## Seminar program for Spring 2022

Our seminar continues with a mixture of in person and virtual talks. As usual, we meet on (most) Thursdays at 3.00pm EST unless otherwise noted. In person talks will be in MW154.

For virtual talks, the Zoom link can be obtained from the organizers, Andreas Koutsogianis and Dan Thompson. For most virtual talks, video will be posted afterwards, and will remain viewable on Zoom for 120 days after the talk.

The following is our current schedule; more talks might be announced soon.

Jan 27: Virtual –  Ben Call (OSU)

Feb 10: Virtual – Anthony Quas (University of Victoria, Canada)

Feb 17: Virtual – Richard Sharp (University of Warwick, UK)

Feb 24: Virtual – Ethan Ackelsberg (OSU)

Mar 3: Virtual – John Griesmer (Colorado School of Mines)

Mar 10: Virtual – Anh N. Le (OSU)

Mar 24: In Person – Andreas Koutsogiannis (Aristotle University of Thessaloniki, Greece)

Mar 31: In Person – Dong Chen (PennState)

April 7: Virtual – Konstantinos Tsinas (University of Crete, Greece)

Apr 14: Virtual – Yun Yang (Virginia Tech)

Apr 21: Virtual – Rigo Zelada Cifuentes (University of Maryland)

## Seminar program for Fall 2021

This year, our seminar will be a mixture of in person and virtual seminars, with the mix anticipated to trend towards in person later in the year, and virtual early in the year. As usual, we meet on Thursdays at 3.00pm EST unless otherwise noted. In person talks will be in MW154.

For virtual talks, the Zoom link can be obtained from the organizers, Andreas Koutsogianis and Dan Thompson. For most virtual talks, video will be posted afterwards, and will remain viewable on Zoom for 120 days after the talk.

The following is our current schedule, and more talks will be announced soon.

Aug 17th: In person –  Federico Rodriguez Hertz (Penn State)

Aug 26: Virtual – Aurelia Dymek (Nicolaus Copernicus University, Torun, Poland)

Sept 9: Virtual – Christian Wolf (City College of New York)

Sept 16: Virtual – Andreu Ferre Moragues (Nicolaus Copernicus University, Torun, Poland)

Sept 30: Virtual – Pablo Shmerkin (UBC, Canada)

Oct 8 (Friday, 12.00pm, note unusual day and time): Virtual – Ryokichi Tanaka (Kyoto University, Japan)

Oct 19 (Tues, note unusual day): In Person – Keith Burns (Northwestern)

Oct 21: Virtual – Alejandro Maass (University of Chile, Chile)

Oct 28: Virtual – Wenbo Sun (Virginia Tech)

Nov 18: In Person – Dick Canary (Michigan)

Dec 9: Virtual – Giulio Tiozzo (University of Toronto, Canada)

## Seminar program for Spring 2021

We are pleased to resume our online seminar program. As usual, we meet on Thursdays at 3.00pm EST unless otherwise noted.

The following is our current schedule, and more talks will be announced soon.

Feb 4th: No seminar due to the one-day workshop ‘Hyperbolic Day Online‘ organized by Andrey Gogolev (Ohio State) and Rafael Potrie (Universidad de la Republica)

Feb 11th: Sebastian Donoso (University of Chile)

Feb 18th: Daniel Glasscock (UMass Lowell)

Feb 25th: Florian Richter (Northwestern)

Mar 04th: Claire Merriman (The OSU)

Mar 11th: Dominik Kwietniak (Jagiellonian University in Krakow)

Mar 18th: Donald Robertson (University of Manchester)

Mar 25th: Mariusz Lemańczyk (Nicolaus Copernicus University)

Apr 1st: Break

April 8th: Jonathan DeWitt (The University of Chicago)

Apr 15th: Joel Moreira (University of Warwick)

Apr 22nd: Steve Cantrell (The University of Chicago)

Apr 29th: Dmitry Kleinbock (Brandeis University)

## New Ohio State Online Ergodic Theory Seminar

UPDATE: We will continue our program in Spring 2021. However, we are taking a brief Winter hiatus. We expect to resume in February.

