The University of Georgia

Past seminars

**Seminar on ****October 29, 2024**

**Speaker:** Dr. **Zoe Nieraeth** (she/her), the University of the Basque country (UPV/EHU); https://sites.google.com/view/zoe-nieraeth

**The title of the lecture:** “A lattice approach to singular integrals”

**Abstract:** In this talk, we discuss the problem of characterizing the boundedness of singular integrals in Banach lattices such as weighted Morrey and variable Lebesgue spaces. Moreover, we apply this lattice viewpoint to the setting of matrix weights to obtain new characterizations and Rubio de Francia extrapolation results for matrix Muckenhoupt weights.

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**Seminar on ****October 15, 2024**

**Speaker:** Prof. **Sundaram Thangavelu**, Indian Institute of Science, Bangalore; https://math.iisc.ac.in/~veluma/

**The title of the lecture:** “How fast can the Fourier transform of a compactly supported function decay?”

**Abstract:** It is well known that the Fourier transform of a compactly supported function on \( \mathbb{R}^n \) cannot have compact support. A natural question is therefore what is the best possible decay for such functions? A classical theorem of Ingham answers this question. In this talk we survey the recent developments on the analogues of this theorem when Fourier transform is replaced by Helgason Fourier transform on Riemannian symmetric spaces or the group Fourier transform on Heisenberg groups.

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**Seminar on ****June 11, 2024**

**Speaker:** Prof. Dr. **Carsten Trunk**, TU Ilmenau; https://www.tu-ilmenau.de/universitaet/fakultaeten/fakultaet-mathematik-und-naturwissenschaften/profil/institute-und-fachgebiete/institut-fuer-mathematik/profil/fachgebiet-angewandte-funktionalanalysis

**The title of the lecture:** “Essential spectra of Sturm–Liouville operators and their indefinite counterpart”

**Abstract:** We show new perturbation results and invariance of essential spectra in terms of the real, locally integrable coefficients \( p \), \( q \), \( r \) for general Sturm-Liouville differential expressions of the form

\( \displaystyle \frac{1}{r} \left( -\frac{d}{dx} p \frac{d}{dx} + q \right)\,, \)

with a.e. positive functions \( p \) and \( r \). If one allows sign changes of the weight function \( r \), then the situation changes dramatically but one keeps control over the essential spectrum and some properties of the isolated point spectrum can still be proved.

This talk is based on a joint works with J. Behrndt (Graz), G. Teschl (Vienna), and P. Schmitz (Ilmenau).

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**Seminar on ****May 28, 2024**

**Speaker:** Prof. **Zhirayr Avetisyan**, Ghent University; https://www.z-avetisyan.com/

**The title of the lecture:** “Universal Functions and Sets”

**Abstract:** Universal objects in mathematics are those which are capable of, in one sense or another, representing most other objects of the same category. In the context of Fourier analysis, universality is understood in the sense of approximations by orthogonal or Fourier series.

The first part of the talk will be devoted to universal functions. Roughly speaking, a function is universal if its Fourier series has universal properties, e.g., its partial sums converge to any given function in a space. There are variations of this, including possible rearrangements and changes of signs. We will establish very general results about the existence of universal functions for minimal systems in Banach spaces, which obey very basic approximation properties. On the other hand, it is clear that in a Banach space, the partial sums of a Fourier series cannot converge to different functions. Therefore we introduce the slightly weaker notion of asymptotic universality and obtain existence results under fairly mild assumptions.

The second part of the talk will be concerned with universal sets. In addition to approximation in the norm, one often needs to approximate a given function spatially, i.e., the approximating function should agree with the original one identically on a large subset of the domain. In this respect, the classical example is Luzin’s theorem, where measurable functions are approximated by continuous ones. However, the arbitrarily large subset of coincidence in Luzin’s theorem depends on the original function. It is remarkable, that for certain kinds of approximation, the subset can be chosen a priori, independent of the function being approximated, making it universal. While results in this direction abound for classical systems, we will introduce sweeping generalisations, as well as covariant constructions in homogeneous spaces of compact topological groups.

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**Seminar on ****May 14, 2024**

**Speaker:** Prof. **Sergei Grudsky**, CINVESTAV, Mexico City; https://www.math.cinvestav.mx/~grudsky/index.html

**The title of the lecture:** “Asymptotics of eigenvalues and eigenvectors of Toeplitz matrices”

**Abstract:** Analysis of the asymptotic behaviour of the spectral characteristics of Toeplitz matrices as the dimension of the matrix tends to infinity and has a history of over 100 years. For instance, quite a number of versions of Szegö's theorem on the asymptotic behaviour of eigenvalues and of the so-called strong Szegö theorem on the asymptotic behaviour of the determinants of Toeplitz matrices are known. Starting in the 1950s, the asymptotics of the maximum and minimum eigenvalues were actively investigated. However, investigation of the individual asymptotics of all the eigenvalues and eigenvectors of Toeplitz matrices started only quite recently: the first papers on this subject were published in 2009-2010. A survey of this new field is presented here.

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**Seminar on ****April 30, 2024**

**Speaker:** Prof. Dr. **Uwe Kähler**, University of Aveiro; https://sweet.ua.pt/ukaehler/Webpage/Main.html

**The title of the lecture:** “Global pseudo-differential operator calculus over spin groups”

**Abstract:** In this talk, we present a construction of a global symbol calculus of pseudo-differential operators on spin groups in the sense of Ruzhansky-Turunen-Wirth, with special attention given to the case of Spin(4). Using representations of Spin(4) we construct a group Fourier transform and establish the calculus of left-invariant differential operators and of difference operators on the group Spin(4). Afterwards, we apply this calculus to give criteria for subellipticity and global hypoellipticity of pseudo-differential operators in terms of their matrix-valued full symbols. Several examples of first and second-order globally hypoelliptic differential operators are given, including somewhere the criteria of hypoellipticity will be reduced to the problem of approximation distance between irrational and rational numbers.

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**Seminar on ****April 2, 2024**

**Speaker:** Prof. em. **Alessia Kogoj**, University of Urbino; https://www.researchgate.net/profile/Alessia-Kogoj

**The title of the lecture:** “Subelliptic Liouville Theorems”

**Abstract:** Several Liouville-type theorems are presented, related to evolution equations and to their “time”-stationary counterparts. The equations we are dealing with are left translations invariant on a Lie group structure and, in some cases, homogeneous with respect to a group of dilations. In all these cases the operators have smooth coefficients and are hypoelliptic. We also present a “polynomial” Liouville-type theorem for X-elliptic operators with nonsmooth coefficients, by extending to this new setting a celebrated result by Colding and Minicozzi related to the Laplace-Beltrami operator on Riemannian manifolds.

The results are contained in a series of papers in collaboration with A. Bonfiglioli, E. Lanconelli, Y. Pinchover, S. Polidoro and E. Priola.

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**Seminar on ****March 5, 2024**

**Speaker:** Prof. **Alex Iosevich**, The University of Rochester, USA; https://people.math.rochester.edu/faculty/iosevich/

**The title of the lecture:** “Restriction Theory, Uncertainty Principles and Signal Recovery”

**Abstract:** Let \(f: {\mathbb Z}_N^d \to {\mathbb C}\) and define \(\widehat{f}(m)=N^{-d} \sum_{x \in {\mathbb Z}_N^d} \chi(-x \cdot m) f(x)\), the discrete Fourier transform, where \(\chi(t)=e^{\frac{2 \pi i t}{N}}\). Suppose that the signal \(f\) is transmitted via its Fourier transform and that some of the transmission is lost, i.e the values \({\{\widehat{f}(m)\}}_{m \in S}\) are unobserved for some \(S \subset {\mathbb Z}_N^d\). The question, raised by Donoho and Stark in the late \(80\)s is, are there reasonable assumptions on the signal \(f\) and the missing set of frequencies \(S\) such that \(f\) can be recovered exactly, despite the signal loss? Donoho and Stark showed that the uncertainty principle for the Fourier transform can be used to derive a set of sufficient conditions. In this talk, we are going to see that discrete restriction theory for the Fourier transform can be brought to bear on this problem. We are also going to discuss some discretization procedures that can be used to speed up the recovery process at the cost of a small error.

