Opened 8 years ago

Closed 8 years ago

#16022 closed defect (fixed)

simon_two_descent sometimes gives wrong answers

Reported by: pbruin Owned by:
Priority: critical Milestone: sage-6.2
Component: elliptic curves Keywords: simon_two_descent
Cc: cremona, mmasdeu Merged in:
Authors: Peter Bruin Reviewers: Marc Masdeu
Report Upstream: Reported upstream. No feedback yet. Work issues:
Branch: 275e4be (Commits, GitHub, GitLab) Commit: 275e4bef0c5027517163a50248cf417308d6bde7
Dependencies: Stopgaps:

Status badges

Description

The current version of Denis Simon's 2-descent program (in Sage since #11005) has a bug:

sage: K.<y> = NumberField(x^4 + x^2 - 7);
sage: E = EllipticCurve(K, [1, 0, 5*y^2 + 16, 0, 0])
sage: E.simon_two_descent(lim1=2, limtriv=3)
[0, 0, []]

The rank is in fact 1, and a correct return value would be (1, 1, [(-369/25*y^3 + 539/25*y^2 - 1178/25*y + 1718/25 : -27193/125*y^3 + 39683/125*y^2 - 86816/125*y + 126696/125 : 1)]).

This ticket is just to fix this as quickly as possible; it should eventually be fixed in a new version of Simon's program.

(See #15608 for a list of tickets related to simon_two_descent.)

Change History (6)

comment:1 Changed 8 years ago by pbruin

  • Branch set to u/pbruin/16022-simon_two_descent_bug
  • Commit set to 275e4bef0c5027517163a50248cf417308d6bde7
  • Status changed from new to needs_review

comment:2 Changed 8 years ago by pbruin

To reproduce this directly in GP:

\r ell.gp
K = bnfinit(y^4 + y^2 - 7);
a = Mod(y, K.pol);
E = [1, 0, 5*a^2 + 16, 0, 0];
DEBUGLEVEL_ell = 2;
LIM1 = 2;
LIMTRIV = 3;
bnfellrank(K, E)

comment:3 Changed 8 years ago by mmasdeu

  • Reviewers set to Marc Masdeu
  • Status changed from needs_review to positive_review

comment:4 follow-up: Changed 8 years ago by wuthrich

Comment moved from #10745:

I see that you changed ell.gp. Is this good ? It is very very likely that the next update of simon's scripts will forget to make this patch-fix of a upstream file. Would it not be better to tell the author to change this in his version and we update our file ? This is a genuine question as I am not sure what is better. One thing to bear in mind is that it seems that Denis has not been very active on the bugs in his script recently.

comment:5 in reply to: ↑ 4 Changed 8 years ago by pbruin

Replying to wuthrich:

I see that you changed ell.gp. Is this good ? It is very very likely that the next update of simon's scripts will forget to make this patch-fix of a upstream file. Would it not be better to tell the author to change this in his version and we update our file ? This is a genuine question as I am not sure what is better. One thing to bear in mind is that it seems that Denis has not been very active on the bugs in his script recently.

I reported the bug to Denis and sent him the same patch used for this ticket. Something similar happened with #15483. So far, these are the only two tickets in which changes to ell.gp were made. Hopefully he will make these fixes (or alternative ones) in the next version. (We don't know when that will be, but the last version of ell.gp dates from 2011, while all his other programs were updated last January.)

comment:6 Changed 8 years ago by vbraun

  • Branch changed from u/pbruin/16022-simon_two_descent_bug to 275e4bef0c5027517163a50248cf417308d6bde7
  • Resolution set to fixed
  • Status changed from positive_review to closed
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