Embedding functors and their arithmetic properties

Abstract

In this article, we focus on how to embed a torus $\mathrm{T}$ into a reductive group $\mathrm{G}$ with respect to a given root datum $\Psi$ over a scheme $\mathrm{S}$. This problem is also related to embedding an étale algebra with involution into a central simple algebra with involution (cf. \[PR10]). We approach this problem by defining the embedding functor, which is representable and is a left homogeneous space over $\mathrm{S}$ under the automorphism group of $\mathrm{G}$. In order to fix a connected component of the embedding functor, we define an orientation $u$ of $\Psi$ with respect to $\mathrm{G}$. We show that the oriented embedding functor is also representable and is a homogeneous space under the adjoint action of $\mathrm{G}$. Over a local field, the orientation $u$ and the Tits index of $\mathrm{G}$ determine the existence of an embedding of $\mathrm{T}$ into $\mathrm{G}$ with respect to the given root datum $\Psi$. We also use the techniques developed in Borovoi’s paper \[Bo99] to prove that the local-global principle holds for oriented embedding functors in certain cases. Actually, the Brauer–Manin obstruction is the only obstruction to the local-global principle for the oriented embedding functor. Finally, we apply the results on oriented embedding functors to give an alternative proof of Prasad and Rapinchuk’s Theorem, and to improve Theorem 7.3 in \[PR10].