Connection Blocking in $SL(n,R)$ Quotients

Abstract

Let $G$ be a connected Lie group and $\Gamma \subset G$ a lattice. Connection curves of the homogeneous space $M=G/\Gamma$ are the orbits of one parameter subgroups of $G$. To \textit{block} a pair of points $m_1,m_2 \in M$ is to find a \textit{finite} set $B \subset M\setminus \{m_1, m_2 \}$ such that every connecting curve joining $m_1$ and $m_2$ intersects $B$. The homogeneous space $M$ is \textit{blockable} if every pair of points in $M$ can be blocked. \par In this paper we investigate blocking properties of $M_n=SL(n,R)/\Gamma$, where $\Gamma=SL(n,Z)$ is the integer lattice. We focus on $M_2$ and show that the set of bloackable pairs is a dense subset of $M_2 \times M_2$, and we conclude manifolds $M_n$ are not blockable. Finally, we review a quaternionic structure of $SL(2,R)$ and a way for making co-compact lattices in this context. We show that the obtained quotient homogeneous spaces are not finitely blockable.