Polynomial hyperbolicity and products of free groups

Abstract

In this article, we define a locally finite graph $X$ as $η$-polynomially hyperbolic if there exists a Lipschitz map $φ: X \to Z$ to some hyperbolic space $Z$ satisfying the following condition: there exists $C \geq 0$ such that $$|B(p,R_1) \cap φ^{-1} (B(q,R_2))| \leq (C R_1)^{η(C R_2)} \text{ for all } p,q \in X, R_1,R_2 \geq 0.$$ The picture to keep in mind is that coarse fibres of $φ$ have polynomial growth with a degree coarsely controlled by $η$ as the thickness of the fibres grows. The map $η$ quantifies how brutal we have to be in order to turn $X$ into a hyperbolic space. Our main result is that, among cocompact special groups, being $\mathrm{lin}$-polynomially hyperbolic amounts not to contain $\mathbb{F}_2 \times \mathbb{F}_2$ as a subgroup. Consequently, containing $\mathbb{F}_2 \times \mathbb{F}_2$ as a subgroup turns out to be quasi-isometric invariant for cocompact special groups.