Ask Your Question
2

inverse image under ring homomorphism

asked 2021-01-04 15:12:56 +0100

Karina gravatar image

updated 2023-01-09 23:59:54 +0100

tmonteil gravatar image

Hi there!

Just starting to learn Sage and can't get why both inverse_image calls below give an error saying the given element does not have a preimage, while y^2 clearly has xbar as preimage.

Referring to the generators of R1 and R2 explicitly doesn't help either.

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([y^2, y^3], R2)
sage: h
Ring morphism:
  From: Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (-x^3 + y^2)
  To:   Quotient of Multivariate Polynomial Ring in x, y over Rational Field by the ideal (x)
  Defn: xbar |--> y^2
        ybar |--> y^3

sage: h.inverse_image(y)
Traceback (most recent call last)
...
ValueError: element y does not have preimage

sage: h.inverse_image(y^2)
Traceback (most recent call last)
...
ValueError: element y^2 does not have preimage

Thanks!

edit retag flag offensive close merge delete

1 Answer

Sort by ยป oldest newest most voted
1

answered 2021-01-04 16:13:59 +0100

rburing gravatar image

updated 2021-01-17 16:31:47 +0100

Thanks for reporting this bug; it is now tracked at trac ticket #31178.

Edit: It was fixed on the same day, and the bugfix has since been merged into SageMath. The first beta that includes this fix is 9.3.beta6, which will be released soon.

edit flag offensive delete link more

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

Question Tools

1 follower

Stats

Asked: 2021-01-04 15:12:56 +0100

Seen: 381 times

Last updated: Jan 17 '21