Dynamics Archives for Fall 2018 to Spring 2019

Infinite mixing for one-dimensional maps with an indifferent fixed point

When: Thu, August 31, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Marco Lenci (Universita' di Bologna) - http://www.dm.unibo.it/~lenci/
Abstract: In the first part of the talk, I will give some background on the
question of mixing for dynamical systems preserving an infinite
measure (a.k.a. 'infinite mixing'). Then I will recall and discuss the
definitions of 'infinite-volume mixing' that I have introduced in
recent years, with a survey on some examples of dynamical systems
which verify or do not verify such definitions. Among these examples
there will be one-dimensional intermittent maps, the subject of recent
work with C. Bonanno and P. Giulietti.
In the second part of the talk, I will better state the results for
the intermittent maps: they comprise a class of expanding maps of
[0,1] with a 'strongly neutral' fixed point in 0 and a class of
expanding maps of the real line with strongly neutral fixed point at
infinity. I will give a sketch of how some of the definitions of
infinite-volume mixing are proved or disproved. Finally I will show
how one property, called global-local mixing, entails certain limit
theorems for our intermittent maps.

Ground states for Frenkel-Kontorova models on quasicrystals

When: Thu, September 7, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Rodrigo Trevino (UMD) - http://trevino.cat
Abstract: The Frenkel-Kontorova model was first proposed in the 1930's to describe the structure and dynamics of a crystal lattice in the vicinity of a dislocation core, and by now has found many uses outside of solid state physics. Viewed from a dynamical systems point of view, it exhibits a lot of rich behavior tied to all sorts of great theories (e.g. KAM theory and Aubry-Mather theory) and fundamental open questions (e.g. Lyapunov exponents for the standard map).

I will talk about this model in the setting where the crystal is aperiodic. In this setting, most of the dynamics are no longer available, but some tools developed to study the (periodic) classical model are still useful. I will talk about how one of them in particular, the so-called anti-integrable limit, is useful to find ground states (also known as equilibrium configurations). No background on the model will be assumed.

Mixing and local limit theorem for some hyperbolic flows

When: Thu, September 28, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Peter Nandori (UMD) - http://math.umd.edu/~pnandori/
Abstract: We consider a special flow over a mixing map with some hyperbolicity.
In case the roof function is square integrable, we find a set of conditions, under which the flow is mixing and also satisfies the local limit theorem. In case the roof function is non-integrable, we identify another set of conditions that imply Krickeberg mixing. The most important condition is the local limit theorem for the underlying map. We check that the conditions are satisfied for some examples, such as Axiom A flows, Sinai billiards, geometric Lorenz attractors (finite measure case) and suspensions over Pomeau-Manneville maps (finite and infinite measure cases). The talk is based on joint work with Dmitry Dolgopyat.

Fourier decay and spectral gaps on hyperbolic surfaces - UNUSUAL TIME

When: Tue, October 17, 2017 - 2:00pm
Where: Kirwan Hall 1308
Speaker: Semyon Dyatlov (MIT) - http://math.mit.edu/~dyatlov/
Abstract: Let $M$ be a nonelementary convex co-compact hyperbolic surface. It is well-known that the Selberg zeta function $Z_M(s)$ has a zero at $s=\delta$ and no zeroes to the right of it, where $0 \delta-\varepsilon\}$. An application is an asymptotic counting formula for lengths of closed geodesics with remainder of relative size $O(e^{-\varepsilon t})$.

The key ingredient of the proof is a Fourier decay bound for the Patterson-Sullivan measure on the limit set. This bound relies on the nonlinearity of the transformations generating the corresponding group as well as bounds on exponential sums which follow from the discretized sum-product theorem. The Fourier decay bound implies a fractal uncertainty principle for the limit set, which in turn gives the gap. This talk will include an introduction to transfer operators on Schottky groups, which are used throughout the proofs.

This talk is based on joint works with Jean Bourgain and Maciej Zworski.

Can you hear the shape of a drum and deformational spectral rigidity of convex axis-symmetric planar domains.

