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Commentarii Mathematici Helvetici


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Volume 92, Issue 4, 2017, pp. 777–800
DOI: 10.4171/CMH/424

Published online: 2017-10-24

Generalized $\beta$-transformations and the entropy of unimodal maps

Daniel J. Thompson[1]

(1) The Ohio State University, Columbus, USA

Generalized $\beta$-transformations are the class of piecewise continuous interval maps given by taking the $\beta$-transformation $x \mapsto \beta x$ (mod 1), where $\beta >1$, and replacing some of the branches with branches of constant negative slope. If the orbit of 1 is finite, then the map is Markov, and we call $\beta$ (which must be an algebraic number) a generalized Parry number. We show that the Galois conjugates of such $\beta$ have modulus less than 2, and the modulus is bounded away from 2 apart from the exceptional case of conjugates lying on the real line. We give a characterization of the closure of all these Galois conjugates, and show that this set is path connected. Our approach is based on an analysis of Solomyak for the case of $\beta$-transformations. One motivation for this work is that the entropy of a post-critically finite (PCF) unimodal map is the logarithm of a generalized Parry number. Thus, our results give a mild restriction on the set of entropies that can be attained by PCF unimodal maps.

Keywords: Interval maps, expansions of real numbers, topological entropy

Thompson Daniel: Generalized $\beta$-transformations and the entropy of unimodal maps. Comment. Math. Helv. 92 (2017), 777-800. doi: 10.4171/CMH/424