Opened 21 months ago
Closed 18 months ago
#20790 closed enhancement (fixed)
Computing plane curve models for algebraic curves
Reported by:  gjorgenson  Owned by:  

Priority:  minor  Milestone:  sage7.4 
Component:  algebraic geometry  Keywords:  gsoc2016 
Cc:  bhutz, mmarco  Merged in:  
Authors:  Grayson Jorgenson  Reviewers:  Miguel Marco, Ben Hutz 
Report Upstream:  N/A  Work issues:  
Branch:  679338b (Commits)  Commit:  679338b2f7fd3888f06fa4f5d1b69310339e1fd7 
Dependencies:  Stopgaps: 
Description
Given a generic algebraic curve, compute a plane curve birational to it.
Change History (34)
comment:1 Changed 20 months ago by
 Branch set to u/gjorgenson/ticket/20790
comment:2 Changed 20 months ago by
 Commit set to 309eb0316c74e84cb8b7bd2fad0ab5514cf4d894
comment:3 Changed 20 months ago by
 Commit changed from 309eb0316c74e84cb8b7bd2fad0ab5514cf4d894 to 0de1323a77147059de0f82c7a8642c44c31552ed
Branch pushed to git repo; I updated commit sha1. New commits:
0de1323  20790: implemented projections for affine curves.

comment:4 Changed 20 months ago by
 Status changed from new to needs_review
I implemented a function to project affine curves that allows the user to specify coordinates to use in the projection. If the given coordinates yield a projection that doesn't have a curve as its image, an error will be raised. I also implemented a plane_projection function that doesn't take any user input but just tries to find two coordinates that will yield a projection that has a plane curve as its image. I think such a projection should always exist, but right now my approach to finding one is just trying all possible projections to two coordinates.
For projective curves, I added some functionality for working with curves defined over finite fields. Now the projection function will test all of the points of the ambient space of a curve over a finite field until it finds a point not on the curve. An error is returned if the curve contains all of the ambient space points.
comment:5 Changed 20 months ago by
A few questions:
 It could make sense in some cases to consider projections even if the image is not a curve (or if it is, but some components could be mapped to points). I think we should allow that, but maybe raising a warning about it.
 In python (and sage), indices start in 0. Shouldn't we follow that approach when selecting the coordinates we project to? MAybe we could also allow a list (or tuple) of variables as input.
 don't modify the input
indices
inside the function. That could have undesired side effects. Instead, create an internal copy.
 I think you go into too much trouble to define the elimination ideal. Just calling
elimination_ideal
on it should give you the ideal defining the image. Something like this:
sage: A.<x,y,z> = AffineSpace(QQ,3) sage: C = Curve([y^7  x^2 + x^3  2*z, z^2  x^7  y^2], A) sage: I = C.defining_ideal() sage: IE = I.elimination_ideal(z) sage: IE Ideal (y^14 + 2*x^3*y^7  2*x^2*y^7  4*x^7 + x^6  2*x^5 + x^4  4*y^2) of Multivariate Polynomial Ring in x, y, z over Rational Field sage: A2 = AffineSpace(QQ, 2, (x,y)) sage: H = C.Hom(A2) sage: phi = H((x,y)) sage: phi Scheme morphism: From: Affine Curve over Rational Field defined by y^7 + x^3  x^2  2*z, x^7  y^2 + z^2 To: Affine Space of dimension 2 over Rational Field Defn: Defined on coordinates by sending (x, y, z) to (x, y) sage: C2 = Curve([A2.coordinate_ring()(i) for i in IE.gens()],A2) sage: C2 Affine Plane Curve over Rational Field defined by y^14 + 2*x^3*y^7  2*x^2*y^7  4*x^7 + x^6  2*x^5 + x^4  4*y^2
should be enough to construct both the morphism and the image curve. Something similar could be done for the projective case.
 Do we want to create new coordinates in the case of affine projection? or would it be better to keep the original ones? Maybe have an option for this?
 I think it would me slightly more pythonic to return a tuple instead of a list.
 In the projective case, ¿would it make sense to have an option to project to smaller subspaces?
 In the plane projection method, instead of breaking the loop, just return
L
. Also, you could avoid the double loop by just iterating over the Subsets of the coordinates (maybe this is slower, but I think it would make the code easyer to read).
 I am confused by this part of the code in the projective projection:
l = list(PP.gens()) for i in range(n+1): l[i] = 1 while(F(l) == 0): l[i] = l[i] + 1
I think that, in general, you could get simpler projections if you start with l[i] = 0 (for instance, most times you will get the projection from point [0:0:...: 0:1].
 In this code, why do you create H in the first time? You redefine it just later without using it:
H = Hom(PP, PP) # only need the first n coordinates of the change of coordinates map coords = [PP.gens()[i]  Q[i]/Q[n]*PP.gens()[n] for i in range(n)] # create the projection map onto the first n coordinates PP2 = ProjectiveSpace(self.base_ring(), n  1) H = Hom(self, PP2)
comment:6 Changed 20 months ago by
 Commit changed from 0de1323a77147059de0f82c7a8642c44c31552ed to d9d149f0b2aca63e2b4f97ee0d5d9e0b4763657b
Branch pushed to git repo; I updated commit sha1. New commits:
d9d149f  20790: various changes from first review