When: Thu, October 19, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Vadim Kaloshin (UMD) - https://www.math.umd.edu/~vkaloshi/
Abstract: M. Kac popularized the question 'Can you hear the shape of a drum?'. Mathematically, consider a bounded planar domain $\Omega$ and
the associated Dirichlet problem
\Delta u+\lambda^2 u=0, u|_{\partial \Omega}=0.
The set of $\lambda$’s such that this equation has a solution, denoted
$\mathcal L(\Omega)$ is called the Laplace spectrum of $\Omega$.
Does Laplace spectrum determines $\Omega$? In general, the answer is negative.

Consider the billiard problem inside $\Omega$. Call the length spectrum the closure
of the set of perimeters of all periodic orbits of the billiard. Due to deep properties of
the wave trace function, generically, the Laplace spectrum determines the length
spectrum. In the space of strictly convex axis-symmetric domains we shall discuss
Sarnak's question whether one can deform such a domain without changing
its spectrum. This is based joint works with J. De Simoi, A. Figalli and Q. Wei.


When: Thu, October 26, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Alex Blumenthal (UMD) - http://math.umd.edu/~alexb123/

Statistical properties of the Standard map with increasing coefficient

When: Thu, November 2, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Alex Blumenthal (UMD) - http://math.umd.edu/~alexb123/
Abstract: The Chirikov standard map $F_L$ is a prototypical example of a one-parameter family of volume-preserving maps for which one anticipates chaotic behavior on a non-negligible (positive-volume) subset of phase space for a large set of parameters. Analysis in this direction is notoriously difficult, and it remains an open question whether this chaotic region, the stochastic sea, has positive Lebesgue measure for any value of L.

I will discuss two related results on a more tractable version of this problem. The first is a kind of ‘finite-time mixing estimate, indicating that for large L and on a suitable timescale, the map $F_L$ is strongly mixing. The second pertains to statistical properties of compositions of standard maps with increasing parameter L: when the parameter L increases at a sufficiently fast polynomial rate, we obtain asymptotic decay of correlations estimates, a Strong Law, and a CLT, all for Holder observables.

On Siegel's question and density of collisions in the Restricted Planar Circular Three Body Problem - UNUSAL DATE

When: Tue, November 7, 2017 - 2:00pm
Where: Kirwan Hall 1308
Speaker: Jianlu Zhang (UMD) -
Abstract: For the Restricted Circular Planar 3-Body Problem, we show that there exists a full dimensional open set $U$ in phase space independent of the mass ratio $\mu$, where the set of initial points which lead to collision is $O(\mu ^{1/20} )$ dense as $\mu \rightarrow 0$.

Homoclinic points for geodesic flows and billiards

When: Thu, November 9, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Zhihong Jeff Xia (Northwestern) - http://www.math.northwestern.edu/~xia/
Abstract: It is believed and conjectured that, for a typical Hamiltonian system, every hyperbolic periodic point has a homoclinic point. This is indeed the case in many situations. However, for geodesic flow and billiards, the usual perturbation techniques are no longer available, since there is no local perturbation. We will show that for geodesic flows on two-sphere and for convex billiards, it is still true. The proof uses prime ends and relies more on global analysis of stable and unstable manifolds, rather than perturbation techniques.

Asymptotic behavior of Markov operator corresponding to non-expanding piecewise linear maps

When: Thu, November 16, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Fumihiko Nakamura (Hokkaido University ) -
Abstract: The non-expanding piecewise linear map, known as the Nagumo-Sato (NS)
model, is described as $S_{\alpha,\beta}(x)=\alpha x+\beta ({\rm
mod} 1)$, where $0

Poisson boundary of random walks on free semigroups

When: Thu, November 30, 2017 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Behrang Forghani (University of Connecticut) - https://sites.google.com/site/behrangforghani/
Abstract: In early 60, Furstenberg employed the theory of Poisson boundary of random walks on groups to obtain several fundamental rigidity results for lattices in Lie groups. One of the main questions in the theory of random walks on groups is how to describe the Poisson boundary of a concrete random walk on a concrete group. In particular, for an arbitrary random walk on a finitely generated free group, it is conjectured that the space of infinite irreducible words equipped with the hitting measure is the Poisson boundary.

The conjecture has been solved by Dynkin-Maljutov for a first neighborhood random walk, by Derriennic for a finitely supported random walk, and by Kaimanovich for a random walk whose both entropy and logarithmic moment are finite.

