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.multiplication_by_m_isogeny() fails for negative m #34727

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yyyyx4 opened this issue Nov 7, 2022 · 8 comments
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.multiplication_by_m_isogeny() fails for negative m #34727

yyyyx4 opened this issue Nov 7, 2022 · 8 comments
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c: elliptic curves p: minor / 4 t: bug
Milestone
sage-9.8

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@yyyyx4
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yyyyx4 commented Nov 7, 2022 

sage: EllipticCurve([5,5]).multiplication_by_m_isogeny(-1)
# ...
AssertionError: bug in multiplication_by_m_isogeny()
sage: EllipticCurve([5,5]).multiplication_by_m_isogeny(-2)
# ...
NotImplementedError: Kohel's algorithm currently only supports cyclic isogenies (except for [2])
sage: EllipticCurve([5,5]).multiplication_by_m_isogeny(-3)
# ...
ValueError: n must be a positive integer (or -1 or -2)

All of these are because .multiplication_by_m_isogeny() calls .torsion_polynomial() with m, which may be negative, rather than abs(m). As the last error message indicates, the values -1 and -2 additionally have special meaning for .torsion_polynomial(), which made debugging a bit more confusing...

Component: elliptic curves

Author: Lorenz Panny

Branch/Commit: a6e99a7

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/34727
@yyyyx4 yyyyx4 added this to the sage-9.8 milestone Nov 7, 2022
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yyyyx4 commented Nov 7, 2022

New commits:

a6e99a7use abs(m) to compute m-division polynomial

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yyyyx4 commented Nov 7, 2022

Branch: public/34727

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yyyyx4 commented Nov 7, 2022

Author: Lorenz Panny

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yyyyx4 commented Nov 7, 2022

Commit: a6e99a7

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Reviewer: Frédéric Chapoton

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comment:2

ok

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vbraun commented Nov 20, 2022

Changed branch from public/34727 to a6e99a7

@vbraun vbraun closed this as completed in 1b9ac4d Nov 20, 2022
@yyyyx4 yyyyx4 mentioned this issue Dec 3, 2022
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c: elliptic curves p: minor / 4 t: bug
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sage-9.8
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