Cartan Projections of Some Non-reductive Subgroups and Proper Actions on Homogeneous Spaces

Abstract

Kobayashi (Duke Math. J. 67:653–664, 1992) gave a necessary condition for the existence of compact Clifford–Klein forms in terms of Cartan projections and non-compact dimensions of reductive subgroups. We extend his method to non-reductive subgroups and give some examples of homogeneous spaces of reductive type that do not admit compact Clifford–Klein forms by comparing Cartan projections and non-compact dimensions of reductive subgroups with those of non-reductive subgroups.