**The seminar is joint for the division of Analysis and Probability. Unless otherwise indicated, the seminar takes place Tuesdays at 1515-1615 in MVL:14.**

**18-10-30**

**Andrew McKee**

*Multipliers and approximation properties*

The talk is aimed at an audience

**familiar with the basics of groups and abstract harmonic analysis**.

**18-11-06**

**Genkai Zhang**

**18-11-08 (Thursday at 15:15 in MVL:14)**

**Eero Saksman (**

**University of Helsinki)**

*On imaginary chaos*

The talk will contain both probability theory and analysis and so could be of interest to anyone in the division.

**Some familiarity with probability will be assumed**.

**Past Seminars:**

**18-01-23**

**Jérémie Joudioux**

**(Radboud University Nijmegen)**

*The vector-field method for the relativistic transport equation with application to the Einstein-Vlasov system*

The vector-field method, developed by Klainerman in the 80s was the key to understand the global existence and asymptotic behaviour of solutions to some nonlinear wave equation. This method is based on the construction of appropriate commutators with the wave equation. Hence, coercive estimates for a solution of a wave equation can be extended to derivatives of this solution, and pointwise weighted estimates can be derived. I will briefly present in this talk this vector-field method, and explain how it extends to relativistic transport equations. Then, after briefly introducing the stability problems in General Relativity, I will explain how this method can be used to prove the stability of the "ground state" of the Einstein equations in the presence of a matter field modelled by a transport equation (Einstein-Vlasov system). This is joint work with David Fajman (Vienna) and Jacques Smulevici (Orsay-ENS) (arXiv:1707.06141).

The first two parts of the talk will be kept as elementary as possible to an

**audience familiar with basic PDE estimates**. The last part of the talk will give a quick and simplified overview on the proof of the stability of Minkowski space.

**18-01-30**

**Hasse Carlsson**

*Estimates of the renewal measure*

Let $\mu$ be a nonlattice probability meassure on the line, and $\nu=\sum_0^\infty \mu^{n*}$ its renewal measure. Blackwell's renewal theorem states that

$$

\lim_{x\to+\infty} \nu(x+I)=|I|/\mu_1,

$$

where $\mu_1=\int xd\mu(x)$ is the first moment of $\mu$ and $|I|$ the length of the interval $I$.

A lot of papers have studied the rate of this convergence. My talk is yet another attempt in this quest. In particular I am intersted in the case where $\mu$ has finite moments of order $\alpha$ (i.e. $\int|x|^\alpha d\mu(x)<\infty$) when $1>\akpha<2$.

The methods are Fourier analytic.

Download Abstract

The talk is aimed at the

**whole division**.

**18-02-06**

**Peter Sjögren**

*Estimates for some operators associated with the Laplacian with drift in Euclidean space*

Let $v\neq 0$ be a vector in $\mathbb{R}^n$. Consider the Laplacian on $\mathbb{R}^n$ with drift $\Delta_v=\Delta+2v\cdot\Nabla$ and the measure $d\mu(x)=e^{2\langle v,x\rangle}dx$, with respect to which $\Delta_v$ is self-adjoint. This measure has exponential growth with respect to the Euclidean distance. We study weak type $(1,1)$ and other sharp endpoint estimates for the related Riesz transforms of any order, and also for the vertical and horizontal Littlewood-Paley-Stein functions for the associated heat and Poisson semigroups.

This is joint work with Hong-Quan Li, Shanghai.

Download Abstract

The talk is intended for a

**general audience of analysts**.

**18-02-20**

**Przemys**

**ł**

**aw Ohrysko**

*Spectrally reasonable measures*

Let us recall that the measure on the circle group is said to have a natural spectrum iff its spectrum is equal to the closure of the image of the Fourier-Stieltjes transform. As the set of all measures does not possess good algebraic properties (it is not closed under addition, in particular) we introduced the concept of spectrally reasonable measures, i. e. a measure perturbing any measure with a natural to spectrum to a measure with a natural spectrum. It appears that the collection of all such measures has a Banach algebra structure. In my talk I will describe a wide class of examples of measures from this class including absolutely continuous ones. Moreover, I will prove that (except trivial cases) no discrete measure is reasonable. The presentation is based on the paper: ‘Spectrally reasonable measures’, St. Petersburg Math. J. 28 (2017), 259-271 written in collaboration with Michał Wojciechowski.

The talk is aimed at **everybody familiar with the basics of harmonic analysis and the Banach algebra theory**.

**18-03-13**

**Ramiz Reza (Indian Institute of Science, Bangalore)**

*Curvature inequalities for operators in the Cowen-Douglas class of a planar domain*

The talk is aimed at an audience with **background and interests in Operator Theory and Complex Analysis**.

