Triple factorisations of the general linear group and their associated geometries

Abstract

Triple factorisations of finite groups $G$ of the form $G=PQP$ are essential in the study of Lie theory as well as in geometry. Geometrically, each triple factorisation $G=PQP$ corresponds to a $G$-flag transitive point/line geometry such that `each pair of points is incident with at least one line'. We call such a geometry \emph{collinearly complete}, and duality (interchanging the roles of points and lines) gives rise to the notion of \emph{concurrently complete} geometries. In this paper, we study triple factorisations of the general linear group $\mathrm{GL}(V)$ as $PQP$ where the subgroups $P$ and $Q$ either fix a subspace or fix a decomposition of $V$ as $V_1\oplus V_2$ with $\dim (V_1)=\dim (V_2)$.