### Talks in 2023

#### Kolloquium zum 50. Geburtstag von Johann Brauchart

**Title:**

**Speaker:**()

**Date:**20.10.2023

**Room:**HS BE01, Steyrergasse 30, EG

**Abstract:**

\section*{Programm:}

\begin{description}

\item{10:00-11:00} Peter Kritzer (JKU Linz)

Quasi-Monte Carlo using points with non-negative local discrepancy}

\item{11:00-12:00} Christoph Aistleitner (TU Graz)

TBA}

\item{12:00-14:00} Gemeinsames Mittagessen

\item{14:00-15:00} Martin Ehler (Universität Wien)

TBA}

\item{15:00-16:00} Josef Dick (University of New South Wales, Sydney)

TBA}

\end{description}

#### Combinatorics Seminar

**Title:**Majority bootstrap percolation on product graphs

**Speaker:**Anna Geisler (Graz University of Technology)

**Date:**Friday 6th October 12:30

**Room:**AE06, Steyrergasse 30

**Abstract:**

Bootstrap percolation} is a spreading process on graphs where starting with an initial set $A$ of infected vertices, other vertices become infected once a certain threshold $r$ of their neighbors are infected. We say that the set $A$ percolates if eventually all vertices of the graph are infected. When $r = d(v)/2$, where $d(v)$ is the degree of a vertex, that is, when a vertex becomes infected once half of its neighbors are already infected, we call the process majority bootstrap percolation}.

We analyse this process when the initial set $A$ is chosen randomly, with each vertex being assigned to $A_p$ with a fixed probability $p$ independently, which in some way represents the ‘typical’ behaviour for sets of density $p$. Balogh, Bollob\'{a}s and Morris considered this process on the hypercube, a well-studied geometric graph, and demonstrated a sharp threshold for the property that the set $A_p$ percolates. We determine a similar sharp threshold for a broader class of product graphs}, those arising as the cartesian product of many graphs of fixed order.

Joint work with Mauricio Collares, Joshua Erde and Mihyun Kang.

#### Combinatorics Seminar

**Title:**The (second-)largest component in spatial inhomogeneous random graphs

**Speaker:**Joost Jorritsma (Eindhoven University of Technology)

**Date:**Friday 29th September 16:15-16:45

**Room:**AE06 Steyrergasse 30, EG

**Abstract:**

In a series of papers with J\'ulia Komj\'athy and Dieter Mitsche, we uncover the deep relation between three types of connected components in a large class of supercritical spatial random graphs that includes long-range percolation, geometric inhomogeneous random graphs, and the age-dependent random connection model. Let $\mathcal{C}_n^\sss{(1)}$ and $\mathcal{C}_n^\sss{(2)}$ denote the largest and second-largest component in the graph restricted to a $d$-dimensional box/torus of volume $n$, and let $\mathcal{C}(0)$ be the component in the infinite graph that contains a vertex at the origin. We identify $\zeta\in(0,1)$ such that

\begin{align}

\Prob\big(|\mathcal{C}_n^\sss{(1)}|\le (1-\varepsilon)\E\big[|\mathcal{C}_n^\sss{(1)}|\big]\big)=\exp\big(-\Theta\big(n^{\zeta}\big)\big),\label{eq:ltld}

|\mathcal{C}_n^\sss{(2)}|\, \big/\, (\log n)^{1/\zeta}=\Theta_{\zeta}(1),\nonumber

\Prob\big(n\le |\mathcal{C}(0)|<\infty\big)=\exp\big(-\Theta\big(n^{\zeta}\big)\big),\nonumber

\end{align}

as $n$ tends to infinity.

In words, there is a single exponent $\zeta$ (depending only on a few parameters of the model) that is guiding the speed of the lower tail of large deviations for the giant, the size of the second largest component, and the decay of the finite cluster size distribution of the origin.

