# 2018-19

#### The seminars are held on Tuesdays at 14:00 in Room B3.02 - Mathematics Institute

(unless stated otherwise)

##### Organizers: Polina Vytnova and Selim Ghazouani

### Term 1

- 2 October

**Richard Sharp**(Warwick)

Title:*Periodic orbit growth on covers of Anosov flows*

Abstract: Suppose we have the lift of an Anosov flow to a regular cover. By considering periodic orbits intersecting some bounded region in the cover, one may define a Gurevic entropy which is less that or equal to the topological entropy of the Anosov flow. In the special case of a geodesic flow over a compact manifold with negative sectional curvatures, we have equality if and only if the cover is amenable. This result fails for other Anosov flows but we will discuss a natural generalisation. This is joint work with Rhiannon Dougall. - 9 October

**Michele Triestino**(Dijon)

Title:*Smoothening singular group actions on manifolds*

Abstract: Motivated by the recent results around Zimmer’s program, we study group actions on manifolds, with singular regularity (we require that every element is differentiable at all but countably many points). The groups under considerations have a fixed point property, named*FW*, which generalizes Kazhdan’s property (*T*) (in particular we can consider actions of lattices in higher rank simple Lie groups).

The main result is that if a group*G*has property*FW*, any singular action of*G*on a closed manifold

1) either has a finite orbit,

2) or is conjugate to a differentiable action, up to changing the differentiable structure of the manifold.

This is a joint work with Yash Lodha and Nicolas Matte Bon. - 16 October

**Etienne Le Masson**(Universite de Cergy-Pontoise)

Title:*Quantum ergodicity and random waves in the Benjamini-Schramm limit*

Abstract: One of the fundamental problems in quantum chaos is to understand how high-frequency waves behave in chaotic environments. A famous but vague conjecture of Michael Berry predicts that they should look on small scales like Gaussian random fields. We will show how the notion of Benjamini-Schramm convergence of manifolds (originally defined for graphs) can be used to formulate Berry’s conjecture precisely. The Benjamini-Schramm convergence includes the high-frequency limit as a special case but provides a much more general framework that will lead us to consider a case where the frequencies stay bounded and the size of the manifold increases instead. In this alternative setting we will explain how the ergodicity of the Gaussian wave and the mixing of the geodesic flow can be used to prove weaker forms or consequences of the random wave conjecture.

Joint work with Miklos Abert and Nicolas Bergeron. - 17 October (2:00pm, B3.02)

**Hillel Furstenberg**(Jerusalem)

Title:*Affine group actions*

Abstract: When*X*is a compact, convex space, and a group*G*acts on*X*preserving the affine structure, we speak of an "affine action", or, representation. In analogy with linear representation theory, one would like to describe all minimal — or irreducible — affine actions. We develop the theory for Lie groups and focus on*PSL(2,R)*, noting that every bounded harmonic function in the unit disc leads to an irreducible affine representation. It turns out surprisingly that up to equivalence, this group has a unique irreducible affine representation. - 23 October
**Stephen Cantrell**(Warwick)

Title:*Comparing word length with displacement for actions on CAT(-1) spaces*

Abstract: Suppose a hyperbolic group acts sufficiently nicely on a complete CAT(-1) geodesic metric space . Fix an origin for . Each group element in displaces this origin by a distance comparable to the word length of . In this talk we discuss various ways in which we can make an averaged comparison between the word length and displacement associated to the action of on . We will also discuss other geometrically interesting real valued functions on hyperbolic groups, for which these comparison results apply. - 30 October
**Dmitry Turaev**(Imperial)

Title:*On wandering domains near a homoclinic tangency*

Abstract: Given a map, we define a wandering domain as an open region such that the diameter of its images by the iterations of the map shrinks to zero but the corresponding limit set is not a periodic point. It is known that many finitely smooth two-dimensional diffeomorphisms have wandering domains while it is not known if a polynomial diffeomorphism of a plane can have one. We discuss wandering domains whose limit sets is a homoclinic tangency. We show the existence of real analytic planar diffeomorphisms with wandering domains and discuss how to find wandering domains for polynomial diffeomorphisms of the three-dimensional space.

