Endomorphism rings of reductions of elliptic curves and Abelian varieties

Yu. G. Zarhin

Algebra i analiz, Tome 29 (2017) no. 1, pp. 110-144.

Résumé

Let $E$ be an elliptic curve without CM that is defined over a number field $K$. For all but finitely many non-Archimedean places $v$ of $K$ there is a reduction $E(v)$ of $E$ at $v$ that is an elliptic curve over the residue field $k(v)$ at $v$. The set of $v$'s with ordinary $E(v)$ has density 1 (Serre). For such $v$ the endomorphism ring $\operatorname{End}(E(v))$ of $E(v)$ is an order in an imaginary quadratic field. We prove that for any pair of relatively prime positive integers $N$ and $M$ there are infinitely many non-Archimedean places $v$ of $K$ such that the discriminant $\boldsymbol\Delta(\mathbf v)$ of $\operatorname{End}(E(v))$ is divisible by $N$ and the ratio $\frac{\boldsymbol\Delta(\mathbf v)}N$ is relatively prime to $NM$. We also discuss similar questions for reductions of Abelian varieties. The subject of this paper was inspired by an exercise in Serre's "Abelian $\ell$-adic representations and elliptic curves" and questions of Mihran Papikian and Alina Cojocaru.

Détail

Keywords: absolute Galois group, Abelian variety, general linear group, Tate module, Frobenius element.

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