Constricting elements and the growth of quasi-convex subgroups

Abstract: Given a group $G$ acting on a geodesic metric space, we consider a preferred collection of paths of the space -- a path system -- and study the spectrum of relative exponential growth rates and quotient exponential growth rates of the infinite index subgroups of $G$ which are quasi-convex with respect to this path system. If $G$ contains a constricting element with respect to the same path system, we are able to determine when the first kind of growth rates are strictly smaller than the growth rate of $G$, and when the second kind of growth rates coincide with the growth rate of $G$. Examples of applications include relatively hyperbolic groups, $CAT(0)$ groups and hierarchically hyperbolic groups containing a Morse element.