ZZ → QQ is not onto

[saraedum]

Currently, this fails

sage: QQ.coerce_map_from(ZZ).is_surjective()
NotImplementedError

To fix this, we make the coercion morphism know it is not surjective.

Component: commutative algebra

Keywords: sd86.5

Author: Julian Rüth

Branch/Commit: 79fd62a

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/23186

[tscrim]

comment:1

You should never, ever give information about specific objects at the category level.

However, you can impart this information in the Z_to_Q morphism class.

[saraedum]

comment:2

Why not do that at the category level?

[saraedum]

Branch: u/saraedum/zz___qq_is_not_onto

[saraedum]

Author: Julian Rüth

[saraedum]

Changed branch from u/saraedum/zz___qq_is_not_onto to none

[tscrim]

comment:5

Because to do something at the category level, you should be doing something for all objects in that category, not for one specific object (in a specific implementation). Now, if you wanted to have something in Rings checking if a morphism to ZZ was surjective returning True, then that would be good because it is true for all rings (of characteristic 0). Although map has a concrete implementation of is_surjective, which is a technical detail that would need to be worked around (which that method probably should be lifted to the category of set morphisms).

The fix LGTM.

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New commits:

79fd62a

ZZ is not onto QQ

[tscrim]

Branch: u/saraedum/zz___qq_is_not_onto

[tscrim]

Reviewer: Travis Scrimshaw

[tscrim]

Commit: 79fd62a

[vbraun]

Changed branch from u/saraedum/zz___qq_is_not_onto to 79fd62a