During the talk, I will explain intuition about the relation between the three quantities through the lens of an example. Time permitting, I discuss also the upper tail of large deviations of the giant component, i.e., $\Prob\big(|\mathcal{C}_n^\sss{(1)}|\ge (1+\varepsilon)\E\big[|\mathcal{C}_n^\sss{(1)}|\big]\big)$, and show that it behaves drastically different compared to the lower tail in~\eqref{eq:ltld}.

#### Combinatorics Seminar

**Title:**q-Counting Set-Valued Tableaux

**Speaker:**Alexander Lazar (Universite Libre de Bruxelles)

**Date:**Friday 29th September 15:00-15:30

**Room:**Webex and AE06

**Abstract:**

Set-valued tableaux are a generalization of young tableaux in which the cells of the tableaux are filled with nonempty sets of integers. These tableaux originated in the study of the $K$-theory of the Grassmannian, and they also have connections to Brill-Noether theory and to dynamical algebraic combinatorics. In this talk I will present some recent work with Sam Hopkins (Howard University) and Svante Linusson (KTH) which is motivated by the connection to combinatorial dynamical systems. Using probabilistic reasoning, we derive elegant product formulas for the $q$-enumeration of "barely set-valued" tableaux (that is, tableaux with exactly one extra element). Time permitting, I will also discuss some ongoing work with Linusson in which we extend our results to arbitrary set-valued tableaux of certain special shapes.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m4b9e22ad5fa0dc60554bf1575845ee90}

\]

#### Combinatorics Seminar

**Title:**SL(4) web bases from hourglass plabic graphs

**Speaker:**Stephan Pfannerer-Mittas (TU Wien)

**Date:**Friday 29th September 13:15-13:45

**Room:**AE06 Steyrergasse 30, EG

**Abstract:**

In 1995, Kuperberg introduced a remarkable collection of trivalent web bases which encode tensor invariants of $SL_3$. Extending these bases to general $SL_r$ has been an open problem ever since. We present a solution to the $r=4$ case by introducing a new generalization of Postnikov's plabic graphs. In this talk I will mostly focus on the beautiful combinatorics of these graphs and their relation to rectangular standard Young tableaux.

This is joint work with Christian Gaetz, Oliver Pechenik, Jessica Striker and Joshua Swanson.

#### Combinatorics Seminar

**Title:**Perfect matchings in random sparsifications of Dirac hypergraphs

**Speaker:**Vincent Pfenninger (University of Birmingham)

**Date:**Friday 29th September 11:30-12:00

**Room:**Webex and AE06

**Abstract:**

We show that, for $k \geq 3$ and $n$ divisible by $k$, if a $k$-uniform hypergraph $H$ on $n$ vertices has large enough minimum $(k-1)$-degree to guarantee a perfect matching, then asymptotically almost surely a $p$-random subhypergraph of $H$ also contains a perfect matching, provided that $p > \frac{C \log n}{ n^{k-1}}$. Our result strengthens Johansson, Kahn, and Vu's seminal solution to Shamir's problem and can be viewed as a 'robust' version of a hypergraph Dirac-type result by Rödl, Ruciński, and Szemerédi.

This is joint work with Dong Yeap Kang, Tom Kelly, Daniela Kühn, and Deryk Osthus.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=mb8d3461791088d2393546b3e2a9d4d8d}

\]

#### Vortrag im Rahmen des Seminars Operator Theory

**Title:**Spectral Estimates for Laplace operators, Geometry of Graphs, and Expanders

**Speaker:**Delio Mugnolo (Fernuniversität Hagen)

**Date:**Donnerstag, 5.10.2023, 12:15 Uhr

**Room:**TU Graz, Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

Abstract:

I will offer an invitation to spectral geometry of metric graphs. Several bounds on the spectral gap of the Laplacian on metric graphs with standard or Dirichlet vertex conditions will be derived. I will especially present estimates based on recently introdcuced metric quantities, such as the length of a shortest cycle (girth), the avoidance diameter, the torsional rigidity and the mean distance. Using known results about Ramanujan graphs, a class of expander graphs, we also prove that some of these metric quantities, or combinations thereof, do not deliver any spectral bounds with the correct scaling.