comment:7 Changed 20 months ago by
Looks good so far.
One addition that would be nice os to allow the user to choose the projection point. It may become useful at some point.
comment:8 Changed 20 months ago by
By the way, instead of
indices[len(indices)  1]
you can just use
indices[1]
comment:9 Changed 20 months ago by
 Commit changed from d9d149f0b2aca63e2b4f97ee0d5d9e0b4763657b to 7e639548c685b5d88c665bd582232abde8c79283
Branch pushed to git repo; I updated commit sha1. New commits:
7e63954  20790: minor changes and option for projective projection to pass a point

comment:10 Changed 20 months ago by
Thanks, changes made!
comment:11 Changed 20 months ago by
 Status changed from needs_review to positive_review
comment:12 Changed 20 months ago by
Reviewer name
comment:13 Changed 20 months ago by
 Reviewers set to Miguel Marco
comment:14 Changed 20 months ago by
 Status changed from positive_review to needs_work
Merge conflict, merge in next beta...
comment:15 Changed 20 months ago by
 Commit changed from 7e639548c685b5d88c665bd582232abde8c79283 to 7e1f6cf2732f6f832d5eb3d79d4785223e6f3335
 Status changed from needs_work to needs_review
New commits:
7e1f6cf  20790: merge with 7.3 beta4

comment:16 Changed 20 months ago by
 Commit changed from 7e1f6cf2732f6f832d5eb3d79d4785223e6f3335 to 98285148ebdb1c2c3cc189ec82716a80784c35f8
Branch pushed to git repo; I updated commit sha1. New commits:
9828514  20790: merge with 7.3 beta5

comment:17 Changed 20 months ago by
 Commit changed from 98285148ebdb1c2c3cc189ec82716a80784c35f8 to ee363e4fc1c9be96b771f9fe1fd5f05ede4862ce
Branch pushed to git repo; I updated commit sha1. New commits:
ee363e4  20790: merge with 7.3 beta6

