#7956: constructing a scheme morphism to an affine curve
----------------------------------+-----------------------------------------
Reporter: wjp | Owner: AlexGhitza
Type: defect | Status: needs_review
Priority: major | Milestone: sage-4.8
Component: algebraic geometry | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author: Michael Orlitzky
Merged: | Dependencies:
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Old description:
> From http://groups.google.com/group/sage-
> devel/browse_thread/thread/1f3d4eca8bbff6c2/d3108ab8f2060050
>
> Ronald van Luijk encountered the following problem:
>
> {{{
> sage: S.<p,q> = QQ[]
> sage: A1.<r> = AffineSpace(QQ,1)
> sage: A1_emb = Curve(p-2)
> sage: type(A1_emb)
> <class 'sage.schemes.plane_curves.affine_curve.AffineCurve_generic'>
> sage: g = A1.hom([2,r],A1_emb)
> TypeError: _point_morphism_class() takes exactly 1 non-keyword argument
> (3 given)
> }}}
>
> We browsed through the schemes module a bit, and the functionality for a
> morphism to an affine curve does seem to exist through functions such as
> {{{AlgebraicScheme_subscheme_affine._point_morphism_class()}}}, but
> is not accessible since {{{AlgebraicScheme_subscheme_affine}}} is not a
> superclass of {{{AffineCurve_generic}}}. Comparing it to the projective
> case, {{{AlgebraicScheme_subscheme_projective}}} _is_ a superclass of
> {{{ProjectiveCurve_generic}}}.
>
> Is this a simple oversight in the class hierarchy for
> {{{AffineCurve_generic}}}, or is there a more fundamental reason why this
> does not yet work?
>
> I made a patch (for sage 4.2) that makes the class hierarchy for affine
> curves similar to that of projective curves, but would appreciate if
> someone familiar with the schemes module could take a look since it is a
> rather invasive change:
>
> http://www.math.leidenuniv.nl/~wpalenst/sage/affine_morphism.patch
>
> The patch also changes the constructor of
> {{{SchemeMorphism_on_points_affine_space}}} to expect a number of
> polynomials equal to the dimension of the ambient space instead of the
> dimension of the curve/subscheme, analogous to a change to
> {{{SchemeMorphism_on_points_projective_space}}} by David Kohel from 2007.
New description:
From http://groups.google.com/group/sage-
devel/browse_thread/thread/1f3d4eca8bbff6c2/d3108ab8f2060050
Ronald van Luijk encountered the following problem:
{{{
sage: S.<p,q> = QQ[]
sage: A1.<r> = AffineSpace(QQ,1)
sage: A1_emb = Curve(p-2)
sage: type(A1_emb)
<class 'sage.schemes.plane_curves.affine_curve.AffineCurve_generic'>
sage: g = A1.hom([2,r],A1_emb)
TypeError: _point_morphism_class() takes exactly 1 non-keyword argument (3
given)
}}}
This is fixed in current Sage (e.g. 5.0.prealpha1). A doctest is added by
* [attachment:sage-trac_7956.patch]
Here's a description of the other patch, which fixes an apparent oversight
in the class hierarchy.
We browsed through the schemes module a bit, and the functionality for a
morphism to an affine curve does seem to exist through functions such as
{{{AlgebraicScheme_subscheme_affine._point_morphism_class()}}}, but
is not accessible since {{{AlgebraicScheme_subscheme_affine}}} is not a
superclass of {{{AffineCurve_generic}}}. Comparing it to the projective
case, {{{AlgebraicScheme_subscheme_projective}}} _is_ a superclass of
{{{ProjectiveCurve_generic}}}.
Is this a simple oversight in the class hierarchy for
{{{AffineCurve_generic}}}, or is there a more fundamental reason why this
does not yet work?
wjp made a patch (for sage 4.2) that makes the class hierarchy for affine
curves similar to that of projective curves, but would appreciate if
someone familiar with the schemes module could take a look since it is a
rather invasive change:
http://www.math.leidenuniv.nl/~wpalenst/sage/affine_morphism.patch
The patch also changes the constructor of
{{{SchemeMorphism_on_points_affine_space}}} to expect a number of
polynomials equal to the dimension of the ambient space instead of the
dimension of the curve/subscheme, analogous to a change to
{{{SchemeMorphism_on_points_projective_space}}} by David Kohel from 2007.
Apply
* [attachment:sage-trac_7956.patch]
--
Comment(by mstreng):
Replying to [comment:3 mjo]:
> It looks like this is working, here's a doctest for it.
You mean that the bug is fixed in current Sage already, not that wjp's
patch is working.
Apply sage-trac_7956.patch only.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/7956#comment:4>
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