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An unspecified power of x is reported to be `O(log(x)**6)` #14384

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normalhuman opened this Issue Mar 4, 2018 · 3 comments

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normalhuman commented Mar 4, 2018

>>> var('x a')
>>> series(x**a, x, 0)
O(log(x)**6)

This should probably return the expression x**a itself, as x**a is not O(log(x)**6) when a is negative.

@normalhuman normalhuman added the series label Mar 4, 2018

skirpichev added a commit to skirpichev/diofant that referenced this issue Mar 4, 2018

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ArighnaIITG commented Mar 7, 2018

@normalhuman ,
I was trying to decipher the series function in sympy.core.expr in order to solve this issue.
Some questions are coming up ::

I believe this particular function call goes to the n-series handling part of the function -
In that , we have

if n is not None:  # nseries handling
       s1 = self._eval_nseries(x, n=n, logx=logx)
        o = s1.getO() or S.Zero

Is the _eval_nseries here being called from the Pow class in sympy.core.power?
And what checkpoints do we provide? Like when a is not explicitly specified, we return the sympified expression, without any order evaluation.
Is Pow the only case?

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normalhuman commented Mar 7, 2018

It's not about adding a checkpoint like "if the expression is Pow, and the exponent is a symbol, and the sign of symbol is unknown, then..." The goal isn't to special-case this particular function; it is to understand and improve the algorithm for computation of power series.

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bibhu107 commented Jul 21, 2018

I am new to sympy and Open Source. I feel this issue interesting, If @skirpichev Is not doing can I take it?

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