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

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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 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).

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

``` Add test for sympy/sympy#9173 ```
`Fixed in f572eea.`
``` 0dd9b7e ```

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

``` Add test for sympy/sympy#9173 ```
`Fixed in f572eea.`
``` 5a510ac ```

### 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)

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```
``` 74ade76 ```

### 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)

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```
``` 48c035e ```