#104 and finally in diofant#316)
    close sympy/sympy#9192 (see 9bf622f)
    close sympy/sympy#7130 (see e068fa3)
    close sympy/sympy#8514 (see b2d543b)
    close sympy/sympy#9334 (see 90de625)
    close sympy/sympy#8229 (see 9755b89)
    close sympy/sympy#8061 (see 7054f06)
    close sympy/sympy#7872 (tested in diofant#6)
    close sympy/sympy#3496 (tested in test_log_symbolic)
    close sympy/sympy#2929 (see da7db7a)
    close sympy/sympy#8203 (oo is not a real, see diofant#36)
    close sympy/sympy#7649 (0 is imaginary since diofant#8)
    close sympy/sympy#7256 (fixed in c0a4549)
    close sympy/sympy#6783 (see cb28d63)
    close sympy/sympy#5662 (is_integer issue fixed in 6bfa9f8, there is no is_bounded anymore)
    close sympy/sympy#5295 (fixed with diofant#354)
    close sympy/sympy#4856 (we now have flake/pep tests)
    close sympy/sympy#4555 (flake8 enabled after diofant#214)
    close sympy/sympy#5773 (cmp_to_key removed after diofant#164 and c9acbf0)
    close sympy/sympy#5484 (see above)

    Added regression tests:
    from https://groups.google.com/forum/#!topic/sympy/LkTMQKC_BOw
    fixes sympy/sympy#8825 (probably via diofant#209)
    fixes sympy/sympy#8635
    fixes sympy/sympy#8157
    fixes sympy/sympy#7872
    fixes sympy/sympy#7599
    fixes sympy/sympy#6179
    fixes sympy/sympy#5415
    fixes sympy/sympy#2865
    fixes sympy/sympy#5907
    fixes sympy/sympy#11722

    Closes diofant#347
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