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**Seminar on ****March 5, 2024**

**Speaker:** Prof. **Joachim Toft**, Linnaeus University (LNU); https://lnu.se/en/staff/joachim.toft/

**The title of the lecture:** “Fractional Fourier transform, harmonic oscillator propagators and Strichartz estimates”

**Abstract:** Using the Bargmann transform, we prove that harmonic oscillator propagators and Fractional Fourier Transforms (FFT) are essentially the same. We deduce continuity properties for such operators on modulation spaces and apply the results to prove Strichartz estimates for the harmonic oscillator propagator when acting on modulation spaces. We also show that general forms of fractional harmonic oscillator propagators are continuous and suitable for so-called Pilipović spaces and their distribution spaces. Especially we show that FFT of any complex order can be defined and that these transforms are continuous on strict Pilipović function and distribution spaces.

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**Seminar on ****February 20, 2024**

**Speaker:** Prof. **Serena Federico**, University of Bologna; https://www.unibo.it/sitoweb/serena.federico2/cv-en

**The title of the lecture:** “Weyl Calculus on graded groups”

**Abstract:** In this talk we will investigate the existence of a Weyl pseudo-differential calculus on any graded Lie group. To start with, we will recall the fundamental properties of the Weyl quantization in the Euclidean setting and of the corresponding pseudo-differential calculus. Afterwards, we will define a family of quantizations on any graded Lie group and develop the corresponding symbolic calculus. Finally, inside this family of quantizations we will identify a possible Weyl quantization on any graded group. In the end, we will see that the identified quantization is the uniquely determined Weyl quantization in the case of the Heisenberg group.

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**Seminar on ****February 06, 2024**

**Speaker:** Prof. em. **Elias Wegert**, Institute of Applied Analysis, TU Bergakademie Freiberg, 09596 Freiberg, Germany; https://de.wikipedia.org/wiki/Elias_Wegert

**The title of the lecture:** “Numerical Range, Blaschke Products, and Poncelet Polygons”

**Abstract:** In 2016, Gau, Wang and Wu conjectured that a partial isometry \( A \) acting on a \( n \)-dimensional complex Hilbert space cannot have a circular numerical range with a non-zero centre. In this talk, we verify this for operators with \( \mathrm{rank}\,A = n - 1 \).

The proof is based on the unitary similarity of \( A \) to a compressed shift operator generated by a finite Blaschke product \( B \). We then use the description of the numerical range by Poncelet polygons associated with \( B \), a special representation of Blaschke products related to boundary interpolation, and an explicit formula for the barycenters of the vertices of Poncelet polygons involving elliptic functions.

The talk is a mixture of operator theory, plain geometry, and elementary complex analysis. In particular, we give a short introduction to the visualization of complex functions by „phase plots“ and demonstrate how they can help to discover relevant properties of these functions.

**Full abstract see here:** https://drive.google.com/file/d/1u44q9liZP6IxAwECBwRS7Lpkq9hmoSIz/view?usp=sharing

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**Seminar on ****January 23, 2024**

**Speaker:** Prof. Dr. Reinhold Schneider, Technical University of Berlin; https://scholar.google.de/citations?user=3CmBwQcAAAAJ&hl=de

**The title of the lecture:** “Numerical Solution of high-dimensional Hamilton Jacobi Bellmann (HJB) Equations, Mean Field Games and Compositional Tensor Networks”

**Abstract:** The partial differential equations of Mean Field Games introduced by Lasry & Lions describe the solution of feedback optimal control problems as well as optimal transport problems. These equations play a fundamental role in optimal control, optimal transport, computational finance and machine learning. Therefore solving these kinds of equations seems to be of utmost importance in future science and technology. However for solving these non-linear and high dimensional equations, one has to deal with two major difficulties, namely 1) the curse of dimensions and secondly, 2) possible lack of regularity. Here we focus only on the first issue. (The second problem can be relaxed by adding randomness, which is always present in practice.) We consider Potential Mean Field Games and are focusing on the (deterministic/stochastic) HJB. In order to compute semi-global solutions, we consider control affine dynamical systems and quadratic cost for the control as a prototype example. We follow a Lagrangian approach perspective and a recent approximation concept of compositional sparsity. In contrast to our earlier published semi-Lagrangian approaches, we describe a direct minimization of the total averaged cost overall initial values, first introduced by Kunisch & Walter (2021). Approximately, the true total cost is replaced by averaging oversampled initial values, where the sought approximate value function is parametrized in tensor form. Compositional sparsity has been inspired by Deep Neural Networks. The individual neural network layers are replaced by tree-based tensor networks (HT/TT) or sparse polynomials, which improves the stability for a numerical treatment of the optimization problem.

Related literature: A machine learning framework for solving high-dimensional mean field game and mean field control problems, Ruthotto, Osher, Li, Nurbekyan, Fung - Proceedings of the National Academy of Sciences, 2020

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**Seminar on ****December 12, 2023**

**Speaker:** Prof. Claudia Garetto, Queen Mary University of London; https://www.qmul.ac.uk/maths/profiles/claudiagaretto.html

**The title of the lecture:** “Higher order hyperbolic equations with multiplicities”

**Abstract:** In this talk, I will discuss Gevrey and C∞ well-posedness for linear higher-order hyperbolic equations with multiplicities. I will review the different methods employed for time- and/or x-dependent coefficients and the conditions needed on the lower-order terms. Work in collaboration with Michael Ruzhanksy (Ghent/QMUL) and Bolys Sabibtek (QMUL).

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**Seminar on ****November 28, 2023**

**Speaker:** Dr. Nazar Miheisi, King’s Kollege London; https://www.kcl.ac.uk/people/nazar-miheisi

**The title of the lecture:** “Completeness of Systems of Inner Functions”

**Abstract:** An inner function is an analytic function on the unit disk whose boundary values have modulus 1 almost everywhere – these play a special role in operator theory and function theory. In this talk, I will discuss the following problem: for which inner functions ϕ and ψ, are the powers ϕm, ψn (m, n ∈ Z) complete in the weak-∗ topology of L∞? This problem was first considered in 2011 by Hedenmalm and Montes-Rodríguez who gave a complete solution for atomic inner functions with one singularity. I will give a recent extension of this result to a much wider class of inner functions. If time permits, I will also discuss a connection with a problem in Fourier uniqueness.

