A l’occasion de la journée Systèmes dynamiques, probabilité, statistiques à Quimper, j’ai présenté la notion de facteur de Bowen et son utilité pour l’étude des difféomorphismes de surfaces. On peut ainsi rendre injective les extensions finies construites par Omri Sarig, en ne perdant qu’une partie faiblement errante (union dénombrable de parties errantes).

## Posts Tagged ‘surface dynamics’

## “Facteurs de Bowen” à Quimper

Posted in Dynamics, talks, tagged smooth ergodic theory, surface dynamics, symbolic dynamics on December 17, 2017| Leave a Comment »

## Dilation factors of pseudo-Anosov homeomorphisms

Posted in Dynamics, talks, tagged Dynamics, flat surfaces, moduli space, pseudo-Anosov, smooth dynamics, surface dynamics, topology on January 14, 2010| Leave a Comment »

Erwan Lanneau gave a talk in Orsay about his work on surfaces of translation. Pseudo-Anosov homeomorphisms are homeomorphisms of such surfaces which are affine away from the singular set of the surface and whose differential is hyperbolic. The dilation factor is the dominant eigenvalue of that differential. It is a Perron number.

The minimum of the dilation factor for given genus is known for geni 1 and 2 only (the techniques behind could be extended to genus 3 but not farther). One also knows that .

The main result of the talk is that the above does not hold when restricted to a given type of translation surfaces. More precisely, the moduli space of translation surfaces of given genus splits into connected components (at most three), one of them corresponding to hyperellipticity and the following holds:

**Theorem (Boissy-Lanneau)** *Let be a pseudo-Anosov on a hyperelliptic translation surface of genus g admitting an involution with fixed points). Assume that has a unique singularity. Then its dilation is strictly greater than (but approach this value as ).*

## Discontinuity of the topological entropy for Lozi maps

Posted in Dynamics, news, papers, tagged continuity of the entropy, Dynamics, entropy, examples, hyperbolicity, Lozi maps, piecewise affine dynamics, surface dynamics on December 10, 2009| Leave a Comment »

I have shown that, like diffeomorphisms, piecewise affine surface homeomorphisms are approximated in entropy by horseshoes, away from their singularities. It follows in particular that their topological entropy is lower-semicontinuous: a small perturbation cannot cause a macroscopic drop in entropy.

The continuity of the entropy for such maps had been an open problem for some time. Rigorous numerical estimates by Duncan SANDS and Yutaka ISHII seemed to suggest some discontinuous drops, but investigation at a small scale suggested these drops to be steep yet continuous variations.

Izzet B. YILDIZ has solved this question by finding for Lozi maps on , small numbers such that, setting , for all :

- ;
- .

The verification turns out to be quite simple (once you know where to look!). The non-wandering set of is shown to be reduced to be reduced to the fixed points of its fourth iterates, yielding the zero entropy immediately. on the other hand is shown to admit 2 disjoint closed quadrilaterals such that hyperbolically crosses both and and hyperbolically crosses . This means that the sides of and can be branded alternatively *s* and *u* with the following property. The image of a *u* side crosses each of it meets, intersecting both their *s* sides and none of their *u* sides. This again yields the entropy estimate.