conversion from SR.series to PowerSeries

[rws]

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/

[rws]

New commits:

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

[rws]

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]

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.

[rws]

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.

[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

[rws]

New commits:

​3ece975

16203: conversion from SR.series to PowerSeries

[git]

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

​8c4acea	Merge branch 'develop' into t/16203/public/16203-1
​84bc4fb	16203: sync with 17659

[git]

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

​1d363e0

16203: fixes

[git]

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

​50cb33c

Merge branch 'develop' into t/16203/public/16203-1

[rws]

Pending because #17659 is pending.

[rws]

New commits:

​9cb0cc9

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

[maurimo]

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

[rws]

New commits:

​ec8c797

16203: conversion from SR.series to PowerSeries

[git]

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

​aa9c05c

16203: fix string cmp

[rws]

Thanks!