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**Seminar on ****November 14, 2023**

**Speaker:** Pedro Gonçalves Ramos, Postdoctoral researcher at École Polytechnique Fédérale de Lausanne; https://sites.google.com/view/gionnoramos/

**The title of the lecture:** “Time-frequency localization operators, their eigenvalues and relationship to elliptic PDE”

**Abstract:** In the classical realm of time-frequency analysis, an object of major interest is the short-time Fourier transform of a function. This object is a modified Fourier transform of a signal \( f\left( x \right) \), changed by a certain ’window function’, in order to make simultaneous analysis on frequency and time more feasible. Since the pioneering work of Daubechies, time-frequency localisation operators have been of extreme importance in that analysis. These operators are defined to measure how much

the short-time Fourier transform of a function concentrates in the time-frequency plane, and thus the study of the eigenvalues and eigenfunctions of such operators is intimately connected to how well one can perform the simultaneous analysis of signals mentioned above. In this talk, we will explore the case where the window function is a Gaussian. We will discuss some classical and recent results on domains of maximal time-frequency concentration, their eigenvalues, and stability/inverse problems associated with such properties. During this investigation, we shall see that many of these problems possess some rather unexpected connections with calculus of variations, overdetermined elliptic boundary value problems and free boundary problems.

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**Seminar on July 10**

**Speaker:** Prof. Alex Iosevich, The University of Rochester, USA; https://www.sas.rochester.edu/mth/people/faculty/iosevich-alex/index.html

**The title of the lecture:** “Finite point configurations and learning theory”

**Abstract:** The basic question we ask is, how large does the Hausdorff dimension of a subset of Euclidean space need to be to ensure that it contains a congruent copy of the finite point configuration of a given type? This problem is strongly connected with the Erdos distance problem in combinatorics, the Falconer distance problem in geometric measure theory, and Furstenberg-type configuration problems in ergodic theory. Connections with complexity problems in learning theory will also be discussed.

This talk was delivered both in person, at the University of Georgia in Tbilisi, Georgia, and remotely, via the Webex platform.

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**Seminar on June 13, 2023**

**Speaker:** Doctoral student Duvan Cardona Sanchez, Ghent Analysis and PDE Center; https://sites.google.com/site/duvancardonas/

**The title of the lecture:** "Continuity properties for some operators arising in non-commutative harmonic analysis"

**Abstract:** The non-commutative harmonic analysis on nilpotent Lie groups after the developments by Folland and Stein in the 70s has been fundamental for the analysis of hypoelliptic problems on graded Lie groups. They started the program of generalising in the setting of nilpotent Lie groups the results available in the Eucliden harmonic analysis. Several fundamental results in this area have been obtained in the last 50 years. In this setting, we review some recent results about the boundedness of oscillating Fourier multipliers, pseudo-differential operators and other operators arising in this setting. The results presented in this talk are part of my joint work with M. Ruzhansky (Ghent) and J. Delgado (Colombia).

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**Seminar**** on May 30, 2023**

**Speaker:** Prof. Harm Bart, Erasmus University Rotterdam; https://www.researchgate.net/scientific-contributions/Harm-Bart-2013767776

**The title of the lecture:** „The Rouché Theorem for Fredholm operator-valued functions: an enhanced version“

Abstract: The well-known classical Rouché Theorem is concerned with the perturbation of scalar analytic functions. Roughly speaking: if the perturbation is small enough, the perturbed function has the same number of zeros as the original one. In the 1971 paper [GS], I.C. Gohberg and E.I. Sigal generalized the theorem to a result involving Fredholm operator-valued functions. Although just one of them is indicated in [GS] (and in [GGK], Section XI.9 as well), there are actually two versions of the generalization, due to the fact that bounded linear operators as a rule do not commute. So a commutativity issue manifests itself here.

There is another one. The Rouché Theorems involve the logarithmic residues of the functions involved, i.e., a contour integral of their logarithmic derivatives. In the scalar case such a logarithmic is unambiguously determined; in the non-scalar setting, it is not. There, again, two possibilities present themselves, depending on which order one takes in the product of the derivative and the inverse. Generally, these options do not come down to the same.

In the lecture, an approach will be presented that yields an encompassing (strictly) stronger variant of the results indicated above.

[GS] I.C. Gohberg, E.I. Sigal, An operator generalization of the logarithmic residue theorem and the theorem of Rouché, Mat. Sbornik 84 (126) (1971), 607-629 (Russian), English Transl. in Math. USSR Sbornik 13 (1971), 603–625.

[GGK] I. Gohberg, S. Goldberg, M.A. Kaashoek, Classes of Linear Operators, Vol. I, Operator Theory: Advances and Applications, OT 49, Birkhäuser Verlag, Basel 1990.

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**Seminar on May 16, 2023**

**Speaker:** Prof. Dr. Davit Natroshvili, Department of Mathematics, Georgian Technical University, Tbilisi, Georgia; https://my.gtu.ge/Personal/473

**The title of the lecture:** “An alternative potential method for mixed boundary value problems”

**Abstract:** We consider an alternative potential method to investigate a mixed boundary value problem (BVP) for the Lamé system of elasticity in the case of a three-dimensional bounded domain when the boundary surface is divided into two disjoint parts, where the Dirichlet and Neumann type boundary conditions are prescribed respectively for the displacement vector and the stress vector. Our approach is based on the potential method. We look for a solution to the mixed boundary value problem in the form of a linear combination of the single-layer and double-layer potentials with densities supported respectively on the Dirichlet and Neumann parts of the boundary.

This approach reduces the mixed BVP under consideration to a system of integral equations which contain neither extensions of the Dirichlet or Neumann data, nor the Steklov-Poincaré type operator. Moreover, the right-hand sides of the system are vectors coinciding with the Dirichlet and Neumann data of the problem under consideration.

The corresponding matrix integral operator is invertible in the appropriate Bessel potential and Besov spaces, which implies the unconditional unique solvability of the mixed BVP in the corresponding Sobolev spaces and representability of solutions in the form of a linear combination of the single-layer and double-layer potentials with densities supported respectively on the Dirichlet and Neumann parts of the boundary.

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**Seminar on May 02, 2023**

**Speaker: Anastasia Kisil**, University of Manchester, UK; https://anastasiakisil.weebly.com/

**The title of the lecture: **"A generalisation of the Wiener-Hopf methods for an equation in two variables with three unknown functions"

**Abstract:** In the talk, I will present an analytic solution to a generalisation of the Wiener– Hopf equation in two variables and with three unknown functions. This equation arises in many applications, for example, when solving the discrete Helmholtz equation associated with scattering on a domain with a perpendicular boundary. The traditional Wiener–Hopf method is suitable for problems involving boundary data on co-linear semi-infinite intervals, not for boundaries at an angle. This significant extension will enable the analytical solution to a new class of problems with more boundary configurations. Progress is made by defining an underlining manifold that links the two variables. This allows us to meromorphically continue the unknown functions on this manifold and formulate a jump condition. As a result, the problem is fully solvable in terms of Cauchy-type integrals which is surprising since this is not always possible for this type of functional equation.

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**Seminar on March 21, 2023**

**Speaker: ****Guillermo P. Curbera, **Professor of Mathematics, Instituto de Matemáticas IMUS, Universidad de Sevilla; https://euler.us.es/~curbera/

**The title of the lecture: **"The fine spectra of the finite Hilbert transform, beyond the \( L^p \)-spaces"

**Abstract:** We consider the finite Hilbert transform acting on rearrangement invariant spaces over \(\left( 1,1 \right) \). We present results on the spectrum and exemplary spectra, extending the Widom results for the \( L^p \)-spaces.

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**Seminar on February 21, 2023**

**Speaker: ****Jean Lagacé, **King’s College London, UK; https://lagacejean.github.io/

**The title of the lecture: **“Spectral Geometry on rough spaces”

**Abstract:** Spectral geometry is concerned with studying the spectrum (the eigenvalues) of differential operators (like the Laplacian) on surfaces and manifolds, and relating that spectrum with the geometry of the underlying space. Many tools, for instance, pseudo-differential calculus and microlocal analysis, have been developed over the years to study the spectrum when everything is smooth. In this talk, I will focus on the asymptotic distribution of eigenvalues (Weyl’s Law), and explain what we can keep when we do not assume the smoothness of the underlying space. Special attention will be given to the Steklov problem.

