Cartan projections of fiber products and non quasi-isometric embeddings

Abstract

Let $\Gamma$ be a finitely generated group and $N$ be a normal subgroup of $\Gamma$. The fiber product of $\Gamma$ with respect to $N$ is the subgroup $\Gamma \times_N \Gamma=\{(\gamma, \gamma w): \gamma \in \Gamma, w \in N\}$ of the direct product $\Gamma \times \Gamma$. For every representation $\rho:\Gamma \times_N \Gamma \rightarrow \mathsf{GL}_d(k)$, where $k$ is a local field, we establish upper bounds for the norm of the Cartan projection of $\rho$ in terms of a fixed word length function on $\Gamma$. As an application, we exhibit examples of finitely generated and finitely presented fiber products $P=\Gamma \times_N \Gamma$, where $\Gamma$ is linear and Gromov hyperbolic, such that $P$ does not admit linear representations which are quasi-isometric embeddings.