We are pleased to announce that we will be running an online seminar program in Fall 2020. The seminar will take place in our usual time slot unless otherwise noted – Thursdays 3.00pm (EST). Some seminars are scheduled at an alternate time of Friday 12.40pm (EST).

Our current schedule for the semester follows:

Sept 17: Lien-Yung “Nyima” Kao (George Washington University)

Oct 2 (Friday, 1pm EST): Tushar Das (University of Wisconsin)

Oct 9 (Friday, 12.40pm EST): Mark Demers (Fairfield University)

Oct 16 (Friday, 12.40pm EST): Tianyu Wang (Ohio State)

Oct 22: Andrew Best (Ohio State)

Oct 29: Tamara Kucherenko (City College of New York)

Nov 12: Shahriah Mirzadeh (Michigan State)

Nov 19: Yeor Hafuta (Ohio State)

Dec 3: Nikos Frantzikinakis (University of Crete)

Title: Polynomial Ergodic Theorems for Strongly Mixing Commuting Transformations

Speaker:  Rigo Zelada Cifuentes – University of Maryland

Abstract: We present new polynomial ergodic theorems dealing with probability measure preserving $\mathbb Z^L$-actions having at least one strongly mixing element. We prove that, under different conditions, the set of $n\in\mathbb Z$ for which the multi-correlation expressions $$\mu(A_0\cap T_{\vec v_1(n)}A_1\cap \cdots\cap T_{\vec v_L(n)}A_L)$$ are $\epsilon$-independent, must be $\Sigma_m^*$. Here $\vec v_1,…,\vec v_L$ are $\mathbb Z^L$-valued polynomials in one variable and $\Sigma_m^*$, $m\in\N$, is one of a family of notions of largeness intrinsically connected with strongly mixing. We will also present two examples showing the limitations of our results. The existence of these examples suggests further questions dealing with the weakly, mildly, and strongly mixing properties of a multi-correlation sequence along a polynomial path.  This talk is based in joint work with Vitaly Bergelson.

Meeting ID: 938 8598 9739

Recorded Talk:

## Seminar 04.14.22 Yang

Title: Entropy rigidity for 3D Anosov flows

Speaker:  Yun Yang – Virginia Tech

Abstract: Anosov systems are among the most well-understood dynamical systems. Special among them are the algebraic systems. In the diffeomorphism case, these are automorphisms of tori and nilmanifolds. In the flow case, the algebraic models are suspensions of such diffeomorphisms and geodesic flows on negatively curved rank one symmetric spaces. In this talk, we will show that given an integer k ≥ 5, and a C^k Anosov flow Φ on some compact connected 3-manifold preserving a smooth volume, the measure of maximal entropy is the volume measure if and only if Φ is C^{k−ε}-conjugate to an algebraic flow, for ε > 0 arbitrarily small. This is a joint work with Jacopo De Simoi, Martin Leguil and Kurt Vinhage.

Meeting ID: 938 8598 9739

Recorded Talk:

## Seminar 04.07.22 Tsinas

Title:Multiple ergodic theorems for sequences of polynomial growth

Speaker:  Konstantinos Tsinas – University of Crete (Greece)

Abstract: Following the classical results of Host-Kra and Leibman on the polynomial ergodic theorem, it is natural to ask whether we can establish mean convergence of multiple ergodic averages along several other sequences, which arise from functions that have polynomial growth. In 1994, Boshernitzan proved that for a function f, which belongs to a large class of smooth functions (called a Hardy field) and which has polynomial growth, its “distance” from rational polynomials is crucial in determining whether or not the sequence of the fractional parts of f(n) is equidistributed on [0,1]. This, also, implies a corresponding mean convergence theorem in the case of single ergodic averages along the sequence ⌊f(n)⌋ of integer parts. In the case of multiple averages, it was conjectured by Frantzikinakis that a similar condition on the linear combinations of the involved functions should imply mean convergence. We verify this conjecture and show that in all ergodic systems we have convergence to the “expected limit”, namely, the product of the integrals. We rely mainly on the recent joint ergodicity results of Frantzikinakis, as well as some seminorm estimates for functions belonging to a Hardy field. We will also briefly discuss the “non-independent” case, where the L^2-limit of the averages exists but is not equal to the product of the integrals.