This is joint work with Mikhail Karpukhin (University College London) and Iosif Polterovich (Université de Montréal).

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**Seminar on February 21, 2023**

**Speaker: **Prof. **Jani Virtanen, **University of Reading, UK; https://janivirtanen.wordpress.com/

**The title of the lecture: **“On the Berger-Coburn phenomenon”

**Abstract:** In 1987, Berger and Coburn proved that if the Hankel operator with a bounded symbol is compact on the classical Fock space, then so is the Hankel operator with the conjugate symbol. This property is unique for the Fock space, and fails for the Hardy space and the Bergman space, for example. In 2004, Bauer showed that an analogous result remains true if compactness is replaced by being Hilbert-Schmidt. The question of what happens with the other Schatten classes remained open for almost two decades. I report on the recent progress, which answers the question in full.

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**Seminar on ****February 7, 2023**

**Speaker: **Prof. **Alexander Meskhi,** TSU A. Razmadze Mathematical Institute and Kutaisi International University, Georgia; https://www.kiu.edu.ge/?m=316

**The title of the lecture:** “Boundedness criteria for multilinear fractional integral operators”

**Abstract:** Please see the poster

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**Seminar on ****January 24, 2023**

**Speaker: **Prof. Dr.** Dorothee Haroske,** Friedrich-Schiller-Universität Jena, Fakultät für Mathematik & Informatik, Institut für Mathematik, Germany; https://users.fmi.uni-jena.de/~haroske/

**The title of the lecture:** “Morrey smoothness spaces: A new approach”

**Abstract:** In recent years so-called Morrey smoothness spaces attracted a lot of interest. They can (also) be understood as generalisations of the classical spaces \( A^s_{p,q} (\mathbb{R}^n) \), \( A\in \{B,F\} \), where the parameters satisfy \( s\in \mathbb{R} \) (smoothness), \( 0<p \le \infty \) (integrability) and \( 0<q \le \infty \) (summability). In the case of Morrey smoothness spaces additional parameters are involved. In our opinion, among the various approaches at least two scales enjoy special attention, also in view of applications: the scales \( \mathcal{A}^s_{u,p,q} (\mathbb{R}^n) \), with \( \mathcal{A}\in \{\mathcal{N}, \mathcal{E}\} \), \( u\geq p \), and \( A^{s, \tau}_{p,q} (\mathbb{R}^n) \), with \( \tau\geq 0 \).

We reorganise these two prominent types of Morrey smoothness spaces by adding to \( \left( s,p,q \right) \) the so-called slope parameter \( \varrho \), preferably (but not exclusively) with \( -n \le \varrho <0 \). It comes out that \( \left| \varrho \right| \) replaces \( n \), and \( \min \left( \left| \varrho \right|,1 \right) \) replaces \( 1 \) in slopes of (broken) lines in the \( \left( \frac{1}{p},s \right) \)-diagram characterising distinguished properties of the spaces \( A^s_{p,q} \left( \mathbb{R}^n \right) \) and their Morrey counterparts.

Our aim is two-fold. On the one hand, we reformulate some assertions already available in the literature (many of them are quite recent). On the other hand, we establish on this basis new properties, a few of them became visible only in the context of the offered new approach, governed, now, by the four parameters \( \left( s,p,q,\varrho \right) \).

The talk is based on joint work with Hans Triebel (Jena).

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**Seminar on ****December 13, 2022**

**Speaker: **Prof.** Luis Castro,** CIDMA - Center for Research and Development in Mathematics and Applications & Department of Mathematics, University of Aveiro, Portugal; https://www.ua.pt/pt/p/10311888

**The title of the lecture:** “New convolutions generated by Hermite functions and consequent classes of integral operators”

**Abstract:** We will introduce new convolutions generated by multi-dimensional Hermite functions and study some of their properties. Namely, we will analyse classes of integral operators generated by those convolutions. This will also give rise to the study of the solvability of a general class of integral equations whose kernel depends on four different functions. Additional properties will appear along the way, among which we highlight new Young-type inequalities and factorizations (where the new convolutions take a central role). The talk is based on joint work with R.C. Guerra (Coimbra, Portugal) and N.M. Tuan (Hanoi, Vietnam).

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**Seminar on ****November 29, 2022**

**Speaker: **Prof.** Ferenc Weisz,** Eötvös Loránd University, Budapest, Hungary; https://www.researchgate.net/profile/Ferenc-Weisz

**The title of the lecture:** “Hardy spaces in the theory of trigonometric and Walsh-Fourier series and Lebesgue points”

**Abstract:** We introduce higher dimensional martingale and classical Hardy spaces and consider trigonometric and Walsh-Fourier series. We state that the maximal operator of the Fejér or Cesàro means of a higher dimensional function is bounded from the corresponding Hardy space to the Lebesgue space. This implies some almost everywhere convergence of the Cesàro means. We characterize the set of convergence as different types of Lebesgue points.

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**Seminar on ****November 15, 2022**

**Speaker: Peter Kuchment,** University Distinguished Professor, Mathematics Department, Texas A&M University, USA; https://www.math.tamu.edu/~peter.kuchment/

**The title of the lecture:** “Wonderful World of tomography”

**Abstract:** Here I would provide an introduction to non-experts to the multifaceted and booming mathematics (PDEs, harmonic analysis, etc.) of imaging.

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**Seminar on ****November 1, 2022**

**Speaker: **Prof. **Elijah Liflyand,** Department of Mathematics, Bar-Ilan University, Israel; https://u.math.biu.ac.il/~liflyand/

**The title of the lecture:** “Wiener algebras and trigonometric series in a coordinated fashion”

**Abstract:** Let $W_0(\mathbb{R})$ be the Wiener Banach algebra of functions representable by the Fourier integrals of Lebesgue integrable functions. It is proven in the paper that, in particular, a trigonometric series $\sum\limits_{k=-\infty}^\infty c_k e^{ikt}$ is the Fourier series of an integrable function if and only if there exists a $\phi\in W_0(\mathbb R)$ such that $\phi(k)=c_k$, $k\in\mathbb Z$. If $f\in W_0(\mathbb R)$, then the piecewise linear continuous function $\ell_f$ defined by $\ell_f(k)=f(k)$, $k\in\mathbb Z$, belongs to $W_0(\mathbb R)$ as well. Moreover, $\|\ell_f\|_{W_0}\le \|f\|_{W_0}$. Similar relations are established for more advanced Wiener algebras. These results are supplemented by numerous applications. In particular, new necessary and sufficient conditions are proved for a trigonometric series to be a Fourier series and new properties of $W_0$ are established.

This is joint work with R. Trigub.

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**Seminar on ****October 18, 2022**

**Speaker: **Prof. **Diogo Oliveira e Silva,** Departamento de Matemática, Instituto Superior Técnico, Lisboa, Portugal; https://www.math.tecnico.ulisboa.pt/~oliveiraesilva/

**The title of the lecture:** “The Stein-Tomas inequality: three recent improvements”

**Abstract:** The Stein-Tomas inequality dates back to 1975 and is a cornerstone of the Fourier restriction theory. Despite its respectable age, it is fertile ground for current research. The goal of this talk is threefold: we present a recent proof of the sharp endpoint Stein-Tomas inequality in three space dimensions; we present a variational refinement and withdraw some consequences; and we discuss how to improve the Stein-Tomas inequality in the presence of certain symmetries.