comment:18 Changed 19 months ago by
 Status changed from needs_review to needs_work
You now need to use relative import for constructor:
from .constructor import Curve
comment:19 Changed 19 months ago by
 Commit changed from ee363e4fc1c9be96b771f9fe1fd5f05ede4862ce to 16d6207d099a7368a20bb2c0527c120f5d51943c
comment:20 Changed 19 months ago by
 Status changed from needs_work to needs_review
Ok, should be good now
comment:21 Changed 19 months ago by
 Milestone changed from sage7.3 to sage7.4
 Reviewers changed from Miguel Marco to Miguel Marco, Ben Hutz
 Status changed from needs_review to needs_work
All tests currently pass, but in looking through the functionality I don't think the 'newvariables' approach is consistent with other parts of sage. the names of the variables does not specify the space. So even if you have two affine spaces of the same dimension, base_ring and variables, they will be isomorphic, but not the same object in Sage. I think it would be better to have the user give the space to project into.
Similarly for the 'same variables' boolean. They really aren't the same variables. I think this just leads to user confusions since now you have two 'x's that mean different things.
Finally, for plane projection, you don't allow this functionality.
I think in all cases the better solution is to allow the user to specify the space, not the names of the variables. Look, for example, at the affine_patch and projective_embedding functionality for subschemes.
Unrelated: the doc tests do not test the input checks nor do they test the 'newvariables' parameter.
comment:22 Changed 18 months ago by
 Commit changed from 16d6207d099a7368a20bb2c0527c120f5d51943c to 878503201d65e0ace5146466d706417ec383fc83
Branch pushed to git repo; I updated commit sha1. New commits:
8785032  20790: changes from review

comment:23 Changed 18 months ago by
 Status changed from needs_work to needs_review
Ok, the user can now specify an ambient space to project into in each of the projection functions. I also added some input tests.
comment:24 Changed 18 months ago by
 Commit changed from 878503201d65e0ace5146466d706417ec383fc83 to 39108a1cf5c2b84da83b01f7283390236852ae1d
Branch pushed to git repo; I updated commit sha1. New commits:
39108a1  20790: added some tests

comment:25 Changed 18 months ago by
The projection functions now have tests to show passing an ambient space will ensure the projected curves are defined in that space.
comment:26 Changed 18 months ago by
 Status changed from needs_review to positive_review
looks good to me.
comment:27 Changed 18 months ago by
 Commit changed from 39108a1cf5c2b84da83b01f7283390236852ae1d to 67320bb67498749e9a85e7cb77f6bbd51a6fdad3
 Status changed from positive_review to needs_review
Branch pushed to git repo; I updated commit sha1 and set ticket back to needs_review. New commits:
67320bb  Merge branch 'master' into u/gjorgenson/ticket/20790

comment:28 Changed 18 months ago by
Fixed a merge conflict with the imports in affine_curve.py, projective_curve.py.
comment:29 Changed 18 months ago by
 Status changed from needs_review to positive_review
comment:30 Changed 18 months ago by
 Status changed from positive_review to needs_work
Merge conflict, probably #21085
comment:31 Changed 18 months ago by
 Commit changed from 67320bb67498749e9a85e7cb77f6bbd51a6fdad3 to 679338b2f7fd3888f06fa4f5d1b69310339e1fd7
Branch pushed to git repo; I updated commit sha1. Last 10 new commits:
3dd26e7  21085: resolution of singularities implementation, first attempt

4a6bfbf  21085: some minor changes

78e1ce2  21085: changes from review

1cbb355  21085: generalized blowup() to work for any point on a given curve

d1b1edb  Merge branch 'master' into u/gjorgenson/ticket/21085

78fdd81  21085: add import back in that was accidentally removed during merge

50d05de  21285: error in change_ring for affine morphisms

c6b7a42  21085: merge with ticket 21285 for bug fix

bebf7b0  21085: minor improvements

679338b  20790: merge with 21085 to fix conflicts

comment:32 Changed 18 months ago by
 Status changed from needs_work to needs_review
Ok, fixed the conflict with 21085. I don't think there are any other curve tickets closed after sage 7.4 beta1, so this should be good to go.
comment:33 Changed 18 months ago by
 Status changed from needs_review to positive_review
comment:34 Changed 18 months ago by
 Branch changed from u/gjorgenson/ticket/20790 to 679338b2f7fd3888f06fa4f5d1b69310339e1fd7
 Resolution set to fixed
 Status changed from positive_review to closed
Branch pushed to git repo; I updated commit sha1. New commits:
20790: merge with tickets 20697, 20676
20790: First attempt at implementing projections for projecitve curves.