Although the study of random walks on free semigroups is less arduous than the one of free groups, the conjecture remains unsolved for arbitrary random walks on free semigroups. In this talk, I will show the conjecture holds whenever the random walk on a free semigroup has finite entropy or finite logarithmic moment or finite w-logarithmic moment for some finite words. This talk is based on a joint work with Giulio Tiozzo from the University of Toronto.

Disjointness criterion for parabolic flows

When: Thu, January 25, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Adam Kanigowski (Penn State) -
Abstract: We will state a general disjointness criterion for two ergodic systems. We will then show how this criterion can be used to study disjointness in the class of parabolic systems such as unipotent flows and their time changes, nil-flows and their time-changes, smooth surface flows.

Invariant Random Subgroups of Full Groups of Bratteli diagrams

When: Thu, February 15, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Kostya Medynets (Naval Academy) - https://www.usna.edu/Users/math/medynets/index.php
Abstract: In the talk, we will classify the ergodic invariant random subgroups (IRS) of simple AF full groups. AF full groups arise as the transformation groups of Bratteli diagrams that preserve the cofinality of infinite paths in the diagram. AF full groups are complete (algebraic) invariants for the isomorphism of Bratteli diagrams. Given a simple AF full group G, we will prove that every ergodic IRS of G arises as the stabilizer distribution of a diagonal action on X^n for some n, where X is the path-space of the Bratteli diagram associated to G. This is joint work with Artem Dudko.

Stochastic properties of the Z^2-periodic Sinai billiard

When: Thu, February 22, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: (Francoise Pene) - http://lmba.math.univ-brest.fr/perso/francoise.pene/
Abstract: We study stochastic properties of the Z^2-periodic Sinai billiard (recurrence, ergodicity, mixing, decorrelation, limit theorems).

Families of periodic paths on the pentagon

When: Thu, March 1, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Diana Davis (Swarthmore College) - http://www.swarthmore.edu/NatSci/ddavis3/
Abstract: There are infinite "families" of periodic paths on the pentagon, whose members all have a similar appearance but get more and more complicated. I'll show some beautiful examples of these families, and explain how we use the group structure on the set of periodic directions to obtain them.

Automorphisms of the shift: entropy and the dimension representation

When: Thu, March 8, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Scott Schmieding (Northwestern) - https://www.scholars.northwestern.edu/en/persons/scott-edward-schmieding
Abstract: For a shift of finite type $(X,\sigma)$, the automorphism group $Aut(\sigma)$ consists of all homeomorphisms from $X$ to $X$ which commute with the shift map $\sigma$. The group $Aut(\sigma)$ is known to contain a rich structure, and has been heavily studied over the years. To analyze a particular automorphism, one may consider its action on the dimension group, an ordered abelian group associated to the system $(X,\sigma)$. We will describe what this dimension group is, and discuss relationships between various dynamical properties of an automorphism and its action on the associated dimension group. We'll focus on connections between the topological entropy of an automorphism and spectral data coming from its action on the dimension group, and how this relates to an entropy conjecture for shifts of finite type in the spirit of Shub's classical entropy conjecture.

Mather theory and symplectic rigidity.

When: Thu, March 15, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Mads Bisgaard (ETH Zurich, Switzerland) - https://sites.google.com/view/madsbisgaard
Abstract: Aubry-Mather theory studies the flow associated to a convex Hamiltonian H on a cotangent bundle, by finding so-called Mather measures. Symplectic topology studies properties imposed on the flow by the geometry of the cotangent bundle. It is interesting to understand how these theories can play together. In this talk I will show how different techniques from symplectic topology give rise to incarnations of Mather’s $\alpha$-function and that its relation to invariant measures continues to hold: Mather measures exist. I will discuss applications to Hamiltonian system on closed symplectic manifolds, $\mathbb R^{2n}$ and twisted cotangent bundles.

Metric Approach to McGehee blow-up and Marchall's lemma.