Meeting ID: 938 8598 9739

Recorded Talk: https://osu.zoom.us/rec/share/Gf98gFbI9Itd1STAukYTGjTHeePNXMHIsdoCITVDNs0cCpKQbNDEjUaYfEEVHbms.BBHTyrGjdrrvmvPr

## Seminar 03.31.22 Chen – In person

Title: Marked boundary rigidity and Anosov extension

Speaker: Dong Chen – Penn State University

Abstract: In this talk we will show how a sufficiently small geodesic ball in any Riemannian manifold can be embedded into an Anosov manifold with the same dimension. Furthermore, such embedding exists for a larger family of domains even with hyperbolic trapped sets. We will also present some applications to boundary rigidity and related open questions. This is a joint work with Alena Erchenko and Andrey Gogolev.

## Seminar 03.24.22 Koutsogiannis – In person

Title: Convergence of polynomial multiple ergodic averages for totally ergodic systems

Speaker: Andreas Koutsogiannis – Aristotle University of Thessaloniki (Greece)

Abstract: A collection of integer sequences is jointly ergodic if for every ergodic measure preserving system the multiple ergodic averages, with iterates given by this collection of sequences, converge to “the expected limit” in the mean, i.e., the product of the integrals. Exploiting a recent approach of Frantzikinakis, which allows one to avoid deep tools from ergodic theory that were previously used to establish similar results, we study joint ergodicity in totally ergodic systems for integer parts of real polynomial iterates. More specifically, our main results in this direction are a sufficient condition for k terms, and a characterization in the k=2 case. Joint work with Wenbo Sun.

## Seminar 03.10.22 Le

Title: Interpolation sets for nilsequences

Speaker: Anh N. Le – Ohio State University

Abstract: Interpolation sets are classical objects in harmonic analysis whichhave a natural generalization to ergodic theory regardingnilsequences. A set $E$ of natural numbers is an interpolation set fornilsequences if every bounded function on E can be extended to anilsequence on $\mathbb{N}$. By a result of Strzelecki, lacunary setsare interpolation sets for nilsequences. In this talk, I show that nosub-lacunary sets are interpolation sets for nilsequences and theclass of interpolation sets for nilsequences is closed under unionwith finite sets.

Meeting ID: 938 8598 9739

Recorded Talk: https://osu.zoom.us/rec/share/nIo2Tnfv7PMRIP3U_EG7FWw7N1YhFRL4BeJa_gqE0voCXN3enu_jnHuH-tW1H5q2.84ac0THimUVpKQfW

## Seminar 03.03.22 Griesmer

Title: Rigidity sequences for measure preserving transformations

Speaker: John Griesmer – Colorado School of Mines

Abstract:Let $(X,\mu,T)$ be a probability measure preserving system.  An increasing sequence $(n_k)$ of natural numbers is a rigidity sequence for $(X,\mu,T)$ if $\lim_{k\to\infty} \mu(A\triangle T^{-n_k}A)=0$ for every measurable $A\subset X$.  A classical result says that a generic measure preserving transformation is weak mixing and has a rigidity sequence, and it is natural to wonder which sequences are rigidity sequences for some weak mixing system.  Bergelson, del Junco, Lemańczyk, and Rosenblatt (2012) popularized many problems inspired by this question, and interesting constructions have since been provided by T. Adams; Fayad and Thouvenot; Fayad and Kanigowski; Griesmer; Badea, Grivaux, and Matheron; and Ackelsberg, among others.   This talk will summarize the relevant foundations and survey some recent results. We also consider two variations: union rigidity, where $\lim_{K\to\infty} \mu\Bigl(A\triangle \bigcup_{k>K}T^{-n_k}A\Bigr)=0$ for some  $A$ with $0<\mu(A)<1$, and summable rigidity, where $\sum_{k=1}^\infty \mu(A\triangle T^{-n_k}A)$ converges for some $A$ with $0<\mu(A)<1$.