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**Seminar on ****May 09, 2022**

**Speaker: **Prof. Dr. **Volker Mehrmann,** TU Berlin, Germany; https://en.wikipedia.org/wiki/Volker_Mehrmann

**The title of the lecture:** “Modeling analysis and numerical simulation of multi-physical systems: A change of paradigm”

**Abstract:** Most real-world dynamical systems consist of subsystems from different physical domains, modelled by partial differential equations, ordinary differential equations, and algebraic equations, combined with input and output connections. In recent years, the class of dissipative port-Hamiltonian (pH) systems have emerged as a very efficient new modelling methodology to deal with such complex systems. The main reasons are that the network-based interconnection of pH systems is again pH, Galerkin projection in PDE discretization and model reduction preserves the pH structure and the physical properties are encoded in the geometric properties of the flow as well as the algebraic properties of the equations. Furthermore, dissipative pH systems form a very robust representation under structured perturbations and directly indicate Lyapunov functions for stability analysis. We discuss dissipative pH systems and describe, how many classical models can be formulated in this class. We illustrate some of the nice algebraic properties, including local canonical forms, the formulation of an associated Dirac structure, and the local invariance under space-time dependent diffeomorphisms.

The results are illustrated with some real-world examples.

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**Seminar on ****June 20, 2022**

**Speaker: **Prof. **Lars-Erik Persson**, UiT The Arctic University of Norway; https://www.larserikpersson.se/

**The title of the lecture:** “On my life with Hardy and his inequalities”

**Dedication:** This lecture is dedicated to the memory of our dear friend, colleague and important collaborator Professor Vakhtang Kokilashvili.

**Abstract:** First I will describe some background and historical remarks from the beginning of this remarkable story, which started around 100 years ago. After that, I will present a fairly new convexity approach to proving Hardy-type inequalities, which was not discovered by Hardy himself and many others, see [1], [2] and [3]. I continue by presenting some selected parts of the story until 2017, where I myself have been involved to some extent, see [1] and [4]. Finally, I present some examples of remarkable new results after 2017, which show that this area is still a source of inspiration for new research. In particular, to also illustrate my close connection to Georgian mathematics I mention some new Hardy-type results we developed and used in our new book [5]. Several open questions are pointed out.

[1] A. Kufner, L.E. Persson and N. Samko, Weighted Inequalities of Hardy Type, World Scientific, Second Edition, New York, London, etc., 2017 (480 pages).

[2] L.E. Persson, Lecture Notes, Collège de France, Pierre-Louis Lions Seminar, November 2015 (48 pages).

[3] C. Niculescu and L.E. Persson, Convex Functions and Their Applications, CMS Books in Mathematics, Springer, Second Edition, 2018 (431 pages)

[4] V. Kokilashvili, A. Meskhi and L.E. Persson, Weighted Norm Inequalities for Integral transforms with Product Kernels, Nova Scientific Publishers, Inc., New York, 2010 (355 pages).

[5] L.E. Persson, G. Tephnadze and F. Weisz, Martingale Hardy Spaces and Summability of Vilinkin-Fourier Series, Springer, New York, to appear 2022 (610 pages).

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**Seminar on ****June 6, 2022**

**Speaker: **Prof. **Björn Birnir**, CNLS and the University of California at Santa Barbara; https://birnir.math.ucsb.edu/

**The title of the lecture:** “The statistical theory of Stochastic Nonlinear Partial Differential Equations with application to the angiogenesis equations”

**Abstract: **We develop the statistical theory for stochastic nonlinear PDEs with both additive and multiplicative noise. The canonical example is the stochastic Navier-Stokes equation. We solve the Kolmogorov-Hopf equation for the invariant measure determining the statistical quantities. Then the theory is applied to the stochastic angiogenesis equations describing how veins grow through the human body.

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**Seminar on ****May 23, 2022**

**Speaker: **Prof. **Oleksiy Karlovych**, NOVA University Lisbon, Portugal; https://docentes.fct.unl.pt/oyk/

**The title of the lecture:** “Algebras of convolution type operators with continuous data do not always contain all rank one operators”

**Abstract:** See poster

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**Seminar on ****May 9, 2022**

**Speaker: **Prof. **Volker Mehrmann**, TU Berlin, Germany; https://www.bimos.tu-berlin.de/menue/bimos_people/members/professors/volker_mehrmann/, https://en.wikipedia.org/wiki/Volker_Mehrmann

**The title of the lecture:** “Modeling analysis and numerical simulation of multi-physical systems: A change of paradigm”

**Abstract:** Most real-world dynamical systems consist of subsystems from different physical domains, modeled by partial-differential equations, ordinary differential equations, and algebraic equations, combined with input and output connections. To deal with such complex systems, in recent years the class of dissipative port-Hamiltonian (pH) systems have emerged as a very efficient new modeling methodology. The main reasons are that the network-based interconnection of pH systems is again pH, Galerkin projection in PDE discretization and model reduction preserves the pH structure and the physical properties are encoded in the geometric properties of the flow as well as the algebraic properties of the equations. Furthermore, dissipative pH system form a very robust representation under structured perturbations and directly indicates Lyapunov functions for stability analysis. We discuss dissipative pH systems and describe, how many classical models can be formulated in this class. We illustrate some of the nice algebraic properties, including local canonical forms, the formulation of an associated Dirac structure, and the local invariance under space-time dependent diffeomorphisms. The results are illustrated with some real-world examples.

**Download poster:** Link to Download

**Seminar on ****April 25, 2022**

**Speaker: **Prof. **Natasha Samko**, The Arctic University of Norway, Narvik, Norway; http://www.nsamko.com/

**The title of the lecture:** “Weighted boundedness of certain sublinear operators in generalized Morrey spaces on quasi-metric measure spaces under the growth condition”

**Abstract:** We prove the weighted boundedness of Calderón-Zygmund and maximal singular operators in generalized Morrey spaces on quasi-metric measure spaces, in general non-homogeneous, only under the growth condition on the measure, for a certain class of weights. The weights and characteristics of the spaces are independent of each other. The weighted boundedness of the maximal operator is also proved in the case when lower and upper Ahlfors exponents coincide with each other. Our approach is based on two important steps. The first is a certain transference theorem, where without using homogeneity of the space, we provide a condition which ensures that every sublinear operator with the size condition, bounded in Lebesgue space, is also bounded in generalized Morrey space. The second is a reduction theorem which reduces the weighted boundedness of the considered sublinear operators to that of weighted Hardy operators and the non-weighted boundedness of some special operators.

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**Seminar on ****April 11, 2022**

**Speaker: **Professor **Alexander Pushnitski**, King’s College London, UK; https://www.kcl.ac.uk/people/alexander-pushnitski

**The title of the lecture:** “The spectra of some arithmetical matrices”

**Abstract:** I will discuss the spectral theory of a family of infinite arithmetical matrices, whose (n,m)-th entry involves the least common multiple of n and m, denoted LCM(n,m). The simplest example of such a matrix is {1/LCM(n,m)}, where n,m range over natural numbers. It turns out that an explicit formula for the asymptotics of eigenvalues of this matrix can be given. This is recent joint work with Titus Hilberdink (Reading).