**18-03-20**

**Sergey Zuev**

*Self-decomposable point processes*

Self-decomposable (SD) point processes constitute the class of point processes arising as a limit of superpositions of independent point processes. They are subclass of infinitely divisible point processes and contain the class of stable point processes. We characterise SD for the most general scheme involving independent branching operation on points, in particular, thinning. We show that a (regular) point process is SD iff it is representable as a series of branching operations applied to independent realisations of another point process whose scaled distribution correspond to the Levy measure.

The talk is aimed at a** ****general audience familiar with basic probability**.

**18-04-10****Simone Calogero**

*Self-gravitating elastic balls*

**A body is said to be self-gravitating if the only forces acting on the body are due to its internal strain and to its own self-generated gravitational field. The body is in equilibrium when these two forces balance each other at any point of the body. Many astrophysical objects (e.g. stars) are modelled as static self-gravitating bodies. Mathematically they are described as solutions to non-linear systems of elliptic PDEs. In this talk I will show how to prove existence of solutions using variational methods when the body is made up of elastic matter.**

**The talk is intended for a**

**general audience of analysts**.

**18-05-22**

**Johan Tykesson**

*Generalized divide and color models*

In this talk, we consider the following model: one starts with a finite or countable set V, a random partition of V and a parameter p in [0,1]. The corresponding Generalized Divide and Color Model is the {0,1}-valued process indexed by V obtained by independently, for each partition element in the random partition chosen, with probability p, assigning all the elements of the partition element the value 1, and with probability 1-p, assigning all the elements of the partition element the value 0.

A very special interesting case of this is the ``Divide and Color Model'' (which motivates the name we use) introduced and studied by Olle Häggström.

Some of the questions which we study here are the following. Under what situations can different random partitions give rise to the same color process? What can one say concerning exchangeable random partitions? What is the set of product measures that a color process stochastically dominates? For random partitions which are translation invariant, what ergodic properties do the resulting color processes have?

The motivation for studying these processes is twofold; on the one hand, we believe that this is a very natural and interesting class of processes that deserves investigation and on the other hand, a number of quite varied well-studied processes actually fall into this class such as the Ising model, the stationary distributions for the Voter Model, random walk in random scenery and of course the original Divide and Color Model.

The talk is based on joint work with Jeff Steif.

The talk is aimed at a **general probability audience**, but is probably accessible to a general analysis audience as well.

**18-05-24 (Thursday at 15:15 in MVL:14)**

**Artur Alho**

*Generic blow up for solutions of the wave equation towards Big Bang singularities*

We study the behaviour of smooth solutions to the wave-equation $\Box_g\phi=0$ near spacelike cosmological singularities. We consider as fixed backgrounds the explicit spatially homogenous flat Friedmann-Lemaitre-Robertson-Walker (flat FLRW) and Kasner metrics, and provide generic conditions on initial data given at a spacelike Cauchy hypersurface, such that the solutions blow up at polynomial, and logarithmic rates respectively.

This is joint work with Grigorios Fournodavlos (Cambridge) and Anne Franzen (Lisbon).

The talk is aimed at an

**audience familiar with Analysis and Geometry**.

**18-05-29**

**Perla Sousi (University of Cambridge)**

*Capacity of random walk and Wiener sausage in 4 dimensions*

In four dimensions we prove a non-conventional CLT for the capacity of the range of simple random walk and a strong law of large numbers for the capacity of the Wiener sausage. This is joint work with Amine Asselah and Bruno Schapira.

This talk is **primarily for a probability audience** but could be of interest to a wider analysis audience.

**18-06-08**

**(Friday at 13:15 in Pascal)**

**Eric Carlen (Rutgers University)**

*Inequalities for $L^p$-norms that sharpen the triangle inequality and complement Hanner's Inequality*

This is joint work with Rupert Frank and Elliott Lieb

The talk is aimed at a

**general analysis and probability audience**.

**18-06-13**

**(Wednesday at 15:15 in MVL:14)**

**Christian Hagendorf**

**(**

**Université Catholique de Louvain**

**)**

*On the transfer-matrix of the supersymmetric eight-vertex model*

The topic of this talk is the square-lattice eight-vertex model with weights $a,b,c,d$ obeying the relation $(a^2+ab)(b^2+ab)=(c^2+ab)(d^2+ab)$. In this case, the model possesses a hidden supersymmetry. I will show how to use this supersymmetry to prove that the transfer matrix of the model for $L=2n+1$ vertical lines and periodic boundary conditions along the horizontal direction possesses the doubly degenerate eigenvalue $\Theta_n = (a+b)^{2n+1}$. This proves a conjecture by Stroganov.