When: Thu, March 29, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Richard Montgomery (UC Santa Cruz) - https://people.ucsc.edu/~rmont/
Abstract: The N-body problem can be rephrased as theproblem of finding geodesics for a certain one-parameter family of metrics,the Jacobi-Maupertuis [JM] metrics. Marchal's lemma is the basic tool for eliminating collisionswhen using variational methods to establish existence of various "designer" orbits in the N-body problem. We rephrase Marchal's lemma as a lemma in metric geometry. The essential idea can be seen in the standard planar Kepler problem where we show that he zero-energy metric is isometric to the cone of radiius one-half which is flat everywhere except at the cone point which corresponds to collision. Inextendibility of geodesics through the cone point corresponds to the conclusion of Marchal's lemma: action minimizers cannot have collision points in their interior. This work has significant overlap with recent work of Barutello, Terracini, and Verzini. Time permitting I will also discuss a metric perspective on McGehee blow-up.

Variational construction of periodic and connecting orbits in the planar Sitnikov problem

When: Thu, April 12, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Mitsuru Shibayama (Kyoto University, Japan) - http://yang.amp.i.kyoto-u.ac.jp/~shibayama/top_e.html
Abstract: We consider the existence of solution in the planar Sitnikov problem, realizing given symbolic sequences by based on variational method. We also prove the existence of various periodic solutions and connecting orbits between them.

Thermodynamic formalism for some models with countable Markov partitions

When: Thu, April 19, 2018 - 2:05pm
Where: Kirwan Hall 1311
Speaker: Michael Jakobson (UMD) - http://www.math.umd.edu/~jakobson/
Abstract: We review several results about ergodic and statistical properties of hyperbolic attractors and present new results obtained in a joint work with Lucia Simonelli (ICTP, Trieste).

Recent results in diffraction theory

When: Thu, April 26, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: E. Arthur Robinson (George Washington University) - https://blogs.gwu.edu/robinson/
Abstract: A quasicrystal is, by one definition, a solid with non-classical diffraction pattern, e.g, 5-fold rotational symmetry. The first quasicrystal was discovered by D. Schectman at NIST in around 1984, for which he ultimately won the 2011 Chemistry Nobel prize. At the time, several physicists suggested the vertex set of a Penrose tiling (or its 3-dimensional analogue) as a model for the placement of atoms in a quasicrystal. The diffraction theory of vertex sets of Penrose-like tilings is closely tied to the dynamical spectrum (especially the point spectrum) of a corresponding type of dynamical system. In this talk, we will start with an overview of this type of dynamical system and the corresponding diffraction theory. Then we will describe some recent new results in this area, most of which concern the spectral and mixing properties of different types of substitution dynamical systems.

Transport problems in mechanical systems

When: Thu, May 3, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Alex Grigo (U of Oklahoma) -
Abstract: In this talk I will present some basic examples of transport phenomena
in classical physics and provide an overview of certain mechanical
models in which one can rigorously investigate these properties.
In particular I will focus on illustrating how transport phenomena
are related to results in the theory of hyperbolic dynamical systems
such as central limit theorems, linear response theory, and averaging theory. Then I will describe in detail some of my recent works in this area, which are in parts joint work with Leonid Bunimovich.

Hyperbolic periodic orbits, heteroclinic connections, and numerical characterization of rare events in some nongradient systems

When: Thu, May 10, 2018 - 2:00pm
Where: Kirwan Hall 1311
Speaker: Molei Tao (Georgia Tech) - http://people.math.gatech.edu/~mtao8/index.html
Abstract: We consider dynamical system perturbed by small Gaussian noises, with a goal of quantifying how noises can affect the dynamics. More precisely, most likely noise-induced metastable transitions are understood by maximizing transition rate provided by Freidlin-Wentzell large deviation theory. Such transitions in gradient systems were understood and known to cross separatrices at saddle points. We instead investigate nongradient systems (possibly irreversible) using a developed tool of (generalized) orthogonal decomposition. Two examples will be described: (1) A different type of transitions, which cross hyperbolic periodic orbits, will be discussed. Corresponding numerical tools for both identifying such periodic orbits and computing transition paths will be developed. (2) The Langevin model of stochastic mechanical systems will be investigated and extended, with emphasis on how its metastable transition can differ from the overdamped (reversible) case. In addition, numerical approaches for general nongradient systems will be presented. If time permits, I will also mention how these results can help design control strategies.