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**Seminar on March 28****, 2022**

**Speaker: **Doctoral student** Duvan Cardona Sanchez, **Ghent University; https://sites.google.com/site/duvancardonas/

**The title of the lecture:** “Oscillating Fourier multipliers theory: geometric aspects and the role of the symmetries”

**Abstract:** Oscillating Fourier multipliers on the torus and on **R**ⁿ play a fundamental role in analysis and PDE and in the setting of Lie groups are still a subject of intensive research. The classical results by Fefferman and Stein in this direction (published between 1970 and 1972 in Acta Math.) have consolidated a fundamental theory for the harmonic analysis of these operators, even, in a more general setting that contains Calderón-Zygmund singular integrals of convolution type. In this talk, we present some recent results that extend the Fefferman and Stein theory of oscillating Fourier multipliers to arbitrary Lie groups of polynomial growth.

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**Seminar on March 14****, 2022**

**Speaker: **Prof. **Hans Georg Feichtinger,** University of Vienna;

https://www.univie.ac.at/nuhag-php/home/fei.php

**The title of the lecture:** “The Banach Gelfand Triple and its role in Classical Fourier Analysis and Operator Theory”

**Abstract:** The Banach Gelfand Triple (S0, L2, S0*)(Rd) (which arose in the context of Time-Frequency Analysis) is a simple and useful tool, both for the derivation of mathematically valid theorems AND for teaching relevant concepts to engineers and physicists (and of course mathematicians, interested in applications!).

In this context, the basic terms of an introductory course on Linear System’s Theory can be explained properly: Translation invariant systems are viewed as linear operators, which can be described as convolution operator by some impulse response, whose Fourier transform is well defined (and is called transfer function), and there is a kernel theorem:

Operators T : S0(Rd) to S0*(Rd) have a "matrix representation" using some sigma in S0*(R2d). Most importantly, dual space S0 ∗ (Rd), the space of so-called mild distributions, contains all kinds of objects relevant for signal processing: periodic signals, discrete signals, and of course discrete and periodic signals. One can show that the generalized Fourier transform for such functions works well and reduced to the DFT/FFT (Fast Fourier Transform).

An important tool is the STFT (Short-Time Fourier Transform). Mild distributions are exactly those tempered distributions which have a bounded short-time Fourier transform, and the w∗-convergence just corresponds to uniform convergence of the STFT over compact subsets of the time-frequency plane.

**Slides:** Click to see

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**Seminar on ****February 28, 2022**

**Speaker: **Prof. **George Tephnadze,** The University of Georgia;

https://www.researchgate.net/profile/George-Tephnadze

**The title of the lecture:** “Almost everywhere convergence of partial sums of trigonometric and Vilenkin systems and certain summability methods”

**Abstract:** The classical theory of the Fourier series deals with the decomposition of a function into sinusoidal waves. Unlike these continuous waves, the Vilenkin (Walsh) functions are rectangular waves. There are many similarities between these theories, but there exist differences also. Much of these can be explained by modern abstract harmonic analysis, combined with martingale theory.

This talk is devoted to investigating tools that are used to study almost everywhere convergence of the partial sums of trigonometric and Vilenkin systems. In particular, these methods combined with martingale theory helps to give a simpler proof of an analogy of the famous Carleson-Hunt theorem for Fourier series with respect to the Vilenkin system. We also define an analogy of Lebesgue points for integrable functions and we will describe which certain summability methods are convergent in these points.

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**Seminar on ****February 14, 2022**

**Speaker: **Prof. **Paata Ivanisvili,** University of California, USA;

https://sites.google.com/view/paata

**The title of the lecture:** “Convex hull of a space curve”

**Abstract:** Finding a simple description of a convex hull of a set K in n-dimensional Euclidean space is a basic problem in mathematics. When K has some additional geometric structures one may hope to give an explicit construction of its convex hull. A good starting point is when K is a space curve. In this talk I will describe convex hulls of space curves which have a "very" positive torsion. In particular, we obtain parametric representation of the boundary of the convex hull, different formulas for their Euclidean volumes of the convex hull, the area of its boundary, and the solution to a general moment problem corresponding to such curves.

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**Seminar on ****January 31, 2022**

**Speaker: **Prof. **Heiko Gimperlein,** University of Innsbruck and University of Parma;

https://www.macs.hw.ac.uk/~hg94/

**The title of the lecture:** “Boundary regularity for fractional Laplacians: a geometric approach”

**Abstract:** We consider the sharp boundary regularity of solutions to the Dirichlet problem for the fractional Laplacian on a smoothly bounded domain in Euclidean space. The fractional Laplacian is defined via the extension method, as a Dirichlet-to-Neumann operator for a degenerate elliptic problem in a half-space of one higher dimension. We use techniques from geometric microlocal analysis to analyse the regularity of solutions, with particular emphasis on asymptotic expansions and Hölder continuous data. Detailed and sharp results about this problem have been obtained by Gerd Grubb, and we present a complementary approach. Extensions to polygonal domains are mentioned.

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**Seminar on January 17, 2022**

**Speaker:** **Nikolai L. Vasilevski**, Department of Mathematics, CINVESTAV, Mexico City, Mexico;

https://www.math.cinvestav.mx/~nvasilev/

**The title of the lecture:** “On analytic type function spaces and direct sum decomposition of $L_2(D, d\nu)$”

**Abstract:** Link to see abstract

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**Seminar on ****December 20**

**Speaker: ****Prof. Dr. Alexander Mielke, **Weierstrass Institute for Applied Analysis and Stochastics and Humboldt-Universität zu Berlin;

https://www.wias-berlin.de/people/mielke/

**The title of the lecture:** “On a rigorous derivation of a wave equation with fractional damping from a system with fluid-structure interaction”

**Abstract: **We consider a linear system that consists of a linear wave equation on a horizontal hypersurface and a parabolic equation in the half-space below. The model describes longitudinal elastic waves in organic monolayers at the water-air interface, which is an experimental setup that is relevant for understanding wave propagation in biological membranes. We study the scaling regime where the relevant horizontal length scale is much larger than the vertical length scale and provide a rigorous limit leading to a fractionally damped wave equation for the membrane. We provide the associated existence results via linear semigroup theory and show convergence of the solutions in the scaling limit. Moreover, based on the energy–dissipation structure for the full model, we derive natural energy and a natural dissipation function for the fractionally damped wave equation with a time derivative of order 3/2.

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**Seminar on ****December 06**

**Speaker: ****Sergey MIKHAILOV, **Professor of Computational and Applied Mathematics, Dept. of Mathematical Sciences, Brunel University London, UK,

http://people.brunel.ac.uk/~mastssm/

**The title of the lecture:** “Volume and Layer Potentials for the Stokes System with Non-smooth Anisotropic Viscosity Tensor and Some Applications”

**Abstract: Link to see abstract**

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**Seminar on November 22**

**Speaker: Prof. ****Vladimir Rabinovich**, National Polytechnic Institute of Mexico, ESIME Zacatenco,

https://www.researchgate.net/profile/Vladimir-Rabinovich

**The title of the lecture: “Interaction problems for the Dirac operators on ****R**^{n}**”**

**Abstract:** Link to see abstract

Recorded talk: N/A

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**Seminar on November 8**

**Speaker: Prof. Eugine Shargorodsky**, King’s College London, UK,

https://www.kcl.ac.uk/people/eugene-shargorodsky

**The title of the lecture: “Negative eigenvalues of two-dimensional Schrödinger operators”**

**Abstract:** According to the celebrated Cwikel-Lieb-Rozenblum inequality, the number of negative eigenvalues of the Schrödinger operator , $-\Delta-V, V>=0$ on $L_2((R^d), d>=3$, is estimated above by $$const\int_{\R^d}V(x)^{d/2}dx $$ It is well known that this estimate does not hold for d=2. I will present estimates for the number of negative eigenvalues of a two-dimensional Schrödinger operator in terms of weighted $L_1$-norms and $LlogL$ type Orlicz norms of the potential obtained over the last decade and discuss related open problems.

