QQbar decorators should handle sequences

[mwageringel]

The forward_map and reverse_map in sage/rings/qqbar_decorators.py do not currently handle polynomial sequences. Because of that, the call to groebner_basis in a polynomial ring over QQbar only returns a list, not a PolynomialSequence, which can be unexpected.

sage: J = QQbar['x,y'].ideal('x^2 - y')
sage: type(J.groebner_basis())
<class 'list'>

The decorator should handle polynomial sequences and should in particular preserve properties such as immutability.

Component: algebra

Keywords: qqbar

Author: Travis Scholl

Branch/Commit: c3e7ed2

Reviewer: Markus Wageringel

Issue created by migration from https://trac.sagemath.org/ticket/29468

[DaveWitteMorris]

comment:1

I think it will suffice to modify the definitions of forward_map and reverse_map by adding elif clauses to deal with items that are a PolynomialSequence or a Sequence.

[tscholl2]

Branch: u/tscholl2/29468

[tscholl2]

Author: Travis Scholl

[tscholl2]

Commit: ce09d1d

[tscholl2]

comment:2

I attempted to handle Sequences and PolynomialSequences, but I'm not exactly sure how to test things like preserving immutability. Let me know if my small test is not enough or if there is some other problem.

[mwageringel]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
-The `forward_map` and `reverse_map` in `sage/rings/qqbar_decorators.py` do not currently handle polynomial sequences. Because of that, the call to `groebner_basis` in a polynomial ring over `QQbar` only returns a list, not a `Sequence`, which can be unexpected.
+The `forward_map` and `reverse_map` in `sage/rings/qqbar_decorators.py` do not currently handle polynomial sequences. Because of that, the call to `groebner_basis` in a polynomial ring over `QQbar` only returns a list, not a `PolynomialSequence`, which can be unexpected.

sage: J = QQbar['x,y'].ideal('x^2 - y') @@ -6,5 +6,5 @@ <class 'list'>

-The decorator should handle sequences and should in particular preserve properties such as immutability.
+The decorator should handle polynomial sequences and should in particular preserve properties such as immutability.

[mwageringel]

Changed commit from ce09d1d to c3e7ed2

[mwageringel]

Reviewer: Markus Wageringel

[mwageringel]

comment:3

Thank you for working on this. I have made a few more small changes.

The immutability is now checked and preserved via is_immutable(). The constructor for polynomial sequences has additional arguments like cr and cr_str, but these are only syntatic and cannot be checked easily, so I think for now it is not that important to preserve these. In the long run, we might add a .map() method to sequences that retains these options, but we do not need to do this on this ticket.

Moreover, I have removed the handling of Sequence in favour of PolynomialSequence, as a sequence of polynomials over QQbar will always be a PolynomialSequence. I have also added another test case.

Do you agree with the changes?

---

New commits:

c3e7ed2

29468: check immutability and add more tests

[mwageringel]

Changed branch from u/tscholl2/29468 to u/gh-mwageringel/29468

[tscholl2]

comment:4

Replying to @mwageringel:

That sounds good to me! I'm still learning this area of Sage, I will have to look into what cr and cr_str do. I also didn't know that Sequences will always be PolynomialSequences in this context, but I saw the test you added for that.

[mwageringel]

comment:5

Let us get this merged.

[vbraun]

Changed branch from u/gh-mwageringel/29468 to c3e7ed2