**Gabriella Keszthelyi**(Rényi Institute, Budapest) — 3:00pm, B3.02

Title:*Dynamical properties of biparametric skew tent maps*

Abstract: pdf - 6 November
**Federico Rodriguez-Hertz**(Penn State/Lille)

Title:*Classification of Anosov actions and first cohomology*

Abstract: In the late 60’s and early 70’s it was conjectured that Anosov diffeomorphisms have an algebraic origin, indeed that Anosov diffeomorphisms are topologically conjugated to algebraic automorphisms of infranilmanifolds. In this direction, J. Franks and A. Manning showed that if the underlying manifold is an infranilmanifold the Anosov diffeomorphism is topologically conjugated to an algebraic one. When the acting group is higher rank, strong rigidity results are expected and it is conjectured that actions of higher rank lattices with an Anosov element should be on infranilmanifolds and smoothly equivalent to algebraic. In this talk I will discuss some recent advances in this direction, proving results analogous to Franks-Manning in the setting of higher rank abelian actions and higher rank lattices on semisimple Lie groups actions. This is a joint project with A. Brown and Z. Wang. - 13 November
**Jose Alves**(Porto/Loughborough)

Title:*Entropy formula and continuity of entropy for piecewise expanding maps*

Abstract: We consider some classes of piecewise expanding maps in finite dimensional spaces with invariant probability measures which are absolutely continuous with respect to Lebesgue measure. We derive an entropy formula for such measures. Using this entropy formula, we present sufficient conditions for the continuity of that entropy with respect to the parameter in some parametrized families of maps. We apply our results to some families of piecewise expanding maps. Joint work with Antonio Pumariño. - 20 November
**Ariel Rapaport**(Cambridge)

Title:*Dimension of planar self-affine sets and measures.*

Abstract: A compact is called self-affine if there exist affine contractions such that . In the 1980s, Kenneth Falconer introduced a value , called the affinity dimension, which is the ''expected'' value for the dimension of . I will discuss a recent project with Mike Hochman, building on our joint work with Balázs Bárány, in which we establish the equality under mild assumptions. - 21 November (2:00pm, MS.05)

**Yotam Smilansky**(Einstein Institute, Jerusalem)

Title:*Multiscale substitution schemes and Kakutani sequences of partitions.*

Abstract: Substitution schemes provide a classical method for constructing tilings of Euclidean space. Allowing multiple scales in the scheme, we introduce a rich family of sequences of tile partitions generated by the substitution rule, which include the sequence of partitions of the unit interval considered by Kakutani as a special case. In this talk we will use new path counting results for directed weighted graphs to show that such sequences of partitions are uniformly distributed, thus extending Kakutani's original result. Furthermore, we will describe certain limiting frequencies associated with sequences of partitions, which relate to the distribution of tiles of a given type and the volume they occupy. - 27 November

**Björn Winckler**(Imperial)

Title:*Instability of renormalization*

Abstract: In this talk I will discuss renormalization of low-dimensional dynamical systems and associated phenomena such as universality and rigidity. The first part of the talk will be an introduction to the classical results in this field. In the second part I will discuss new phenomena that appear in the article 'Instability of Renormalization' (arXiv:1609.04473). In particular, I will introduce the renormalization operator acting on Lorenz maps (these are one-dimensional maps associated with three-dimensional flows undergoing a homoclinic bifurcation). This operator turns out to have non-trivial dynamics inside topological classes of stationary type which in turns leads to the phenomena of coexistence and as dimensional discrepancy. All of these notions will be explained in the talk. - 4 December

**Mike Whittaker**(Glasgow)

Title:*Correspondences for Smale spaces*

Abstract: Correspondences provide a notion of a generalised morphism between Smale spaces. Using correspondences we define an equivalence relation on all Smale spaces which reduces to shift equivalence for shifts of finite type. In this talk I will outline our construction and introduce the key ingredients. This is joint work with Robin Deeley and Brady Killough.

### Term 2

- 8 January
**Matt Galton**(Warwick) - 15 January
**Igors Gorbovickis**(Bremen) - 17 January
**Artem Dudko**(Warsaw) - 22 January
**Anatoly Neishtadt**(Loughborough) - 24 January
**Dmitry Jakobson**(McGill) - 29 January
**Victor Kleptsyn**(Rennes) and**Ale Jan Homburg**(Amsterdam) - 5 February
- 12 February
**Yuri Lima**(UFC) - 19 February
**Leticia Pardo Simon**(Liverpool) and**Tony Samuel**(Birmingham) - 26 February
**Valery Gaiko**(United Institute of Informatics Problems, Minsk) - 5 March
**Jeroen Lamb**(Imperial) - 12 March
**Jens Marklof**(Bristol)