Recorded talk: N/A

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**Seminar on October 25, 2021**

Speaker: Prof. Grigori Rozenblioum, the Chalmers University of Technology and University of Gothenburg, Sweden; St. Petersburg State University and Leonhard Euler International Mathematical Institute in Saint Petersburg,

https://www.chalmers.se/en/staff/Pages/grigori-rozenblioum.aspx

**The title of the lecture: “**Spectral properties of the Neumann-Poincare operator for the elasticity system and related questions about zero-order pseudodifferential operators**”**

**Abstract: **The Neumann-Poincare operator is the double layer potential. Unlike the well-studied electrostatic problem where this operator is compact, the elasticity system is not compact. For the 3D homogeneous isotropic body with smooth boundary, this operator has an essential spectrum consisting of 3 points determined by Lame parameters. The eigenvalues of this operator may converge only to these points. We developed the machinery of spectral analysis of polynomially compact pseudodifferential operators and use it to find the asymptotics of these eigenvalues.

**Recorded talk**** N/A**

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**Seminar on October 11, 2021**

**Dorina Mitrea,** Professor and Chair Department of Mathematics,

https://www.baylor.edu/math/index.php?id=962935

**The title of the lecture: “On boundedness of Singular Integral Operators on Holder Spaces”**

**Abstract:**A central question in Calderon-Zygmund Theory is that of the L2-boundedness of Singular Integral Operators. An equally relevant issue is that of the boundedness of Singular Integral Operators on the scale of Holder spaces. In this talk I will present results in this regard which are applicable to large classes of Singular Integral Operators on general geometric settings.

**Recorded talk**** N/A**

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**Seminar on June 21**

**Dr. Marius Mitrea**, professor of mathematics and chair of the mathematics department at Baylor University, Waco, Texas, United States;

https://www.baylor.edu/math/index.php?id=962939

**The title of the lecture: “Singular Integrals, Geometry of Sets, and Boundary Problems”**

**Abstract:**Presently, it is well understood what geometric features are necessary and sufficient to guarantee the boundedness of convolution-type singular integral operators on Lebesgue spaces. This being said, dealing with other function spaces where membership entails more than a mere size condition (like Sobolev spaces, Hardy spaces, or the John-Nirenberg space BMO) requires new techniques. In this talk I will explore recent progress in this regard, and follow up the implications of such advances into the realm of boundary value problems.

**Recorded talk**** N/A**

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**Seminar on June 7**

**Prof. Valery Smyshlyaev**, University College London, UK;

https://scholar.google.com/citations?user=ckp2DwMAAAAJ

**The title of the lecture: “High-frequency scattering of whispering gallery waves by boundary inflection: asymptotics and boundary integral equations”**

**Abstract:**The talk is on a long-standing problem of scattering of a high-frequency whispering gallery wave by boundary inflection. Like Airy ODE and associated Airy function are fundamental for describing transition from oscillatory to exponentially decaying asymptotic behaviors, the boundary inflection problem leads to an arguably equally fundamental canonical inner boundary-value problem for a special PDE describing transition from a ``modal'' to a ``scattered'' high-frequency asymptotic regimes. An additional recent motivation comes from the problem seemingly holding the keys for numerical analysis of Galerkin-type methods for boundary integral equations (BIE) in high-frequency scattering by smooth non-convex obstacles. The talk first reviews the background, on asymptotically reducing a problem described by Hemlholtz equation to the inner problem. The latter is a Schr\"odinger equation on a half-line with a potential linear in both space and time, and was first formulated and analysed by M.M. Popov starting from 1970-s, and has been intensively studied since then (see [1] for a review and some further references). The associated solutions have asymptotic behaviors with a discrete spectrum at one end and with a continuous spectrum at the other end, and of central interest is to find the map connecting the above two asymptotic regimes. We report recent result in [1] proving that the solution past the inflection point has a ``searchlight'' asymptotics corresponding to a beam concentrated near the limit ray. This is achieved by a non-standard perturbation analysis at the continuous spectrum end, and the result allows interpretations in terms of a unitary scattering operator connecting the modal and the scattered asymptotic regimes.We also review some most recent progress on a reducing the inner problem to one-dimensional boundary integral equations and their further analysis. The integral equations are of improper weakly singular Volterra type of both first and second kinds (with appropriate jump conditions for the latter) and can be shown to be well-posed. Their subsequent regularization allows to express the solution in term of limit of uniformly convergent Neumann series with anticipated further benefits for the problem's asymptotic and possibly numerical analyses. Some parts of the work are joint with Ilia Kamotski, and with Shiza Naqvi.

**Recorded talk**** N/A**

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**Seminar on May 24**

**Prof. Simon Chandler-Wilde**, University of Reading, UK;

http://www.personal.reading.ac.uk/~sms03snc/

**The title of the lecture: “Do Galerkin methods converge for the classical 2nd kind boundary integral equations in polyhedra and Lipschitz domains? ”**

**Abstract:**The boundary integral equation method is a popular method for solving elliptic PDEs with constant coefficients, and systems of such PDEs, in bounded and unbounded domains. An attraction of the method is that it reduces solution of the PDE in the domain to solution of a boundary integral equation on the boundary of the domain, reducing the dimensionality of the problem. Second kind integral equations, featuring the double-layer potential operator, have a long history in analysis and numerical analysis. They provided, through C. Neumann, the first existence proof to the Laplace Dirichlet problem in 3D, have been an important analysis tool for PDEs through the 20th century, and are popular computationally because of their excellent conditioning and convergence properties for large classes of domains. A standard numerical method, in particular for boundary integral equations, is the Galerkin method, and the standard convergence analysis starts with a proof that the relevant operator is coercive, or a compact perturbation of a coercive operator, in the relevant function space. A long-standing open problem is whether this property holds for classical second kind boundary integral equations on general non-smooth domains. In this talk we give an overview of the various concepts and methods involved, reformulating the problem as a question about numerical ranges. We solve this open problem through counterexamples, presenting examples of 2D Lipschitz domains and 3D Lipschitz polyhedra for which coercivity does not hold. This is joint work with Prof Euan Spence, Bath.

**Recorded talk**** N/A**

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**Seminar on April 19**

**Prof. Gerd Grubb**, Department of Mathematical Sciences, University of Copenhagen (UCPH);

https://www.math.ku.dk/english/staff/?pure=en/persons/64670

**The title of the lecture: “Sharp regularity results for electromagnetic fields in Lipschitz domainsBoundary problems for fractional-order operators”**

**Abstract:**There has recently been an upsurge of interest in the fractional Laplacian ${\left( -\Delta \right)}^{a}$ $\left( 0<a<1 \right)$ and other fractional-order pseudodifferential operators $P$, because of applications in financial theory and probability (and also in differential geometry and mathematical physics). The boundary problems for $P$ on subsets $\Omega$ of ${R}^{n}$ are challenging since $P$ is nonlocal; here there have mostly been used real methods from potential theory and integral operator theory, or probabilistic methods. As we know, the pseudodifferential point of view should be useful too, in view of Vishik and Eskin's work in the sixties, and many later works. I shall tell about a circle of results developed in the last 8 years, based on the mu-transmission condition introduced by Hörmander (in his book '85 and in a lecture note '66), telling also how they differ from the results in Eskin's book '81. The pseudodifferential methods have not been popular in the applications community, partly because Fourier transform techniques (and complex functions) do not seem to be part of the toolbox, partly because the ps.d.o. methods originally have

