Natural extensions of unimodal maps: prime ends of planar embeddings and semi-conjugacy to sphere homeomorphisms

Philip Boyland, André de Carvalho, Toby Hall

Published 2017-04-21Version 1

Let $f\colon I\to I$ be a unimodal map with topological entropy $h(f)>\frac12\log2$, and let $\widehat{f}\colon\widehat{I}\to\widehat{I}$ be its natural extension, where $\widehat{I}=\varprojlim(I,f)$. Subject to some regularity conditions, which are satisfied for tent maps and quadratic maps, we give a complete description of the prime ends of the Barge-Martin embedding of $\widehat{I}$ into the disk, and identify the prime ends rotation number with the height of $f$. We also show that $\widehat{f}$ is semi-conjugate to a sphere homeomorphism by a semi-conjugacy for which all fibers except one contain at most three points. In the case where $f$ is a post-critically finite tent map, we show that the corresponding sphere homeomorphism is a generalized pseudo-Anosov map.