We give a concrete presentation for the general linear group defined over a ring which is a finitely generated free $\mathbb{Z}$-module or the integral Clifford group $\Gamma_n(\mathbb{Z})$ of invertible elements in the Clifford algebra with integral coefficients. We then use this presentation to prove that the elementary linear group over $\Gamma_n(\mathbb{Z})$ has a non-trivial decomposition as a free product with amalgamated subgroup the elementary linear group over $\Gamma_{n-1}(\mathbb{Z})$. This allows to obtain applications to the unit group $\mathcal{U}(\mathbb{Z} G)$ of an integral group ring $\mathbb{Z} G$ of a finite group $G$. In particular, we prove that $\mathcal{U} (\mathbb{Z} G)$ is hereditary (FA), i.e. every subgroup of finite index has property (FA), or is commensurable with a non-trivial amalgamated product. In the case $\mathcal{U}(\mathbb{Z} G)$ is not hereditary (FA), we investigate subgroups of finite index in $\mathcal{U}(\mathbb{Z} G)$ that have a non-trivial 查看全文>>