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

mforbes · 2015-03-20T03:23:13Z

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

The text was updated successfully, but these errors were encountered:

mforbes · 2015-11-20T07:23:16Z

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

jksuom · 2015-11-20T19:21:20Z

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).

Fixed in f572eea.

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

mohitacecode · 2020-02-20T06:34:17Z

I think this issue is resolved in the current master.
IMO it can be closed.

>>> var('p_0 p_1 p_2 p_3 b_0 b_1 b_2')
(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)
b_2*y**2 + 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)
>>> Q.diff(y, y).limit(y, 0).simplify()
2*b_2
>>>

sachin-4099 · 2020-06-01T15:40:17Z

I can confirm @mohitacecode 's results. This only needs a test.

Initially the issue was the failure of series/limit unless the expression is simplified first, but it is resoved in current master. So I have added test case addressing this one.

Shardul555 · 2020-12-29T10:08:52Z

I run the code for this expression in the terminal having installed sympy version 1.7.1 but here the coefficients of y**2 and y are not simplified to final answer. Anyone guide me regarding this issue whether it is done with some purpose or this should be fixed?

>>> from sympy import *
>>> from sympy.abc import y
>>> var('p_0 p_1 p_2 p_3 b_0 b_1 b_2')
(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*(b_0*p_2/p_3 + b_0*(-p_2/p_3 + b_1/b_0)) + y**2*(b_0*p_1/p_3 + b_0*p_2*(-p_2/p_3 + b_1/b_0)/p_3 + b_0*(-p_1/p_3 + (p_2/p_3 - b_1/b_0)**2 + b_1*p_2/(b_0*p_3) + b_2/b_0 - b_1**2/b_0**2)) + b_0 + O(y**3)
>>> simplify(y*(b_0*p_2/p_3 + b_0*(-p_2/p_3 + b_1/b_0)))
b_1*y
>>> simplify(y**2*(b_0*p_1/p_3 + b_0*p_2*(-p_2/p_3 + b_1/b_0)/p_3 + b_0*(-p_1/p_3 + (p_2/p_3 - b_1/b_0)**2 + b_1*p_2/(b_0*p_3) + b_2/b_0 - b_1**2/b_0**2)))
b_2*y**2

anutosh491 · 2022-06-27T13:55:30Z

I came across this issue while trying to address something similar and I realize this now ends up giving an even more complicated result than before and even if we simplify first .... we don't get the correct result which is a worse condition than what we had before !

>>> 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*(b_0*p_2/p_3 + b_0*(-p_2/p_3 + b_1/b_0)) + y**2*(b_0*p_1/p_3 + b_0*p_2*(-p_2/p_3 + b_1/b_0)/p_3 + b_0*(-p_1/p_3 + (p_2/p_3 - b_1/b_0)**2 + b_1*p_2/(b_0*p_3) + (b_0*b_2 - b_1**2)/b_0**2)) + b_0 + O(y**3)
>>>
>>> Q.simplify().series(y, n=3)
y*(b_0*p_2/p_3 + b_0*(-p_2/p_3 + b_1/b_0)) + y**2*(b_0*p_1/p_3 + b_0*p_2*(-p_2/p_3 + b_1/b_0)/p_3 + b_0*(-p_1/p_3 + (p_2/p_3 - b_1/b_0)**2 + b_1*p_2/(b_0*p_3) + (b_0*b_2 - b_1**2)/b_0**2)) + b_0 + O(y**3)
>>>
>>> # Expected - b_2*y**2 + b_1*y + b_0 + O(y**3)
>>> 
>>> # Previously gave - 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)

I guess this was closed without adding a test hence we haven't come across this before, hence I guess we should reopen this now . Feel free to correct me if I'm missing something here !

arhaque09 · 2024-02-04T05:01:06Z

I would like to work on this issue! Let me know if its available so I can get assigned!

arhaque09 · 2024-02-05T20:55:06Z

Hi! I am working on adding tests right now!

Qaaee123 · 2024-04-23T02:27:55Z

Hello, I saw this issue and I checked some of the other linked threads. In particular, I saw that in #20724 the issue was closed because it was solved elsewhere, but when testing locally on versions 1.8+, the issue still persisted. Locally, I edited expr.py to include the same proposed change as #20724, and it seemed to solve the issue. However, it has started to fail testcases and I am unsure about how to proceed. I would greatly appreciate any guidance.

