Dynamics of Actions of Automorphisms of Discrete Groups $G$ on Sub$_G$ and Applications to Lattices in Lie Groups

Abstract: For a discrete group $G$ and the compact space Sub$G$ of (closed) subgroups of $G$ endowed with the Chabauty topology, we study the dynamics of actions of automorphisms of $G$ on Sub$_G$ in terms of distality and expansivity. We also study the structure and properties of lattices $\Gamma$ in a connected Lie group. In particular, we show that the unique maximal solvable normal subgroup of $\Gamma$ is polycyclic and the corresponding quotient of $\Gamma$ is either finite or admits a cofinite subgroup which is a lattice in a connected semisimple Lie group with certain properties. We also show that Sub$^c\Gamma$, the set of cyclic subgroups of $\Gamma$, is closed in Sub$\Gamma$. We prove that an infinite discrete group $\Gamma$ which is either polycyclic or a lattice in a connected Lie group, does not admit any automorphism which acts expansively on Sub$^c\Gamma$, while only the finite order automorphisms of $\Gamma$ act distally on Sub$^c_\Gamma$. For an automorphism $T$ of a connected Lie group $G$ and a $T$-invariant lattice $\Gamma$ in $G$, we compare the behaviour of the actions of $T$ on Sub$G$ and Sub$\Gamma$ in terms of distality. We put certain conditions on the structure of the Lie group $G$ under which we show that $T$ acts distally on Sub$G$ if and only if it acts distally on Sub$\Gamma$. We construct counter examples to show that this does not hold in general if the conditions on the Lie group are relaxed.