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Geometry and ergodic theory of non-hyperbolic exponential maps
Authors:Mariusz Urbanski  Anna Zdunik
Institution:Department of Mathematics, University of North Texas, P.O. Box 311430, Denton, Texas 76203-1430 ; Institute of Mathematics, Warsaw University, ul. Banacha 2, 02-097 Warszawa, Poland
Abstract:We deal with all the maps from the exponential family $ \{\lambda e^z\}$ such that the orbit of zero escapes to infinity sufficiently fast. In particular all the parameters $ \lambda\in (1/e,+\infty)$ are included. We introduce as our main technical devices the projection $ F_{\lambda}$ of the map $ f_{\lambda}$ to the infinite cylinder $ Q=\mathbb{C}/2\pi i\mathbb{Z}$ and an appropriate conformal measure $ m$. We prove that $ J_r(F_\lambda)$, essentially the set of points in $ Q$ returning infinitely often to a compact region of $ Q$ disjoint from the orbit of $ 0\in Q$, has the Hausdorff dimension $ h_\lambda\in (1,2)$, that the $ h_\lambda$-dimensional Hausdorff measure of $ J_r(F_\lambda)$ is positive and finite, and that the $ h_\lambda$-dimensional packing measure is locally infinite at each point of $ J_r(F_\lambda)$. We also prove the existence and uniqueness of a Borel probability $ F_\lambda$-invariant ergodic measure equivalent to the conformal measure $ m$. As a byproduct of the main course of our considerations, we reprove the result obtained independently by Lyubich and Rees that the $ \omega$-limit set (under $ f_\lambda$) of Lebesgue almost every point in $ \mathbb{C}$, coincides with the orbit of zero under the map $ f_\lambda$. Finally we show that the the function $ \lambda\mapsto h_\lambda$, $ \lambda\in (1/e,+\infty)$, is continuous.

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