Ergodic Theory and Dynamical Systems 201213
The seminars are held on Tuesdays at 14:00 in Room B3.02  Mathematics Institute
For the schedule of the minicourses please visit this page
Term 3

April 23, 2013Gunther Cornelissen (University of Utrecht)
Title: Dynamical systems and point countingAbstract: Counting the points of a curve over finite fields doesn't determine the curve up to isomorphism; this is analogous to the famous fact that you cannot "hear the shape of a drum". I will show how to remedy this by counting points in a weighted way (namely, using abelian Lseries). The underlying structure is a dynamical system that models actions from class field theory, and  curiously  arose from a study of a quantum statistical mechanical system. 
April 30, 2013Uwe Grimm (Open University)
Title: Singular Continuous Diffraction of the Squiral TilingAbstract: The ThueMorse system is a paradigm of singular continuous diffraction in one dimension. After a short introduction to mathematical diffraction theory, I shall briefly discuss the ThueMorse system and some onedimensional generalisations. As my main example, I am then going to consider in detail a planar system constructed by a bijective block substitution rule, which is equivalent to the inflation rule of the squiral tiling. For balanced weights, the corresponding diffraction measure is purely singular continuous, whereas the dynamical spectrum also contains a pure point component. The diffraction measure is a twodimensional Riesz product that can be calculated explicitly. 
May 7, 2013Jaap Eldering (Imperial College)
Title: Normally hyperbolic invariant manifolds: noncompactness and geometryAbstract: As generalization of hyperbolic fixed points, normally hyperbolic invariant manifolds (NHIMs for short) are an important tool to globally study perturbations of dynamical systems. In this talk, I will first briefly review the theory of NHIMs and their use in applications. Then I will formulate the theorem on persistence of NHIMs and show how my result generalizes the classical compact to a noncompact differential geometric setting. Noncompactness requires us to introduce the concept of Riemannian manifolds of bounded geometry. These can be viewed as the class of uniformly C^k manifolds; I will illustrate this concept with images. Finally, I will describe why bounded geometry is needed and try to say a few words about how it enters the proof of persistence of NHIMs. 
May 14, 2013Markus Fraczek (University of Warwick)Title: Spectra of transfer operators and zeta functions
Abstract: The transfer operator method is one of the very few methods enabling the evaluation of certain zeta functions, such as the Selberg zeta function and the Ruelle zeta function for the KacBaker model. To evaluate these zeta functions we have to determine the spectra of the corresponding transfer operators. Since there is no great hope of an analytical approach to the spectrum of the transfer operator, we study its spectrum mainly using numerical methods. We will construct nuclear representations for certain transfer operator and approximate them by finite matrices, which can be used for numerical computations. Numerical results obtained by implementing this method will be also presented. 
May 21, 2013Charles Walkden (University of Manchester)Title: Instances of stochastic stability for generalised IFSs

May 28, 2013Bram Mesland (University of Warwick)Title: Limit sets, PattersonSullivan measure and noncommutative geometry
Abstract: The limit set of a Kleinian group is the locus for complicated dynamics of this group. Canonically associated to the action of the group on hyperbolic nspace is its limit set and the PattersonSullivan measure. The properties of this measure at infinity reflect the geometry of the action on hyperbolic nspace. To study the ergodic action on the limit set, we replace the (illdefined) quotient space by a noncommutative C*algebra, the crossed product. We show how the underlying hyperbolic geometry can be used to define interesting dynamics on this C*algebra. This will result in a noncommutive metric structure, or spectral triple, for the crossed product algebra. Such spectral triples canonically induce functionals on the Ktheory of the C*algebra, which in this case, by a result of ManinMarcolli, is related to the homology of the associated hyperbolic quotient manifold. This is a report on work in progress.

June 18, 2013Peter Hazard (University of Warwick)Title: Renormalisation of Henonlike Maps in Dimension Two
Abstract: I will discuss recent developments in the renormalization theory for Henonlike maps in two dimensions by Lyubich, Martens, de Carvalho and myself. Even in the strongly dissipative case there are surprising differences with the unimodal picture. For example, universal phenomena observed in the onedimensional unimodal setting for infinitely renormalisable maps persists in the Henonlike setting, but the attractors are no longer rigid. I will discuss this and some other results and, time permitting, mention a few conjectures suggested by numerical experiments.
Term 2
 January 15, 2013
Mike Todd (University of St Andrews)
Title: Dynamical systems with holes: escape rates, equilibrium states and dimension
Abstract: Given a smooth interval map f:I > I with one or more critical points, we consider
the dynamical behaviour of the system when a hole is punctured in the system, allowing
mass to escape. Work by Bruin, Demers and Melbourne studied the escape of Lebesgue
measure, obtaining a conditionally invariant measure absolutely continuous w.r.t. Lebesgue,
as well as a related measure on the survivor set (the set of points which never escape).
