Non-tangential limits and the slope of trajectories of holomorphic semigroups of the unit disc

Abstract:

Let $\Delta \subsetneq \mathbb {C}$ be a simply connected domain, let $f:\mathbb {D} \to \Delta$ be a Riemann map, and let $\{z_k\}\subset \Delta$ be a compactly divergent sequence. Using Gromov’s hyperbolicity theory, we show that $\{f^{-1}(z_k)\}$ converges non-tangentially to a point of $\partial \mathbb {D}$ if and only if there exists a simply connected domain $U\subsetneq \mathbb {C}$ such that $\Delta \subset U$ and $\Delta$ contains a tubular hyperbolic neighborhood of a geodesic of $U$ and $\{z_k\}$ is eventually contained in a smaller tubular hyperbolic neighborhood of the same geodesic. As a consequence we show that if $(\phi _t)$ is a non-elliptic semigroup of holomorphic self-maps of $\mathbb {D}$ with Koenigs function $h$ and $h(\mathbb {D})$ contains a vertical Euclidean sector, then $\phi _t(z)$ converges to the Denjoy-Wolff point non-tangentially for every $z\in \mathbb {D}$ as $t\to +\infty$. Using new localization results for the hyperbolic distance, we also construct an example of a parabolic semigroup which converges non-tangentially to the Denjoy-Wolff point but is oscillating, in the sense that the slope of the trajectories is not a single point.