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# On elliptic curves with an isogeny of degree 7

## Titelangaben

Greenberg, Ralph ; Rubin, Karl ; Silverberg, Alice ; Stoll, Michael:
On elliptic curves with an isogeny of degree 7.
In: American Journal of Mathematics. Bd. 136 (2014) Heft 1 . - S. 77-109.
ISSN 0002-9327
DOI: 10.1353/ajm.2014.0005

## Abstract

We show that if $E$ is an elliptic curve over ${\bf Q}$ with a ${\bf Q}$-rational isogeny of degree $7$, then the image of the $7$-adic Galois representation attached to $E$ is as large as allowed by the isogeny, except for the curves with complex multiplication by ${\bf Q}(\sqrt{-7})$. The analogous result with $7$ replaced by a prime $p > 7$ was proved by the first author. The present case $p = 7$ has additional interesting complications. We show that any exceptions correspond to the rational points on a certain curve of genus $12$. We then use the method of Chabauty to show that the exceptions are exactly the curves with complex multiplication. As a by-product of one of the key steps in our proof, we determine exactly when there exist elliptic curves over an arbitrary field $k$ of characteristic not $7$ with a $k$-rational isogeny of degree $7$ and a specified Galois action on the kernel of the isogeny, and we give a parametric description of such curves.

## Weitere Angaben

Publikationsform: Artikel in einer Zeitschrift Ja Fakultäten > Fakultät für Mathematik, Physik und Informatik > Mathematisches Institut > Lehrstuhl Mathematik II (Computeralgebra)Fakultäten > Fakultät für Mathematik, Physik und Informatik > Mathematisches Institut > Lehrstuhl Mathematik II (Computeralgebra) > Lehrstuhl Mathematik II (Computeralgebra) - Univ.-Prof. Dr. Michael StollFakultätenFakultäten > Fakultät für Mathematik, Physik und InformatikFakultäten > Fakultät für Mathematik, Physik und Informatik > Mathematisches Institut Ja 500 Naturwissenschaften und Mathematik > 510 Mathematik 20 Nov 2014 10:58 20 Nov 2014 10:58 https://eref.uni-bayreuth.de/id/eprint/3750