This was related to the study of equilibrium states. In this talk, motivated by Bowen's
formula for the dimension of dynamically defined sets, I'll consider escape w.r.t. other
natural measures and thus give an expression for the Hausdorff dimension of the survivor set.
This is joint work with M. Demers.
 January 22, 2013
Sebastian van Strien (Imperial College London)
Title: Stochastic stability
 January 29, 2013
Oleg Kozlovski (University of Warwick)
Title: HilbertArnold problem for 1D maps
Abstract: In this talk we will discuss if maps in a "typical" family of maps of an interval or
a circle have bounded number of attracting trajectories. The answer depends on what
"typical" means, and on the smoothness of the maps.
 February 5, 2013
Jon Aaronson (Tel Aviv University)
Title: On multiple recurrence and other properties of "nice" infinite measure preserving transformations
 February 12, 2013
Sofia Trejo (University of Warwick)
Title: How to construct complex box mappings
Abstract: I will talk about the construction of complex box mappings for real analytic
interval maps. More specifically, I will show that given a real analytic map and a point
in its postcritical set it is possible to construct complex box mappings, associated to
the real first return maps, with complex bounds for arbitrarily small scales.In the case the map is a nonrenormalizable polynomial (not necessarily real) with only
hyperbolic periodic points, the complexbox mapping can be constructed using the
Yoccoz puzzle. For real analytic maps we can not guarantee the existence of a Yoccoz
puzzle. For this reason the construction of the box mapping and the proof of complex
bounds in this case requires more work.Complex bounds are fundamental for the proofs of quasiconformal rigidity, renormalization
results and ergodic properties.  February 19, 2013
Shirali Kadyrov (University of Bristol)
Title: Semicontinuity of entropy in homogeneous spaces, escape of mass and Hausdorff dimension.
Abstract: We discuss, under the action of a diagonal element, the relation between semicontinuity
of entropy and escape of mass in noncompact homogeneous spaces of rank one. In a noncompact
case, a limit of a sequence of probability invariant measures may be the zero measure. However,
if the entropies of the measures in this sequence are uniformly greater than the half of the maximal
entropy then we show that the limit measure cannot be the zero measure. Moreover, we provide
bounds on the escaping mass and, as an application, we show that the Hausdorff dimension of the
set of orbits under diagonal action which miss a fixed open set is not full. This is a joint work with
M. Einsiedler and A. Pohl.
 February 26, 2013
Sergey Dobrokhotov (Moscow Institute of Physics and Technology)
Title: Librations, normal forms and tunneling in quantum double well with magnetic field
Abstract: A problem for a Schrödingerparticle placed into a symmetric onedimensional doublewell
potential is considered in many textbooks on Quantum mechanics. It is well known that the level
splitting in some cases can be computed semiclassically taken into account the presence of the
small parameter h. A multidimensional analogue of the problem has been intensively studied by
V. Maslov, A. Poljakov, E.M. Harrel, B. Helfer, J. Sjostrand, B. Simon etc. for lowest energy levels.
The main result of this study is socalled splitting formula for energy E = Aexp(−Jh) with the phase
J based on a certain classical trajectories known as instanton. The constructive but complicated
formula of the amplitude A also connected with the instanton was given in the papers by
S. Yu. Dobrokhotov, V. Kolokoltsov and V. Maslov. In this talk we show that the final splitting
formula takes more natural and simple form if one changes the instanton by socalled libration
(unstable closed trajectories) and use the normal forms coming from classical mechanics
(J. Bruning, S. Yu. Dobrokhotov, E. S. Semenov, Unstable closed trajectories, librations and
splitting of the lowest eigenvalues in quantum double well problem, Regular and Chaotic
Dynamics, 11, 2, (2006), 167180). Also we discuss the following (non trivial) question: what happens
with the splitting in the presence of magnetic field and consider the application to a problem about
propagation of wave packets in nanowires.
This work was done together with J.Brüning and R.V.Nekrasov.
 March 5, 2013
Dalia Terhesiu (University of Roma Tor vergata)
Title: Mixing for some nonuniformly hyperbolic systems with infinite measure
Abstract: Abstract: In work in progress with Carlangelo Liverani, we obtain mixing and
mixing rates for the class of nonuniformly hyperbolic systems with a countable Markov
partition, preserving an infinite measure. Our method consists of a combination of the
framework of operator renewal theory (as introduced in the context of dynamical systems
by Sarig) with the framework of function spaces of distributions (developed in the recent
years along the lines of Blank, Keller and Liverani). In this sense, an important part of our
work is devoted to establishing the spectral gap for the induced map of the original system.
Although, at present we only have results for the Markov case, we are confident that the
method can be generalised to other important classes of transformations. In this talk, I will
present the main results and describe the main steps of the construction.
