#10888: problem in evaluation dual isogeny
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Reporter: wuthrich | Owner: cremona
Type: defect | Status: needs_work
Priority: major | Milestone: sage-4.7
Component: elliptic curves | Keywords: isogenies,
Author: wuthrich | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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Changes (by defeo):
* cc: defeo (added)
* status: needs_review => needs_work
Comment:
The patch works and I'm willing to give a positive review, but the doctest
takes 2.6 cpu secs on my laptop (64-bit Intel Core2 U9400 @ 1.40GHz...
not the fastest cpu ever, but still decent), it would make sense to tag
this test as long.
Alternatively, though I don't know if this is acceptable for Sage
standards, the doctest could be slightly changed. The code below fails in
4.6.2 and is fixed by this patch; it takes only 0.2 secs, because it uses
Kohel's algorithm (instead of Vélu's formulae) to compute `phi` and
because avoids the call to `phi.dual()` (which uses Stark's algorithm). It
tests exactly the same bug, because it crosses the same conditional
branches modified by the patch.
{{{
sage: K.<th> = NumberField(x^2+3)
sage: E = EllipticCurve(K,[7,0])
sage: _.<X> = K[]
sage: phi = E.isogeny(X)
sage: phi.set_pre_isomorphism(E.automorphisms()[1])
sage: phi.set_post_isomorphism(phi.codomain().automorphisms()[1])
sage: P = E(-3,4*th)
sage: phi(P)
(-16/3 : 8/9*th : 1)
}}}
Finally, the doctest does not show up in the reference manual, because it
is in the docstring of the method `__call__`. I don't know if this really
is a problem.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/10888#comment:7>
Sage <http://www.sagemath.org>
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