Possible bug report: Incorrect SymbolicSeries expansion (spurious 1/x term)

[TBK]

To Whom it May Concern:

Per the "Reporting Bugs" procedure, I am emailing sage-devel@ with a suspected bug. I am happy to open a TRAC ticket, if appropriate.

I'm getting some seemingly incorrect series expansions. In particular, adding a constant is sometimes introducing an $x^{-1}$ term instead of an $x^0$ term. I have included below a fairly minimal repro. Sage correctly expands `(1-sqrt(1-x))/x`. However, sage insists that adding `123` introduces a `123*x^-1` term. Using `taylor()`, I get the expected expansion in both cases. 

This seems obviously incorrect, but I am new to sage so apologize if this is just pilot error. I looked for existing threads on power series bugs. The closest I found was this message asserting that the combinatorial species code has long been riddled with bugs. If this is a bug, is taylor() believed to be less bug-riddled?

Repro:

{{{

    sage: ((1-sqrt(1-x))/x + 0).series(x,3)                                                  

    1/2 + 1/8*x + 1/16*x^2 + Order(x^3)  # correct

    sage: ((1-sqrt(1-x))/x + 123).series(x,3)                                                

    123*x^(-1) + 1/2 + 1/8*x + 1/16*x^2 + Order(x^3)  # ???

# taylor() correctly expands both

    sage: taylor((1-sqrt(1-x))/x + 0,x,0,2)                                                  

    1/16*x^2 + 1/8*x + 1/2 

    sage: taylor((1-sqrt(1-x))/x + 123,x,0,2)                                                

    1/16*x^2 + 1/8*x + 247/2 

}}}

Version:

    │ SageMath version 9.2, Release Date: 2020-10-24                     │

    │ Using Python 3.8.5. Type "help()" for help.                        │

 OS: macOS Catalina (10.15.7) (64-bit x86)

Best,

Thomas

[Vincent Delecroix]

Thanks for the bug report.

If you know how to proceed, please open a ticket and post the url here.

- series goes through the ginac library (which is full of bugs and we
are trying to get rid off it)
- taylor goes through maxima which is much more reliable

The sympy library offers a nice alternative for symbolic computations.

Best
Vincent

Le 09/04/2021 à 16:01, TBK a écrit :
> To Whom it May Concern:
>
> Per the "Reporting Bugs" procedure

> <https://doc.sagemath.org/html/en/developer/trac.html#reporting-bugs>, I am

> emailing sage-devel@ with a suspected bug. I am happy to open a TRAC
> ticket, if appropriate.
>
> I'm getting some seemingly incorrect series expansions. In particular,
> adding a constant is sometimes introducing an $x^{-1}$ term instead of an
> $x^0$ term. I have included below a fairly minimal repro. Sage correctly
> expands `(1-sqrt(1-x))/x`. However, sage insists that adding `123`
> introduces a `123*x^-1` term. Using `taylor()`, I get the expected
> expansion in both cases.
>
> This seems obviously incorrect, but I am new to sage so apologize if this
> is just pilot error. I looked for existing threads on power series bugs.
> The closest I found was this message

> <https://groups.google.com/g/sage-devel/c/_rB24fZSHKs/m/XneaemQ4AgAJ> asserting

> that the combinatorial species code has long been riddled with bugs. If
> this is a bug, is taylor() believed to be less bug-riddled?
>
>

> *Repro:*
> {{{

> sage: ((1-sqrt(1-x))/x + 0).series(x,3)
>
> 1/2 + 1/8*x + 1/16*x^2 + Order(x^3) # correct
> sage: ((1-sqrt(1-x))/x + 123).series(x,3)
>
> 123*x^(-1) + 1/2 + 1/8*x + 1/16*x^2 + Order(x^3) # ???
> # taylor() correctly expands both
> sage: taylor((1-sqrt(1-x))/x + 0,x,0,2)
>
> 1/16*x^2 + 1/8*x + 1/2
> sage: taylor((1-sqrt(1-x))/x + 123,x,0,2)
>
> 1/16*x^2 + 1/8*x + 247/2
> }}}
>

> *Version:*

> │ SageMath version 9.2, Release Date: 2020-10-24 │
> │ Using Python 3.8.5. Type "help()" for help. │

> *OS*: macOS Catalina (10.15.7) (64-bit x86)
>
> Best,
> Thomas
>
>

[Dima Pasechnik]

On Fri, Apr 9, 2021 at 4:09 PM Vincent Delecroix

<20100.d...@gmail.com> wrote:
>
> Thanks for the bug report.
>
> If you know how to proceed, please open a ticket and post the url here.
>
> - series goes through the ginac library (which is full of bugs and we
> are trying to get rid off it)

more precisely, it's our fork of ginac, pynac, that is buggy, and it
has diverged from ginac quite a bit,
so it's unfair to blame ginac, the original upstream;
unfortunately the people who used to develop pynac have more or less
left the project.

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[dmo...@deductivepress.ca]

Thanks for reporting the bug. It is now trac ticket #31645, and I expect to upload a fix soon.

[Tom Kneeland]

Thank you all for the speedy turnaround on this! In particular:

 

Vincent and Dima: Thank you for the informative 'code archaeology' and helpful pointers 

Dave: Thank you for filing (and fixing!) #31645

Best,

Thomas

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