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Galois Representations with Non-Surjective Traces
Published online by Cambridge University Press: 20 November 2018
Abstract
Let $E$ be an elliptic curve over
$\mathbb{Q}$
, and let
$r$ be an integer. According to the Lang-Trotter conjecture, the number of primes
$p$ such that
${{a}_{p}}\left( E \right)=r$ is either finite, or is asymptotic to
${{C}_{E,r}}\sqrt{x}/\log x$ where
${{C}_{E,r}}$ is a non-zero constant. A typical example of the former is the case of rational
$\ell $-torsion, where
${{a}_{p}}\left( E \right)=r$ is impossible if
$r\equiv 1\,\left( \bmod \,\ell \right)$. We prove in this paper that, when
$E$ has a rational
$\ell $-isogeny and
$\ell \ne 11$, the number of primes
$p$ such that
${{a}_{p}}\left( E \right)\equiv r\,\left( \bmod \,\ell \right)$ is finite (for some
$r$ modulo
$\ell $) if and only if
$E$ has rational
$\ell $-torsion over the cyclotomic field
$\mathbb{Q}\left( {{\zeta }_{\ell }} \right)$
. The case
$\ell =11$ is special, and is also treated in the paper. We also classify all those occurences.
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- Copyright © Canadian Mathematical Society 1999
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