#19030: fix small bug in elliptic curve isogeny matrix of maps
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Reporter: cremona | Owner:
Type: defect | Status: new
Priority: minor | Milestone: sage-6.9
Component: elliptic curves | Keywords: isogeny class
Merged in: | Authors: John Cremona
Reviewers: | Report Upstream: N/A
Work issues: | Branch:
Commit: | Dependencies:
Stopgaps: |
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When constructing the isogeny class C of an elliptic curve (over Q or a
number field) a partial matrix of maps between the curves is computed,
available as C.isogenies(). However, some of the known maps were being
overwriiten by 0 (used to indicate that the map was not known.
Example:
{{{
K.<a> = NumberField(x^2-2)
E = EllipticCurve([1, -1, 0, 3*a, -2*a - 2])
C = E.isogeny_class()
C.isogenies()
}}}
used to show only 2 nonzero entries, but now shows 4.
I did not add new doctests but two existing ones had to be changed, and
these illustrate the improvement.
Why this matters: it is important that there is a curve on the class from
which you can get to every other curve by following the stored maps, for
example to map across generators. This was not the case i nthe above
example, but now is.
--
Ticket URL: <http://trac.sagemath.org/ticket/19030>
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