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A fix for upstream bug 2441 and gentoo bug 892035, with the changes to
CHANGES and the test suite stripped out (they don't apply cleanly).
From 6f556664bea5dae9d7101533be65f10b9d6332e2 Mon Sep 17 00:00:00 2001
From: Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>
Date: Fri, 20 Jan 2023 11:13:46 +0100
Subject: [PATCH 1/1] ellcard(ellinit([1,1]*ffgen([41,16])))-> impossible
inverse [#2441]
find_isogenous_from_Atkin: check that the kernel divides the division polynomial
---
CHANGES | 1 +
src/basemath/ellsea.c | 3 ++-
src/test/32/ellff | 3 ++-
src/test/in/ellff | 3 +++
4 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/src/basemath/ellsea.c b/src/basemath/ellsea.c
index a6871fa6a7..05f148eadd 100644
--- a/src/basemath/ellsea.c
+++ b/src/basemath/ellsea.c
@@ -852,7 +852,8 @@ find_isogenous_from_Atkin(GEN a4, GEN a6, ulong ell, struct meqn *MEQN, GEN g, G
GEN a4t, a6t, h;
a4a6t(&a4t, &a6t, ell, E4t, E6t, T, p);
h = find_kernel(a4, a6, ell, a4t, a6t, pp1, T, p, pp, e);
- if (h) return gerepilecopy(ltop, mkvec3(a4t, a6t, h));
+ if (h && signe(Fq_elldivpolmod(a4, a6, ell, h, T, pp))==0)
+ return gerepilecopy(ltop, mkvec3(a4t, a6t, h));
}
}
pari_err_BUG("find_isogenous_from_Atkin, kernel not found");
--
2.30.2
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