Meeting ID: 938 8598 9739

Recorded Talk: https://osu.zoom.us/rec/share/LhoRfB_gvaAVAFyou-BQhRojLm0dQ0sk4uFbeQuWVXu1g5ytspNTkgGS25Li1a8Z.anEF2zLVmvlunkCo

## Seminar 02.24.22 Ackelsberg

Title: Large intersections for multiple recurrence in abelian groups

Speaker: Ethan Ackelsberg – Ohio State University

Abstract: With the goal of a common extension of Khintchine’s recurrence theorem and Furstenberg’s multiple recurrence theorem in mind, Bergelson, Host, and Kra showed that, for any ergodic measure-preserving system (X, ℬ, μ, T), any measurable set A ∈ ℬ, and any ε > 0, there exist (syndetically many) n ∈ ℕ such that μ(A ∩ TnA ∩ … ∩ TknA) > μ(A)k+1 – ε if k ≤ 3, while the result fails for k ≥ 4. The phenomenon of large intersections for multiple recurrence was later extended to the context of ⊕𝔽p-actions by Bergelson, Tao, and Ziegler. In this talk, we will address and give a partial answer to the following question about large intersections for multiple recurrence in general abelian groups: given a countable abelian group G, what are necessary and sufficient conditions for a family of homomorphisms φ1, …, φk : G → G so that for any ergodic measure-preserving G-system (X, ℬ, μ, (Tg)gG), any A ∈ ℬ, and any ε > 0, there is a syndetic set of g ∈ G such that μ(A ∩ Tφ1(g)A ∩ … ∩ Tφk(g)A) > μ(A)k+1 – ε? We will also discuss combinatorial applications in ℤd and (ℕ, ·). (Based on joint work with Vitaly Bergelson and Andrew Best and with Vitaly Bergelson and Or Shalom.)

Meeting ID: 941 3609 7274

Recorded Talk: https://osu.zoom.us/rec/share/TY64JIVXsqzNP_i1eNUIiwC0LriToGI6PVmOqPdJGnNuvNFRKkSLVvXiRP27RPU-.lyS_YtUQpBEuOhpC

## Seminar 02.17.22 Sharp

Title: Helicity and linking for 3-dimensional Anosov flows

Speaker: Richard Sharp – University of Warwick, UK

Abstract: Given a volume-preserving flow on a closed 3-manifold, one can, under certain conditions, define an invariant called the helicity. This was introduced as a topological invariant in fluid dynamics by Moffatt and measures the total amount of linking of orbits. When the manifold is a real homology 3-sphere, Arnold and Vogel identified this with the so-called asymptotic Hopf invariant, obtained by taking the limit of the normalised linking number of two typical long orbits. We obtain a similar result for null-homologous volume preserving Anosov flows, in terms of weighted averages of periodic orbits. (This is joint work with Solly Coles.)

Meeting ID: 941 3609 7274

Recorded Talk: https://osu.zoom.us/rec/share/cYO8hmX37fCGqR5DfrnHAnCNK04udNHsLvehiztiGOKAOEiByu-F2FpNPl7GDCGZ.WyVU6UdNkxaxw0fQ

## Seminar 02.10.22 Quas

Title: Lyapunov Exponents for Transfer Operators

Speaker: Anthony Quas – University of Victoria, Canada

Abstract: Transfer operators are used, amongst other ways, to study rates of decay of correlation in dynamical systems. Keller and Liverani established a remarkable result, giving conditions in which the (non-essential) part of the spectrum of a transfer operator changes continuously under small perturbations to the operator. This talk is about an ongoing project with Cecilia Gonzalez-Tokman in which we aim to develop non-autonomous versions of this theory.

Meeting ID: 941 3609 7274

## Seminar 01.27.22 Call

Title: Uniqueness and the K-property of equilibrium states for the geodesic flow on translation surfaces

Speaker: Benjamin Call – Ohio State University

Abstract: In the general setting of CAT(0) spaces, Ricks has provided necessary and sufficient conditions for uniqueness and mixing of the measure of maximal entropy for the geodesic flow. I will discuss recent work establishing uniqueness and the K-property of a class of equilibrium states for the geodesic flow on translation surfaces, a subclass of CAT(0) spaces. This result builds on the orbit-decomposition machinery developed by Climenhaga and Thompson, and is joint work with Dave Constantine, Alena Erchenko, Noelle Sawyer, and Grace Work.

