#31178 closed defect (fixed)

inverse_image of element fails for morphism of quotient rings

Reported by: rburing Owned by:
Priority: major Milestone: sage-9.3
Component: algebra Keywords: inverse_image, hom, quotient
Cc: gh-mwageringel Merged in:
Authors: Markus Wageringel Reviewers: Ricardo Buring
Report Upstream: N/A Work issues:
Branch: 1e5b979 (Commits, GitHub, GitLab) Commit: 1e5b9796e14b40003d870384d8115e58857099a5
Dependencies: Stopgaps:

Status badges


Reported in Ask SageMath question #55099:

sage: R.<x, y> = QQ[]
sage: R1 = R.quotient(R.ideal(y^2 - x^3))
sage: R2 = R.quotient(R.ideal(x))
sage: h = R1.hom([R2(y^2), R2(y^3)], R2)
sage: h(R1(x))
sage: h.inverse_image(R2(y^2))
ValueError: element ybar^2 does not have preimage

Change History (3)

comment:1 Changed 11 months ago by gh-mwageringel

  • Authors set to Markus Wageringel
  • Branch set to u/gh-mwageringel/31178
  • Commit set to 1e5b9796e14b40003d870384d8115e58857099a5
  • Status changed from new to needs_review

Thanks for reporting this. The problem was that quotient rings can have generators which are constant:

sage: R2.gens()
(0, ybar)

Because of this, the check for testing whether an element is contained in the subring generated by certain variables failed.

I have replaced the check by a better and clearer one, which should resolve the issue.

New commits:

1e5b97931178: fix inverse image for quotient rings

comment:2 Changed 11 months ago by rburing

  • Reviewers set to Ricardo Buring
  • Status changed from needs_review to positive_review

Thanks for the fast reply. The new check looks good.

comment:3 Changed 11 months ago by vbraun

  • Branch changed from u/gh-mwageringel/31178 to 1e5b9796e14b40003d870384d8115e58857099a5
  • Resolution set to fixed
  • Status changed from positive_review to closed
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