Fields of definition of building blocks with quaternionic multiplication

TL;DR

This paper characterizes the fields of definition of certain abelian varieties with quaternionic multiplication, revealing cases where these fields are smaller than expected and providing practical computational methods.

Contribution

It extends Ribet's results by characterizing fields of definition for building blocks with quaternionic multiplication using non-abelian group extensions.

Findings

Fields of definition can be strictly smaller with quaternionic multiplication.

Provides a group extension framework for the problem.

Includes practical computational methods for these fields.

Abstract

This paper investigates the fields of definition up to isogeny of the abelian varieties called building blocks. A result of Ribet characterizes the fields of definition of these varieties together with their endomorphisms, in terms of a Galois cohomology class canonically attached to them. However, when the building blocks have quaternionic multiplication, then the field of definition of the varieties can be strictly smaller than the field of definition of their endomorphisms. What we do is to give a characterization of the fields of definition of the varieties in this case (also in terms of their associated Galois cohomology class), by translating the problem into the language of group extensions with non-abelian kernel. We also make the computations that are needed in order to calculate in practice these fields from our characterization.