Hausdorff dimension of directional limit sets for self-joinings of hyperbolic manifolds

Abstract

The classical result of Patterson and Sullivan says that for a non-elementary convex cocompact subgroup $\Gamma<\text{SO}^\circ (n,1)$, $n\ge 2$, the Hausdorff dimension of the limit set of $\Gamma$ is equal to the critical exponent of $\Gamma$. In this paper, we generalize this result for self-joinings of convex cocompact groups in two ways. Let $\Delta$ be a finitely generated group and $\rho_i:\Delta\to \text{SO}^\circ(n_i,1)$ be a convex cocompact faithful representation of $\Delta$ for $1\le i\le k$. Associated to $\rho=(\rho_1, \cdots, \rho_k)$, we consider the following self-joining subgroup of $\prod_{i=1}^k \text{SO}(n_i,1)$: $$\Gamma=\left(\prod_{i=1}^k\rho_i\right)(\Delta)=\{(\rho_1(g), \cdots, \rho_k(g)):g\in \Delta\} .$$ (1). Denoting by $\Lambda\subset \prod_{i=1}^k \mathbb{S}^{n_i-1}$ the limit set of $\Gamma$, we first prove that $$\text{dim}_H \Lambda=\max_{1\le i\le k} \delta_{\rho_i}$$ where $\delta_{\rho_i}$ is the critical exponent of the subgroup $\rho_{i}(\Delta)$. (2). Denoting by $\Lambda_u\subset \Lambda$ the $u$-directional limit set for each $u=(u_1, \cdots, u_k)$ in the interior of the limit cone of $\Gamma$, we obtain that for $k\le 3$, $$ \frac{\psi_\Gamma(u)}{\max_i u_i }\le \text{dim}_H \Lambda_u \le \frac{\psi_\Gamma(u)}{\min_i u_i }$$ where $\psi_\Gamma:\mathbb{R}^k\to \mathbb{R}\cup\{-\infty\}$ is the growth indicator function of $\Gamma$.