**Recorded talk**** N/A**

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**Seminar on March 22**

**Prof. Dr. Martin Costabel**, IRMAR, Institut Mathématique Université de Rennes 1;;

https://perso.univ-rennes1.fr/martin.costabel/

**The title of the lecture: “Sharp regularity results for electromagnetic fields in Lipschitz domains”**

**Abstract:** The standard energy spaces for the time-harmonic Maxwell equations in a bounded 3-dimensional domain are the Hilbert spaces of square-integrable vector fields whose divergence and curl are also square-integrable and whose tangential or normal components are zero on the boundary. Elements of these spaces are known to have additional regularity. The classical Gaffney inequality (1951) states that their gradients are square-integrable if the domain is smooth, and it is clear that there is less regularity if the boundary has corners and edges. For Lipschitz domains, regularity in the Sobolev space of order 1/2 has been known for 30 years, and recently a domain has been constructed that shows that in the scale of Sobolev spaces this is sharp. By the principle that Maxwell singularities are carried by gradients, this regularity result can also be equivalently formulated as a regularity result for the classical Dirichlet and Neumann problems of the Laplacian on bounded Lipschitz domains. I will describe the context and motivation for the result and explain the construction of the domain and some of its additional properties.

**Recorded talk**** N/A**

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**Seminar on March 8**

**Speaker: Prof. Dr. Pavel Exner,**Doppler Institute for Mathematical Physics and Applied Mathematics;

__https://gemma.ujf.cas.cz/~exner/personal.html__

**The title of the lecture: “Product formulae related to Zeno quantum dynamics”**

**Abstract:** We present a new class of product formulae which involve a unitary group generated by a positive self-adjoint operator and a continuous projection-valued function. The problem is motivated by quantum description of decaying systems, in particular, the Zeno effect coming from frequently repeated measurements. Applied to it, the formula expresses the dynamics of such a system. An example of a permanent position ascertaining leading to the effective Dirichlet condition is given.

**Recorded talk**** N/A**

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**Seminar on February 22**

**Speaker: Prof. Paata Ivanishvili,**North Carolina State University (NC), USA;

__https://sites.google.com/view/paata__

**The title of the lecture: “Rademacher type and Enflo Type Coincide”**

**Abstract:** Pick any finite number of points in a Hilbert space. If they coincide with vertices of a parallelepiped then the sum of the squares of the lengths of its sides equals the sum of the squares of the lengths of the diagonals (parallelogram law). If the points are in a general position then we can define sides and diagonals by labeling these points via vertices of the discrete cube {0,1}n. In this case the sum of the squares of diagonals is bounded by the sum of the squares of its sides no matter how you label the points and what n you choose. In a general Banach space we do not have parallelogram law. Back in 1978 Enflo asked: in an arbitrary Banach space if the sum of the squares of diagonals is bounded by the sum of the squares of its sides for all parallelepipeds (up to a universal constant), does the same estimate hold for any finite number of points (not necessarily vertices of the parallelepiped)? In the joint work with Ramon van Handel and Sasha Volberg we positively resolve Enflo's problem. Banach spaces satisfying the inequality with parallelepipeds are called of type 2 (Rademacher type 2), and Banach spaces satisfying the inequality for all points are called of Enflo type 2. In particular, we show that Rademacher type and enflo type coincide.

**Recorded talk**** N/A**

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**Seminar on February ****8**

**Speaker: Prof. Michael Ruzhansky,**Queen Mary University of London;

**The title of the lecture: “Nonharmonic operator analysis”**

**Abstract:** In this talk we will give a survey of our recent works on developing the nonharmonic symbolic calculus. This has applications to various questions for non-self-adjoint operators as well as for operators on manifolds with (or without) boundaries.

**Recorded talk**** N/A**

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**Seminar on January ****25**

**Speaker: Prof. Leonid Parnovski,**Department of Mathematics, University College London, UK

__http://www.homepages.ucl.ac.uk/~ucahlep/__

**The title of the lecture: “Floating mats and sloping beaches: spectral asymptotics of the Steklov problem on polygons”**

**Abstract:** I will discuss asymptotic behaviour of the eigenvalues of the Steklov problem (aka Dirichlet-to-Neumann operator) on curvilinear polygons. The answer is completely unexpected and depends on the arithmetic properties of the angles of the polygon.

**Recorded talk**** N/A**

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**Seminar on January ****11**

**Speaker: Prof. Maria J. ESTEBAN**, CEREMADE (CEntre de REcherche en MAthématiques de la DÉcision, French for Research Centre in Mathematics of Decision), Paris Dauphine University,

__https://www.ceremade.dauphine.fr/~esteban/MJEpage-engl.html__

**The title of the lecture: “Magnetic interpolations inequalities in dimensions 2 and 3”**

**Abstract:** In this talk I will present some results concerning magnetic inequalities, similar to Gagliardo-Nirenberg inequalities, but involving magnetic operators. We will first consider the case of a general magnetic field where general results will be proved, but without much concrete information. Then, in the particular cases of constant or Aharonov-Bohm magnetic fields, we will be able to make those results more precise and get better estimates, or even complete information, about the best constants in the inequalities, or about the optimal extremals.

**Recorded talk**** N/A**

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**Seminar on December 14**

**Speaker: Prof. Ari Laptev**, Department of Mathematics, Imperial College London, http://wwwf.imperial.ac.uk/~alaptev/

**The title of the lecture: “Magnetic rings”**

**Abstract:** We study functional and spectral properties of perturbations of a magnetic second-order differential operator on a circle. This operator appears when considering the restriction to the unit circle of a two dimensional Schrödinger operator with the Bohm-Aharonov vector potential. We prove some Hardy-type inequalities and sharp Keller-Lieb-Thirring inequalities.

**Recorded talk:** https://youtu.be/DEs7IWzOyKs

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**Seminar on November 30**

**Speaker: Prof. Mikhail Sodin,** Raymond & Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, https://en-exact-sciences.tau.ac.il/profile/sodin

**The title of the lecture: “Fourier uniqueness and non-uniqueness pairs”**

**Abstract:** Motivated by a remarkable discovery by Radchenko and Viazovska and by a recent work by Ramos and Sousa, we find conditions sufficient for a pair of discrete subsets of the real axis to be a uniqueness or a non-uniqueness pair for the Fourier transform. These conditions are not too far from each other. The uniqueness theorem can be upgraded to the frame bound and an interpolation formula, which in turn produce an abundance of Poisson-like formulas. This is a report on a joint work in progress with Aleksei Kulikov and Fedor Nazarov.

**Recorded talk:** https://youtu.be/D9G3Sp8CkLQ

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**Seminar on November 16**

**Speaker: Prof. Kristian Seip,** Norwegian University of Science and Technology, https://www.ntnu.edu/employees/kristian.seip

**The title of the lecture: “Fourier interpolation with the zeros of the Riemann zeta function”**

**Abstract:** Originating in work of Radchenko and Viazovska, a new kind of Fourier analytic duality, known as Fourier interpolation, has recently been developed. I will discuss the underlying general duality principle and present a new construction associated with the non-trivial zeros of the Riemann zeta function, obtained in joint work Andriy Bondarenko and Danylo Radchenko. I will emphasize how the latter construction fits into the theory of the Riemann zeta function.

**Recorded talk:** Play recording (58 mins)

**Download poster:** Link to Download

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