Connection blocking in homogeneous spaces and nilmanifolds

Abstract

Let $G$ be a connected Lie group acting locally simply transitively on a manifold $M$. By connecting curves in $M$ we mean the orbits of one-parameter subgroups of $G$. To block a pair of points $m_1,m_2\in M$ is to find a 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 blockable if every pair of points in $M$ can be blocked. Motivated by the geodesic security [4], we conjecture that the only blockable homogeneous spaces of finite volume are the tori. Here we establish the conjecture for nilmanifolds.