conversion from SR.series to PowerSeries

[rwst]

sage: R.<x> = PowerSeriesRing(SR)
sage: var('y')
y
sage: ex=1/(1-y)
sage: ex.series(y,20)
1 + 1*y + 1*y^2 + 1*y^3 + 1*y^4 + 1*y^5 + 1*y^6 + 1*y^7 + 1*y^8 + 1*y^9 + 1*y^10 + 1*y^11 + 1*y^12 + 1*y^13 + 1*y^14 + 1*y^15 + 1*y^16 + 1*y^17 + 1*y^18 + 1*y^19 + Order(y^20)
sage: s=R(_); s
1 + y + y^2 + y^3 + y^4 + y^5 + y^6 + y^7 + y^8 + y^9 + y^10 + y^11 + y^12 + y^13 + y^14 + y^15 + y^16 + y^17 + y^18 + y^19 + Order(y^20)
sage: type(s)
<type 'sage.rings.power_series_poly.PowerSeries_poly'>
sage: s.list()
[1 + 1*y + 1*y^2 + 1*y^3 + 1*y^4 + 1*y^5 + 1*y^6 + 1*y^7 + 1*y^8 + 1*y^9 + 1*y^10 + 1*y^11 + 1*y^12 + 1*y^13 + 1*y^14 + 1*y^15 + 1*y^16 + 1*y^17 + 1*y^18 + 1*y^19 + Order(y^20)]

The correct result would be 1 + x + x^2.... Obviously Order(20) is not recognized as a marker for the existence of a power series. The whole symbolic expression is taken as a constant.

Also reported in http://ask.sagemath.org/question/24777/how-to-convert-a-taylor-polynomial-to-a-power-series/ and http://ask.sagemath.org/question/24968/coefficients-in-polynomial-ring-over-symbolic-ring/

Depends on #17399
Depends on #17438
Depends on #17659

CC: @kcrisman

Component: algebra

Keywords: series conversion days71

Author: Ralf Stephan

Branch/Commit: aa9c05c

Reviewer: Maurizio Monge, Marc Mezzarobba

Issue created by migration from https://trac.sagemath.org/ticket/16203

[rwst]

Branch: u/rws/conversion_from_sr_series___to_powerseries_sr__gives_unexpected_result

[rwst]

Dependencies: #17399, #17438

[rwst]

Commit: 0a43e21

[rwst]

New commits:

32ae67c	17399: do not let maxima handle ex.series coefficients
0a43e21	16203: preliminary implementation, dependent on 17399, 17438

[rwst]

Changed commit from 0a43e21 to none

[rwst]

Changed branch from u/rws/conversion_from_sr_series___to_powerseries_sr__gives_unexpected_result to none

[rwst]

Branch: public/16203

[rwst]

Commit: 53b2ca4

[rwst]

comment:10

This also resolves #16198

---

New commits:

32ae67c	17399: do not let maxima handle ex.series coefficients
99820cf	17399: roll back previous commit to allow merge of 17428
d7da52c	17438: implement coeff list
9452fa9	17438: deprecate ex.coeff/coeffs()
0fec129	17438: implement ex.list()
168b659	Merge branch 'u/rws/coefficients_of_symbolic_expressions_revamp' of trac.sagemath.org:sage into t/17399/fix_coefficients_for_symbolic_series
6cd5286	17399: handle series in ex.coefficients()
53b2ca4	16203: implement conversion SR.series --> PowerSeries

[tscrim]

comment:11

As currently stated in the description, this is the correct result as the variable for PowerSeriesRing is x, whereas we have some symbolic expression in y (so is naturally an element in the coefficient ring). Having mismatched variable names automatically converting over seems fraught with problems and could lead to subtle bugs.

[rwst]

comment:12

Replying to @tscrim:

As currently stated in the description, this is the correct result as the variable for PowerSeriesRing is x, whereas we have some symbolic expression in y (so is naturally an element in the coefficient ring). Having mismatched variable names automatically converting over seems fraught with problems and could lead to subtle bugs.

I see. So, conversion to an existing power series ring should only be allowed if the series expansion variable has the same name as the ring var. Multiple vars aren't supported by SR.series anyway.

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e1a9039

16203: implement ex.series_variable; disallow ex.series conversion into mismatched PowerSeries ring element

[sagetrac-git]

Changed commit from 53b2ca4 to e1a9039

[rwst]

Author: Ralf Stephan

[rwst]

Changed branch from public/16203 to public/16203-1

[rwst]

Changed dependencies from #17399, #17438 to #17399, #17438, #17659

[rwst]

Changed commit from e1a9039 to 3ece975

[rwst]

New commits:

3ece975

16203: conversion from SR.series to PowerSeries

[rwst]

comment:20

Pending because #17659 is pending.

[rwst]

Changed branch from public/16203-1 to public/16203-2

[rwst]

New commits:

9cb0cc9

Merge branch 'public/16203-1' of trac.sagemath.org:sage into tmp06

[rwst]

Changed commit from 50cb33c to 9cb0cc9

[sagetrac-maurimo]

comment:23

"is" should be "==".
The changes to ring.pyx are unrelated and should be removed.
Everything else looks alright.

[sagetrac-maurimo]

Changed keywords from series conversion to series conversion days71

[sagetrac-maurimo]

Reviewer: Maurizio Monge

[rwst]

Changed branch from public/16203-2 to public/16203-3

[rwst]

Changed commit from 9cb0cc9 to ec8c797

[rwst]

New commits:

ec8c797

16203: conversion from SR.series to PowerSeries

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

aa9c05c

16203: fix string cmp

[sagetrac-git]

Changed commit from ec8c797 to aa9c05c

[mezzarobba]

Changed reviewer from Maurizio Monge to Maurizio Monge, Marc Mezzarobba

[rwst]

comment:28

Thanks!

[vbraun]

Changed branch from public/16203-3 to aa9c05c