Embeddings of Maximal Tori in Classical Groups, Odd Degree Descent and Hasse Principles

The aim of this paper is to revisit the question of local–global principles for embeddings of étale algebras with involution into central simple algebras with involution over global fields of characteristic not 2. A necessary and sufficient condition is given in Bayer-Fluckiger et al. (J Eur Math Soc 20:137–163, 2018). In the present paper, we give a simpler description of the obstruction group. It is also shown that if the étale 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.