A note on relative amenability

Abstract

P.-E. Caprace and N. Monod isolate the class $\mathcal{X}$ of locally compact groups for which relatively amenable closed subgroups are amenable. It is unknown if $\mathcal{X}$ is closed under group extension. In this note, we exhibit a large, group extension stable subclass of $\mathcal{X}$, which suggests $\mathcal{X}$ is indeed closed under group extension. Along the way, we produce generalizations of the class of elementary groups and obtain information on groups outside $\mathcal{X}$.