 March 12, 2013
Anthony Dooley (University of Bath)
Title: CPE actions of amenable groups
Abstract: It is well known that Bernoulli actions (of $\mathbb Z$) of the same entropy are
metrically isomorphic, but if one abandons the hypothesis that the actions are Bernoulli,
Ornstein showed that one can have uncountably many nonisomorphic actions, each with
completely positive entropy, of any given entropy. I shall discuss work with Golodets,
where we study CPE actions of amenable groups; there is a version of Ornstein's result
for discrete amenable groups, and we have recently studied CPE actions of nilpotent Lie
groups in terms of actions of their lattice subgroups. This is also related to Avni's recent
work on the Thouvenot conjecture.
Term 1
 October 9, 2012
Sinisa Slijepcevic (University of Zagreb)
Title: Universal properties of formally gradient systems
Abstract: (Joint work with Th. Gallay, Grenoble) Many important dissipative PDE's have a
gradient structure, which is however on large domains only formal (e.g. the energy fun
ctional diverges). We identify some universal properties of that gradient structure, which
apply to several examples (E.g. the reactiondiffusion equation, the complex Ginzburg
Landau equation, the Navier Stokes equation). By using that, we describe the invariant
measures, relaxation times, etc., and show that in spatial dimensions d=1,2, such systems
behave like gradient ones in a statistical sense, and find counterexamples in d>=3. As an
example, we study implications to the unforced Navier Stokes equation on a cylinder.  October 16, 2012
Richard Sharp (University of Warwick)
Title: Noncommutative geometry and measures on limit sets
Abstract: Noncommutative geometry aims describe a wide range of mathematical objects
in terms of C*algebras. We will discuss how this applies to measures on limit sets, including
PattersonSullivan measures on certain Kleinian groups.  October 23, 2012
Andrew Barwell (University of Bristol)
Title: On Shadowing and Limit Sets in Quadratic Maps
Abstract: Shadowing is an important property in dynamical systems, as it tells us that a
system is stable under small perturbations of its orbits. Related to shadowing is the
notion of an internally chain transitive set: a set in which every pair of points can be
linked either by the orbit of a point, or by a perturbed orbit; a seemingly unrelated
concept is that of an omegalimitset  the attracting set of the orbit of a point in
the space.Using symbolic dynamics, we will show that a quadratic map, whose Julia sets is a
dendrite, has shadowing. Furthermore, for these maps the omegalimit sets are
precisely the internally chain transitive sets. It turns out that this is not a coincidence.
 October 30, 2012
Lara El Sabbagh (University of Warwick)
Title: A Lambdalemma for Normally Hyperbolic Manifolds and an Application to Diffusion
Abstract: Roughly speaking, a lambdalemma for normally hyperbolic manifolds asserts that, given a
smooth manifold M and a diffeomorphism f: M > M and a normally hyperbolic submanifold N of M,
if Gamma is a submanifold that transversely intersects the stable manifold of N, then, under iteration
by f, Gamma ''approaches" the foliation of the unstable manifold of N in a suitable topology.
We will present a lambdalemma for normally hyperbolic manifolds, and deduce a result on diffusion
from it: we prove an existence result on drifting orbits along a transition chain of invariant dynamically
minimal sets contained in a normally hyperbolic manifold. This is without any assumption on the
geometrical nature of the invariant sets (in particular, they do not need to be submanifolds).
As a particular case, we recover the wellknown example of Arnold.  November 6, 2012
Kelly Yancey (University of Illinois at UrbanaChampaign)
Title: Generic Homeomorphisms
Abstract: We will start by discussing the history of generic results in the setting of measurepreserving
automorphisms in ergodic theory. Then we will specifically discuss weakly mixing homeomorphisms that
are uniformly rigid and give generic type results for this class of homeomorphisms defined on the two
torus and the Klein bottle.
 November 13, 2012
Vadim Kaimanovich (University of Ottawa)
Title: Continuity of the asymptotic entropy
Abstract: The talk is based on a joint work with Anna Erschler. We prove continuity of the asymptotic
entropy for a large class of random walks on groups with hyperbolic properties.
 November 20, 2012
Zeng Lian (Loughborough University)
Title: Lyapunov exponents and Multiplicative Ergodic Theorem for random systems in a separable Banach space.
Abstract: Lyapunov exponents play an important role in the study of the behavior of dynamical
systems, which measure the average rate of separation of orbits starting from nearby initial points.
They are used to describe the local stability of orbits and chaotic behavior of systems.
Multiplicative Ergodic Theorem provides the theoretical fundation of Lyapunov exponents, which
gives the fundamental information of Lyapunov Exponents and their associates invariant subspaces.
In this talk, I will report the work on Multiplicative Ergodic Theorem (with Kening Lu), which is
applicable to infinite dimensional random dynamical systems in a separable Banach space.
The system could be generated by, for example, random partial differential equations.
 November 27, 2012
Philip Boyland (University of Florida)
Title: Rotation sets for beta shifts and torus homeomorphisms
Abstract:  December 4, 2012 (Cancelled)
Andrzej Bis (University of Warwick)