Embeddings of maximal tori in classical groups, odd degree descent and Hasse principles

TL;DR

This paper investigates local-global principles for embeddings of étale algebras with involution into central simple algebras with involution over global fields, providing simplified obstructions and conditions for the Hasse principle to hold.

Contribution

It offers a simplified description of the obstruction group and proves the Hasse principle under certain conditions, extending previous results and generalizing to odd degree extensions.

Findings

Hasse principle holds for products of linearly disjoint field extensions

Existence after odd degree extension implies existence over the base field

Simplified obstruction group description

Abstract

The aim of this paper is to revisit the question of local-global principles for embeddings of \'etale algebras with involution into central simple algebras with involution over global fields of characteristic not 2. A necessary and sufficient condition is given in [BLP 18]. In the present paper, we give a simpler description of the obstruction group. It is also shown that if the etale algebra is a product of pairwise linearly disjoint field extensions, then the Hasse principle holds, and that if an embedding exists after an odd degree extension, then it also exists over the global field itself. An appendix gives a generalization of this later result, in the framework of a question of Burt Totaro.