Characterizations of extremal hyperbolic rates via Herglotz measures and Koenigs linearization

Abstract

Let $g:\mathbb{D}\to\mathbb{D}$ be a hyperbolic holomorphic self-map of the unit disk with Denjoy--Wolff point $\tau\in\partial\mathbb{D}$ and angular derivative $\alpha\in(0,1)$. We say that $g$ has an \textit{extremal hyperbolic rate} if its forward iterates satisfy the sharp metric asymptotic \begin{equation*} \rho_{\mathbb{D}}(g^{\circ n}(z),w) = n\log\frac{1}{\alpha} + O(1) \quad\text{as }n\to\infty, \end{equation*} for every $z,w\in\mathbb{D}$. Using the Herglotz--Nevanlinna representation, we prove that $g$ has an extremal hyperbolic rate if and only if the associated boundary measure $\sigma$ satisfies \begin{equation*} \int_{\partial\mathbb{D}\setminus\{\tau\}} \log\frac{1}{|\zeta-\tau|} \,d\sigma(\zeta) < \infty. \end{equation*} We further show that this condition is equivalent to a non-degenerate angular asymptotic of the Koenigs linearization: for any conformal map $\phi_\tau:\mathbb{D}\to\mathbb{H}$ with $\phi_\tau(\tau)=\infty$, \begin{equation*} 0< \left| \angle\lim_{z\to\tau} \frac{h(z)}{\phi_\tau(z)} \right| < \infty, \end{equation*} where $h$ is a Koenigs function. We then extend the extremal-rate theory beyond the unit disk. For finitely connected hyperbolic planar domains, the disk characterizations transfer to ordinary boundary points of the associated deck transformation group. For holomorphic self-maps of the unit ball $\mathbb{B}^n$ and for $K$-quasiconformal self-maps, where no comparable Herglotz representation is available; we establish sufficient boundary regularity conditions that guarantee the extremal hyperbolic rate. The quasiconformal result is further extended to finitely connected planar domains at ordinary boundary points. These results show that, across the settings considered here, extremal hyperbolic growth is governed by the non-degeneracy of the boundary linearization at the Denjoy--Wolff point.