#16456: Bug in descend_to method for elliptic curves
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Reporter: cremona | Owner: cremona
Type: defect | Status: new
Priority: major | Milestone: sage-6.3
Component: elliptic curves | Resolution:
Keywords: elliptic curve base change | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Changes (by cremona):
* owner: => cremona
Old description:
> The function descend_to which was implemented in #9384 is incorrect. The
> twisting parameter d in L* is only defined modulo (L*)^2 and one has to
> determine whether there is a representative which lies in K*, which the
> implementation does not do, so some "positive" results are missed. I
> will post an example of this. The heart of the problem being solved here
> is to determine whether (given that j(E) is in K of course) the twisting
> parameter in L*/(L*)^2 is in the image of K*/(K*)^2 or not, and the
> implementation ignores this. (For j=0 or 1728 the principle is the same
> with squares replced by 6th or 4th powers respectively.)
>
> I can see how to fix this for number fields (one can restrict from
> K*/(K*)^2 to a finite K(S,2) for an easily determined set S of primes) or
> for finite fields, but it may not be possible to have this implemented
> for arbitrary fields, which will cause a problem with the patching.
New description:
The function descend_to which was implemented in #9384 is incorrect. The
twisting parameter d in L* is only defined modulo {{{(L*)^2}}} and one has
to determine whether there is a representative which lies in K*, which the
implementation does not do, so some "positive" results are missed. I will
post an example of this. The heart of the problem being solved here is to
determine whether (given that j(E) is in K of course) the twisting
parameter in {{{L*/(L*)^2}}} is in the image of {{{K*/(K*)^2}}} or not,
and the implementation ignores this. (For j=0 or 1728 the principle is
the same with squares replaced by 6th or 4th powers respectively.)
I can see how to fix this for number fields (one can restrict from
K*/(K*)^2 to a finite K(S,2) for an easily determined set S of primes) or
for finite fields, but it may not be possible to have this implemented for
arbitrary fields, which will cause a problem with the patching.
A second and independent bug is reported by Warren Moore:
{{{
sage: k.<i> = QuadraticField(-1)
sage: E = EllipticCurve(k,[0,0,0,1,0])
sage: E.descend_to(QQ) == None
True
}}}
This caused by a "naked Except" clause plus a call to f.preimage() for a
map f which has no attribute/method "preimage".
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Ticket URL: <http://trac.sagemath.org/ticket/16456#comment:1>
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