**audience working in mathematical physics**.

**18-06-21**

**(Thursday at 15:15 in MVL:14)**

**Robin Deeley (University of Colorado, Boulder)**

*Minimal dynamical systems and groupoids with prescribed K-theory*

I will speak about joint work in progress with Ian Putnam and Karen Strung. The goal of the project is to study the existence of minimal dynamical systems and more generally minimal equivalence relations. In particular, I will discuss the following question: given a compact Hausdorff space does there exist a minimal homeomorphism on it? Although the answer is no, a similar question has a positive answer for any finite CW-complex. This question and the question of which C*-algebras can be realized as groupoid C*-algebras related to minimal dynamical systems are the motivation for our constructions.

The **prerequisites for the first half of the talk are basic metric space theory and equivalence relations**. For the second half some knowledge of C*-algebra theory (in particular crossed products) will be required.

**18-06-28**

**(Thursday at 10:00 in MVL:14)**

**Paola Rioseco (**

**Universidad Michoacana de San Nicolás de Hidalgo**

**)**

*Mixing phenomena in relativistic kinetic gas distributions around black holes*

*As a first approximation to understand this phenomenon, I will explain the mixing and why it occurs for the simplest case of a potential in one dimension and for gas particles that satisfy the Vlasov equation. Subsequently, I will discuss some applications for the evolution of inhomogeneities in dark matter halos. Finally, I show that the same phenomena occurs for a collisionless, relativistic kinetic gas which is trapped in the gravitational potential of a (rotating) Kerr black hole.*

**aimed at an audience familiar with kinetic theory.**

**18-08-28 (at 13:15)**

**Sven Raum (Stockholms universitet)**

*C*-superrigidity of 2-step nilpotent groups*

The talk is aimed at an

**audience familiar with basic functional analysis and group theory**.

18-10-02

18-10-02

**Przemysław Ohrysko**

*Inversion problem in measure and Fourier-Stieltjes algebras*

In my talk, I will present the recent developments on the inversion problem in measures in Fourier-Stieltes algebras proposed by N. Nikolski in the paper ‘In search of the invisible spectrum’. The framework within the Banach algebra of all complex-valued Borel regular measures is as follows: suppose that we are given a measure on a locally compact Abelian group with norm one and infimum of the modulus of its Fourier-Stieltjes transform greater than some fixed constant. What is the minimal value of this constant assuring the invertibility of the measure? What can be said about the inverse? The non-triviliaty of this problem is justified via existence of the Wiener-Pitt phenomenon. Similar questions for Fourier-Stieltjes algebras (associated with non-commutative groups) will be discussed. The presented results can be found on arxiv.org in the preprint with the same title.

The talk is aimed at everybody **familiar with the basics of harmonic analysis and the Banach algebra theory**.

18-10-09

18-10-09

**Gunnar Carlsson**

**(Stanford University)**

*Persistent Homology, Theory and Applications*

*Persistent homology has been developed since roughly 2000 as a tool for "measuring" the shape of data sets and as a tool for feature generation for certain kinds of unstructured data. Ordinary homology has been used for many years to study the shape of spaces where one has complete information about the space, and persistent homology adapts these techniques to sampled situations. I will survey the subject with examples.*

**no particular knowledge of topology is required**. I will assume an

**elementary knowledge of linear algebra**.

**18-10-15 (Monday at 13:15 in MVH:12)**

**Lashi Bandara**

**(University of Potsdam)**

*First-order elliptic boundary value problems beyond self-adjoint*

*induced boundary operators*

The Bär-Ballmann framework is a comprehensive framework to consider elliptic boundary value problems (and also their index theory) for first-order elliptic operators on manifolds with compact and smooth boundary. A fundamental assumption in their work is that the induced operator on the boundary is symmetric. Many operators satisfy this requirement including the Hodge-Dirac operator as well as the Atiyah-Singer Dirac operator. Recently, there has been a desire to study more general operators with the quintessential example being the Rarita-Schwinger Dirac operator, which is an operator that fails to satisfy this hypothesis.

In this talk, I will present recent work with Bär where we dispense the symmetry assumption and consider general elliptic operators. The ellipticity of the operator still allows us to understand the spectral theory of the induced adapted operator on the boundary, modulo a lower order additive perturbation, as bi-sectorial operator. We use a mixture of methods coming from pseudo-differential operator theory, bounded holomorphic functional calculus, semi-group theory as well as methods arising from the resolution of the Kato square root problem to recover many of the results of the Bär-Ballman framework.

The talk is

**aimed at an audience familiar with geometry**(manifolds and bundles),

**some operator**

**theory**(adjoints, maximal operator, elliptic first-order).

**18-10-18 (Thursday at 09:00 in MVL:14)**

**David Fajman (University of Vienna)**

*Stability of the Milne model for the Einstein-Vlasov system*

**familiar with mathematical physics**.