Uniform Perfectness, Geodesic Richness, and Rigidity for Sublinearly Morse Boundaries
Hyungryul Baik
TL;DR
The paper extends the Han–Liu intrinsic characterization of uniform perfectness from Morse boundaries to the sublinear Morse boundary $\partial_{\kappa}X$, introducing $\kappa$-Morse geometry and a renormalization framework via $\rho_{\kappa}$. It establishes the equivalence among uniform perfectness, $\kappa$-Morse geodesic richness, and $\kappa$-center-exhaustiveness, using a sublinear thin-triangle theory and coarse-center machinery. The work yields quantitative dimension estimates for $\kappa$-visual metrics (lower Assouad and, under doubling, Hausdorff) and proves a rigidity result: any quasi-symmetric boundary map between $\partial_{\kappa}X$ and $\partial_{\kappa}Y$ is realized by a sublinear bilipschitz equivalence between the spaces, under a growth condition on $\kappa$. These results provide a robust sublinear-geometry analogue of boundary rigidity and dimension theory, with applications to cocompact spaces and groups and to the study of stable subgroups within the $\kappa$-visual framework.
Abstract
Han and Liu gave a geometric characterization of uniform perfectness for the Morse boundary of a proper geodesic metric space: the Morse boundary is uniformly perfect if and only if the space is Morse geodesically rich, equivalently center--exhaustive. In this paper we prove the analogous statement for the sublinearly Morse boundary $\partial_κX$. Here $κ$ is a fixed concave increasing sublinear function and $\partial_κX$ is the boundary introduced by Qing--Rafi for CAT(0) spaces and extended by Qing--Rafi--Tiozzo to proper geodesic spaces. Assuming that $\partial_κX$ has at least three points, we show that uniform perfectness of $\partial_κX$ (for any $κ$--visual metric based at a fixed basepoint) is equivalent to $κ$--Morse geodesic richness and to $κ$--center--exhaustiveness. The geometric input is a sublinear thin--triangle statement for $κ$--Morse geodesics, together with the renormalization map $ρ_κ(t)=\int_0^t \frac{ds}{κ(s)}$, which converts $κ$--scale errors at radius $R$ into bounded errors in the $ρ_κ$--scale. As applications we obtain quantitative lower bounds on the lower Assouad dimension (and, under doubling hypotheses, on the Hausdorff dimension) of $κ$--visual metrics on $\partial_κX$ in terms of the uniform perfectness constant. Finally, for $κ$--center--exhaustive spaces $X$ and $Y$ satisfying a mild additional growth condition on $κ$, we prove a rigidity statement in the sublinear category: every quasi-symmetric homeomorphism $\partial_κX\to\partial_κY$ is induced by a sublinear bilipschitz equivalence $X\to Y$.