Qaaee123 · 2024-04-25T04:37:20Z

Hello, I managed to fix this bug by incorporating ratsimp and it passes all of the test_expr test cases. However, when tested with tan(x).series(x, 2, 6, "+"), the output is incorrect. Any guidance would be greatly appreciated.

>>> tan(x).series(x, 2, 6, "+")
-2 + tan(2) + (x - 2)*tan(2)**2 + (x - 2)**2*tan(2) + (x - 2)**2*tan(2)**3 + (x - 2)**3*tan(2)**4 + 4*(x - 2)**3*tan(2)**2/3 + (x - 2)**3/3 + 2*(x - 2)**4*tan(2)/3 + 5*(x - 2)**4*tan(2)**3/3 + (x - 2)**4*tan(2)**5 + (x - 2)**5*tan(2)**6 + 2*(x - 2)**5*tan(2)**4 + 17*(x - 2)**5*tan(2)**2/15 + 2*(x - 2)**5/15 + x + O((x - 2)**6, (x, 2))
>>> # correct output
tan(2) + (1 + tan(2)**2)*(x - 2) + (x - 2)**2*(tan(2)**3 + tan(2)) + (x - 2)**3*(1/3 + 4*tan(2)**2/3 + tan(2)**4) + (x - 2)**4*(tan(2)**5 + 5*tan(2)**3/3 + 2*tan(2)/3) + (x - 2)**5*(2/15 + 17*tan(2)**2/15 + 2*tan(2)**4 + tan(2)**6) + O((x - 2)**6, (x, 2))

oscarbenjamin · 2024-04-26T11:19:50Z

I'm not sure that there is an issue here. The example comes out more complicated than it needs to but it is correct. If we want to canonicalise the outputs then we should really use ringseries.

oscarbenjamin · 2024-04-26T11:21:36Z

I think best thing is just to add some test cases since they were not added before.

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

asmeurer added the series label Nov 20, 2015

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

Add test for sympy/sympy#9173

0dd9b7e

Fixed in f572eea.

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

Add test for sympy/sympy#9173

5a510ac

Fixed in f572eea.

sachin-4099 added the Easy to Fix This is a good issue for new contributors. Feel free to work on this if no one else has already. label Jun 1, 2020

Shardul555 mentioned this issue Dec 26, 2020

Added test case regarding failure of series/limit unless the expression is simplified #20674

Open

Shardul555 mentioned this issue Dec 29, 2020

Series is not simplified to final answer in output in sympy 1.7.1 #20697

Closed

0sidharth mentioned this issue Jan 21, 2021

Added checks in _eval_as_leading_term and _eval_nseries #20754

Merged

jksuom closed this as completed in #20754 Feb 16, 2021

anutosh491 reopened this Jun 27, 2022

This was referenced Apr 26, 2024

Fix a bug where series/limit failed unless simplified first #26543

Closed

expr: fixed limit/series incorrect simplification, added regression test #26544

Open

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

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

mforbes commented Mar 20, 2015

mforbes commented Nov 20, 2015

jksuom commented Nov 20, 2015

mohitacecode commented Feb 20, 2020

sachin-4099 commented Jun 1, 2020

Shardul555 commented Dec 29, 2020 •

edited

anutosh491 commented Jun 27, 2022

arhaque09 commented Feb 4, 2024 •

edited

arhaque09 commented Feb 5, 2024

Qaaee123 commented Apr 23, 2024

Qaaee123 commented Apr 25, 2024

oscarbenjamin commented Apr 26, 2024

oscarbenjamin commented Apr 26, 2024

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

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

Comments

mforbes commented Mar 20, 2015

mforbes commented Nov 20, 2015

jksuom commented Nov 20, 2015

mohitacecode commented Feb 20, 2020

sachin-4099 commented Jun 1, 2020

Shardul555 commented Dec 29, 2020 • edited

anutosh491 commented Jun 27, 2022

arhaque09 commented Feb 4, 2024 • edited

arhaque09 commented Feb 5, 2024

Qaaee123 commented Apr 23, 2024

Qaaee123 commented Apr 25, 2024

oscarbenjamin commented Apr 26, 2024

oscarbenjamin commented Apr 26, 2024

Shardul555 commented Dec 29, 2020 •

edited

arhaque09 commented Feb 4, 2024 •

edited