This page is from the Spring 2016 semester. Please visit the current semester's seminar page.

NYC Dynamics Seminar at Yeshiva University is a research level seminar with a broad agenda aimed at research mathematicians and graduate students whose interests include various aspects of the modern theory of dynamical systems and related topics in analysis, geometry, number theory and possibly other subjects. Its aim is to supplement more specialized seminars in the NYC area and provide a meeting place and a venue for discussions for mathematicians associated with various universities and colleges in the NYC metropolitan area working or interested in dynamical systems. The seminar will primarily feature speakers from outside the area specially invited for this purpose as well as mathematicians visiting various NYC universities. Core financial support for the seminar is provided by the Center for Mathematical Sciences at Yeshiva University. Support from other institutions who contribute to funding visits of seminar speakers will be acknowledged.

- Marian Gidea (Yeshiva University)
- Pat Hooper (City College and CUNY Graduate Center)
- Anatole Katok (Penn State and Yeshiva University)

**Location:** Yeshiva University, 215 Lexington Ave, Room 506. The entrance to the building is on the southeast corner of 33rd St and Lexington
Ave. Participants will be asked to present some form of identification to the security to be signed in.

**Meeting Times:** Various Wednesdays at 5pm in Spring 2016 and likely in future semesters. The typical lecture time will be 1 hour, followed by a 30 minute
period for extra time for lecture and/or discussions.

February 24
Barak Weiss
Tel Aviv University
Dynamics on closed subsets of \({\mathbb R}^d\) and questions of Danzer and Gowers
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Let \(X\) denote the collection of closed subsets of \({\mathbb R}^d\). Equipped with the Chabauty-Fell topology, this is a compact metric space on which any group of homeomorphisms of \({\mathbb R}^d\) acts. In joint work with Solan and Solomon, we show that the only minimal sets for the action of volume preserving affine maps, are the two fixed points.

As a consequence we solve a question of Gowers in discrete geometry, which is a variant of the following open question of Danzer: does there exist a discrete subset of the plane, with finite upper density, which intersects every convex subset of area one?

The talk will be elementary and self-contained.

March 2
Boris Hasselblatt
Tufts University
Godbillon-Vey classes for isotropic foliations and rigidity
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For a contact manifold \((M^{2m+1},A)\) and an \(m+1\)-dimensional
\(dA\)-isotropic \(C^2\) foliation, we define *Godbillon--Vey invariants*
\(\{\mathit{GV}_i\}_{i=0}^{m+1}\) inspired by the Godbillon--Vey invariant of
a codimension-one foliation, and we demonstrate the potential of this
family as a tool in geometric rigidity theory. One ingredient for the
latter is the Mitsumatsu formula for geodesic flows on (Finsler) surfaces.

March 9
Rafael de la Llave
Georgia Tech
A posteriori KAM theory. Some applications to regularity.
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We present some recent proofs of KAM theory on the 'a posteriori' format. They show that given an approximate solution of an invariance equation, one can find a true solution nearby.

This proofs lead to efficient algorithms, but they also lead to some theoretical results such as the monogenic or Whitney differentiability with respect to the frequency.

(joint work with R. Calleja and A. Celletti)

April 6
Federico Rodriguez Hertz
Penn State
New developments in smooth rigidity of lattice actions
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In joint work with A. Brown and Z. Wang we were able to show some new rigidity results for higher rank lattice actions that in one way or other display some hyperbolicity. The main technical tool is the notion of resonance that will be discussed in the talk. In particular we prove global rigidity results for Anosov actions on nilmanifolds and also for actions on low dimensions.

April 20
Bassam Fayad
CNRS and University of Paris-6
Spectral properties of mixing flows on surfaces
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How 'chaoticâ€™ can smooth area preserving surface flows be? It is known for a long time (from the works of Kolmogorov, Katok and then Kochergin) that starting from very low regularity these flows, if they do not have fixed points, cannot be mixing. Via PoincarÃ© sections, the latter phenomenon is due to a Denjoy type rigidity of discrete time one dimensional dynamics. However, Kochergin and then Khanin and Sinai showed that these flows can be mixing when they have singularities. Nothing however was known about their spectral type. We will explain why Kochergin flows with one (sufficiently strong) power like singularity typically have a maximal spectral type equivalent to Lebesgue measure on the circle. So, these quasi-minimal flows on the two torus, that have almost the same phase portrait as that of a minimal translation flow, share the same maximal spectral type as Anosov flows! In fact, the Lebesgue spectrum is rather reminiscent of the parabolic paradigm (of horocyclic flows for example) to which the Kochergin flows are related due to the shear along their orbits. We will discuss this relation and its consequences as well as several questions around mixing area preserving flows.

May 4
Omri Sarig
Weizmann Institute of Science
Temporal distributional limit theorems for dynamical systems of low complexity (joint work with D. Dolgopyat)
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The orbits of zero entropy uniquely ergodic map do NOT always all have the same qualitative behavior, but to expose the richness of the orbit structure one needs to look at second order asymptotic behavior such as the error term in the ergodic theorem. "Temporal distributional limit theorems" are a probabilistic tool for doing this.

In the first part of the talk I will explain what these theorems are, and will demonstrate them by examples. These include irrational rotations and horocycle flows. In the second part of the talk I will prove some of the results mentioned in the first part.

This is joint work with D. Dolgopyat.