The Bohr compactification of an arithmetic group

Abstract

Given a group $\Gamma,$ its Bohr compactification $\operatorname{Bohr}(\Gamma)$ and its profinite completion $\operatorname{Prof}(\Gamma)$ are compact groups naturally associated to $\Gamma$; moreover, $\operatorname{Prof}(\Gamma)$ can be identified with the quotient of $\operatorname{Bohr}(\Gamma)$ by its connected component $\operatorname{Bohr}(\Gamma)_0.$ We study the structure of $\operatorname{Bohr}(\Gamma)$ for an arithmetic subgroup $\Gamma$ of an algebraic group $G$ over $\mathbf{Q}$. When $G$ is unipotent, we show that $\operatorname{Bohr}(\Gamma)$ can be identified with the direct product $\operatorname{Bohr}(\Gamma^{\rm Ab})_0\times \operatorname{Prof}(\Gamma)$, where $\Gamma^{\rm Ab}= \Gamma/[\Gamma, \Gamma]$ is the abelianization of $\Gamma.$ In the general case, using a Levi decomposition $G= U\rtimes H$ (where $U$ is unipotent and $H$ is reductive), we show that $\operatorname{Bohr}(\Gamma)$ can be described as the semi-direct product of a certain quotient of $\operatorname{Bohr}(\Gamma\cap U)$ with $\operatorname{Bohr}(\Gamma \cap H)$. When $G$ is simple and has higher $\mathbf{R}$-rank, $\operatorname{Bohr}(\Gamma)$ is isomorphic, up to a finite group, to the product $K\times \operatorname{Prof}(\Gamma),$ where $K$ is the maximal compact factor of the real Lie group $G(\mathbf{R}).$