Natural extensions of unimodal maps: virtual sphere homeomorphisms and prime ends of basin boundaries

Abstract: Let ${f_t\colon I\to I}$ be a family of unimodal maps with topological entropies $h(f_t)>\frac12\log 2$, and ${\widehat{f}}t\colon{\widehat{I}}_t\to{\widehat{I}}_t$ be their natural extensions, where ${\widehat{I}}_t=\varprojlim(I,f_t)$. Subject to some regularity conditions, which are satisfied by tent maps and quadratic maps, we give a complete description of the prime ends of the Barge-Martin embeddings of ${\widehat{I}}_t$ into the sphere. We also construct a family ${\chi_t\colon S^2\to S^2}$ of sphere homeomorphisms with the property that each $\chi_t$ is a factor of ${\widehat{f}}_t$, by a semi-conjugacy for which all fibers except one contain at most three points, and for which the exceptional fiber carries no topological entropy: that is, unimodal natural extensions are virtually sphere homeomorphisms. In the case where ${f_t}$ is the tent family, we show that $\chi_t$ is a generalized pseudo-Anosov map for the dense set of parameters for which $f_t$ is post-critically finite, so that ${\chi{t}}$ is the completion of the unimodal generalized pseudo-Anosov family introduced in [21].