Meeting ID: 941 3609 7274

Recorded Talk: https://osu.zoom.us/rec/share/G56ox7c5B3dudA9ZO303EvX8VW4Fa_z7sCM4S4tdBDsfe4LfolcLJ8p4TGgVgY-X.nY-n-jPdltDSMOCX

## Seminar 12.09.21 Tiozzo

Title: Hitting measures for random walks on cocompact Fuchsian groups

Speaker: Giulio Tiozzo – University of Toronto

Abstract: A recurring question in the theory of random walks on hyperbolic spaces asks whether the hitting (harmonic) measures can coincide with measures of geometric origin, such as the Lebesgue measure. This is also related to the inequality between entropy and drift.  For finitely-supported random walks on cocompact Fuchsian groups with symmetric fundamental domain, we prove that the hitting measure is singular with respect to Lebesgue measure; moreover, its Hausdorff dimension is strictly less than 1. Along the way, we prove a purely geometric inequality for geodesic lengths, strongly reminiscent of the Anderson-Canary-Culler-Shalen inequality for free Kleinian groups.  Joint with P. Kosenko.

Meeting ID: 916 3892 7725

Recorded Talk: https://osu.zoom.us/rec/share/dcqrRfiTn8n_stVOUUooY9q8cCpzFwreq0L3ih3lUrgoR-rdbQefsdxYwfUJu-Y.p0RYOqWCdsYDR1iA

## Seminar 10.28.21 Sun

Title: Joint ergodicity conjecture for systems with commuting transformations

Speaker: Wenbo Sun – Virginia Tech

Abstract: It is well know by the Mean Ergodic Theorem that for any measure preserving system $(X,\mathcal{B},\mu,T)$ and $L^{\infty}$ function f, the time average of $T^{n}f$ converges to the integral of f if and only if T is ergodic. It is a natural question to ask when the average of products of polynomial iterates of  $L^{\infty}$ functions (known as multiple ergodic averages) converges to the product of the integrals of the functions. This question is called the Joint Ergodicity Problem. In this talk, I will introduce some recent advances in this problem. This talk is based on joint works with Sebasti\’an Donoso, Andreu Ferr\’e Moragues and Andreas Koutsogiannis.

Meeting ID: 916 3892 7725

Recorded Talk: https://osu.zoom.us/rec/play/dk–MjWHHK8ex0sdF0ILXOXT338U71LQ1awWnexrtUuyYiEtC-noT76YCLpX4bnCvTAT2mU-xxTPv9d1.OUHqT_H7CAgbLfFM?continueMode=true

## Seminar 10.21.21 Maass

Title: Spectral analysis of topological finite rank systems

Speaker: Alejandro Maass – University of Chile

Abstract: Finite topological rank Cantor minimal systems represent a broad class of sub shifts of zero entropy or odometers [Downarowiz-Maass], it contains well studied systems like substitution sub shifts or linearly recurrent systems. In this talk we will present the study of measure-theoretical and topological eigenvalues for such class of systems, given formulas characterizing them. This work is motivated by the seminal work of Bernard Host where it is proved that measure-theoretical and topological eigenvalues of substitutions systems coincide. This is a joint work with Fabien Durand and Alexander Frank.

Meeting ID: 916 3892 7725

Recorded Talk: https://osu.zoom.us/rec/share/6x1GYGHuPkjR4KFdpi4usJMP1ert17FX_RHSF_MVaxIkD6PrLXjLO83fXN_-CR8u.0rZGFYbboMqO4Ps7

## Seminar 10.08.21 Tanaka

Title: The Manhattan curve and rough similarity rigidity

Speaker: Ryokichi Tanaka – Kyoto University

Abstract: For every non-elementary hyperbolic group, we consider the Manhattan curve, which was originally introduced by M. Burger (1993),
associated with any pair of (say) word metrics. It is convex; we show that it is continuously differentiable and moreover is a straight line if and only if the corresponding two metrics are roughly similar, that is, they are within bounded distance after multiplying by a positive constant. I would like to explain how it is related to the central limit theorem for uniform counting measures on spheres, to ergodic theory of topological flows built on general hyperbolic groups, and to the multifractal structure of Patterson-Sullivan measures. Furthermore, I will present some explicit examples including a hyperbolic triangle group and compute the exact value of the mean distortion for a pair of word metrics by using automatic structures of the group.
Joint work with Stephen Cantrell (University of Chicago).

