Fractal Models for Normal Subgroups of Schottky Groups

For a normal subgroup $N$ of the free group $\F_d$ with at least two generators we introduce the radial limit set $\Lr(N,\Phi)$ of $N$ with respect to a graph directed Markov system $\Phi$ associated to $\F_d$. These sets are shown to provide fractal models of radial limit sets of normal subgroups of Kleinian groups of Schottky type. Our main result states that if $\Phi$ is symmetric and linear, then we have that $\dim_{H}(\Lr(N,\Phi))=\dim_{H} \Lr(\F_d,\Phi))$ if and only if the quotient group $\F_d /N$ is amenable, where $\dim_{H}$ denotes the Hausdorff dimension. This extends a result of Brooks for normal subgroups of Kleinian groups to a large class of fractal sets. Moreover, we show that if $\F_d /N$ is non-amenable then $\dim_{H}(\Lr(N,\Phi))>\dim_{H}(\Lr(\F_d,\Phi))/2$, which extends results by Falk and Stratmann and by Roblin.