Mathematical Research Letters

Volume 30 (2023)

Number 3

Properness of the global-to-local map for algebraic groups with toric connected component and other finiteness properties

Pages: 913 – 943

DOI: https://dx.doi.org/10.4310/MRL.2023.v30.n3.a11

Authors

Andrei S. Rapinchuk (Department of Mathematics, University of Virginia, Charlottesville, Va., U.S.A.)

Igor A. Rapinchuk (Department of Mathematics, Michigan State University, East Lansing, Mich., U.S.A.)

Abstract

This is a companion paper to $\href{ https://doi.org/10.1016/j.jnt.2021.07.001}{[29]}$, where we proved the finiteness of the Tate–Shafarevich group for an arbitrary torus $T$ over a finitely generated field $K$ with respect to any divisorial set $V$ of places of $K$. Here, we extend this result to any $K$-group $D$ whose connected component is a torus (for the same $V$), and as a consequence obtain a finiteness result for the local-to-global conjugacy of maximal tori in reductive groups over finitely generated fields. Moreover, we prove the finiteness of the Tate–Shafarevich group for tori over function fields $K$ of normal varieties defined over base fields of characteristic zero and satisfying Serre’s condition (F), in which case $V$ consists of the discrete valuations associated with the prime divisors on the variety (geometric places). In this situation, we also establish the finiteness of the number of $K$-isomorphism classes of algebraic $K$-tori of a given dimension having good reduction at all $v \in V$ , and then discuss ways of extending this result to positive characteristic. Finally, we prove the finiteness of the number of isomorphism classes of forms of an absolutely almost simple group defined over the function field of a complex surface that have good reduction at all geometric places.

Received 9 March 2021

Received revised 8 August 2021

Accepted 30 August 2021

Published 15 December 2023