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If $E$ is an elliptic curve defined over $\Q$ and $m\ge2$ then the mod-$m$ Galois representation attached to $E$ is the map $\overline\rho_{E,m}: \Gal(\overline{\Q}/\Q) \to \Aut(E[m]),$ giving the action of the absolute Galois group of $\mathbb{Q}$ on the $m$-torsion subgroup $E[m]$.

By identifying the finite abelian group $E[m]$ with $(\Z/m\Z)^2$ we may identify $\Aut(E[m])$ with $\GL(2,\Z/m\Z)$ and hence view the representation as a map $\overline\rho_{E,m}: \Gal(\overline{\Q}/\Q) \to \GL(2,\Z/m\Z),$ defined up to conjugation. In the case where $m=p$ is prime we have $\overline\rho_{E,p}: \Gal(\overline{\Q}/\Q) \to \GL(2,\F_p).$

Furthermore, one can combine the mod-$p^n$ representations attached to $E$ for $n\in \mathbb N$ to define the $p$-adic Galois representation attached to $E$, $\rho_{E,p}: \Gal(\overline{\Q}/\Q) \to \Aut(T_p(E)) \cong \GL(2,\Z_p),$ which describes the action of the absolute Galois group of $\mathbb{Q}$ on $T_p(E)$, the $p$-adic Tate module of $E$.

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• Review status: reviewed
• Last edited by John Jones on 2018-06-18 16:34:48
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