Meeting ID: 916 3892 7725

Recorded Talk: https://osu.zoom.us/rec/share/LPAREOkf7DJ5zT6tKMPY1dZP_Vs824-zAEFnAJ7CTzXJKS-pMk_Bx8o2eD6QXg-y.lFp9Na68FCmloePw

## Seminar 09.30.21 Shmerkin

Title: Beyond Furstenberg’s intersection conjecture

Speaker: Bablo Shmerkin – University of British Columbia (UBC)

Abstract: Hillel Furstenberg conjectured in the 1960s that the intersections of closed ×2 and ×3-invariant Cantor sets have “small” Hausdorff dimension. This conjecture was proved independently by Meng Wu and by myself; recently, Tim Austin found a simple proof. I will present some generalizations of the intersection conjecture and other related results.

Meeting ID: 916 3892 7725

Recorded Talk: https://osu.zoom.us/rec/share/qQEteZ_GglmdyYvMPoZyFwipOFOENgKYvoYIar9tCmhxLm82JhuYaLNr7bNqoFxb.pHzsX46hQRGFTR2y

## Seminar 09.16.21 Ferre Moragues

Title: Polynomial ergodic averages for certain countable ring actions

Speaker: Andreu Ferre Moragues – Nicolaus Copernicus University, Torun, Poland

Abstract: Inspired by a recent result of Frantzikinakis that allows one to establish joint ergodicity of general integer valued sequences, we will take a look at the ring actions setup. I will present new joint ergodicity results for families of independent polynomials when the acting ring is a field of characteristic zero, their corollaries in combinatorics and topological dynamics, and the methods of proof. Based on joint work with Andrew Best.

Meeting ID: 916 3892 7725

## Seminar 09.09.21 Wolf

Title: Computability of topological pressure on compact shift spaces beyond finite type

Speaker: Christian Wolf – CUNY

Abstract: In this talk we discuss the computability (in the sense of computable analysis) of the topological pressure $P_{\rm top}(\phi)$ on compact shift spaces $X$ for continuous potentials $\phi:X\to\bR$. This question has recently been studied for subshifts of finite type (SFTs) and their factors (Sofic shifts). We develop a framework to address the computability of the topological pressure on general shift spaces and apply this framework to coded shifts. In particular, we prove the computability of the topological pressure for all continuous potentials on S-gap shifts, generalized gap shifts, and Beta shifts. We also construct shift spaces which, depending on the potential, exhibit computability and non-computability of the topological pressure. We further show that the generalized pressure function $(X,\phi)\mapsto P_{\rm top}(X,\phi\vert_{X})$ is not computable for a large set of shift spaces $X$ and potentials $\phi$. Along the way of developing these computability results, we derive several ergodic-theoretical properties of coded shifts which are of independent interest beyond the realm of computability. The topic of the talk is joint work with Michael Burr (Clemson U.), Shuddho Das (NYU) and Yun Yang (Virginia Tech).

Meeting ID: 916 3892 7725

Recorded Talk: https://osu.zoom.us/rec/share/dIchmAETAILph7uojccOdxliuEeNlsH4SfDUEzzVuFwqKQiQi3brEW6GvX_Hpj_4.onMqP6LjZX6OSA6_

## Seminar 08.26.21 Dymek

Title: Topological dynamics of (multidimensional) B-free systems

Speaker: Aurelia Dymek – Nicolaus Copernicus University, Torun, Poland

Abstract: Sarnak turned attention to B-free systems. In particular, he investigated the squarefree subshift. Two multidimensional generalizations of B-free systems have been dealt with by Cellarosi, Vinogradov, Baake and Huck. My talk will be concerned with some similarities and differences between B-free systems in the one- and multidimensional case. This is a part of my doctoral dissertation.

Meeting ID: 916 3892 7725