Symmetric subgroups of rational groups of hermitian type

Abstract

A rational group of hermitian type is an algebraic group over the rational numbers whose symmetric space is a hermitian symmetric space. We assume such a group $G$ to be given, which we assume is isotropic. Then, for any rational parabolic $P$ in the group $G$, we find a reductive rational subgroup $N$ closely related with $P$ by a relation we call incidence. This has implications to the geometry of arithmetic quotients of the symmetric space by arithmetic subgroups of $G$, in the sense that $N$ defines a subvariety on such an arithmetic quotient which has special behaviour at the cusp corresponding to the parabolic with which $N$ is incident.