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 $G(\mathbf{ R}).$
Categories: math.GR