On the congruence kernel for simple algebraic groups

Abstract

This paper contains several results about the structure of the congruence kernel C^(S)(G) of an absolutely almost simple simply connected algebraic group G over a global field K with respect to a set of places S of K. In particular, we show that C^(S)(G) is always trivial if S contains a generalized arithmetic progression. We also give a criterion for the centrality of C^(S)(G) in the general situation in terms of the existence of commuting lifts of the groups G(K_v) for v \notin S in the S-arithmetic completion \widehat{G}^(S). This result enables one to give simple proofs of the centrality in a number of cases. Finally, we show that if K is a number field and $G$ is K-isotropic then C^(S)(G) as a normal subgroup of \widehat{G}^(S) is almost generated by a single element.