If time allows, I will briefly discuss the case of graphs with infintely many edges.

#### Combinatorics Seminar

**Title:**Dirac's type Problems for Hypergraphs

**Speaker:**José Diego Alvarado Morales (Universidade de São Paulo)

**Date:**Thursday 28th September 15:30-16:00

**Room:**Webex and AE06

**Abstract:**

In this talk, we have structured our presentation into two distinct parts.

First, we provide a concise review of the literature on Dirac's type problems for uniform hypergraphs,

summarizing results and methodologies.

In the second part, we delve into our ongoing research,

which focuses on the emergence of loose Hamilton cycles within subgraphs of random uniform hypergraphs.

Here, we outline our strategy and discuss the limitations of our proof.

Our main result states that:

the minimum $d$-degree threshold for loose Hamiltonicity relative to the

random $k$-uniform hypergraph $\mathbf{G}^{(k)}(n,p)$ coincides with

its dense analogue whenever $p$ exceeds $n^{- (k-1)/2+o(1)}$ (and the minimum $d$-degree threshold $\Omega(n^{-(k-d)}\log{n}))$.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m000462befb8d64231d5ee72ca1ff5e9e}

\]

#### Vortrag im Rahmen der ÖMG Tagung

**Title:**Primes in arithmetic progressions to smooth moduli

**Speaker:**Julia Stadlmann (University of Oxford)

**Date:**21.09.2023, 16:30 bis 17:00 Uhr

**Room:**HS 11.01 (Uni Graz)

**Abstract:**

The Bombieri-Vinogradov theorem asserts that for $q< x^{1/2} \log(x)^{-A}$, the number of primes $p < x$ with $p \equiv a$ mod $q$ is on average very close to $\pi(x)/\phi(q)$. An extension of this result to $q > x^{1/2}$ is a longstanding open problem. Following Zhang's breakthrough result on bounded gaps between primes~[1], attention turned towards moduli $q$ which are $x^\delta$-smooth. Polymath~[2] showed that the primes have exponent of distribution $0.52\dot{3}$ to smooth moduli.

Following the arguments of Polymath~[3], in this talk we sketch how equidistribution estimates for primes in arithmetic progressions to smooth moduli can be used to give bounds on $H_m = \liminf_{n \rightarrow \infty} (p_{n+m}-p_n)$. We also show how an alternative version of the $q$-van der Corput method can be used to improve the exponent of distribution to smooth moduli to $0.525$.

#### Plenarvortrag im Rahmen der ÖMG Tagung

**Title:**Density of rational points near manifolds

**Speaker:**Damaris Schindler (Goettingen University)

**Date:**21.09.2023, 14 bis 15 Uhr

**Room:**HS 12.01, Univ. Graz

**Abstract:**

tba

#### Öffentlicher Vortrag im Rahmen der ÖMG-Tagung

**Title:**Die Mathematik hinter künstlicher Intelligenz

**Speaker:**Philipp Grohs (Univ. Wien)

**Date:**21.9.2023, 18 Uhr

**Room:**Hörsaal 12.01, Univ. Graz (Heizhaus, Universitätsstr. 2-4)

**Abstract:**

Die Künstliche Intelligenz und Maschinelles Lernen haben in den letzten

Jahrzehnten unglaubliche Fortschritte erfahren, sei es in der Bilderkennung,

der Sprachverarbeitung oder der personalisierten Werbung. Auch in der

Wissenschaft konnten dank dieser Fortschritte in den letzten Jahren

bedeutende Durchbrüche erzielt werden, zum Beispiel in der Simulation der

Proteinfaltung. In diesem Vortrag werden wir die Rolle der Mathematik in der

Künstlichen Intelligenz erörtern, sowie einige grundlegenden mathematischen

Resultate besprechen.

#### Plenarvortrag im Rahmen der ÖMG-Tagung

**Title:**Stochastic Quantisation of Yang-Mills

**Speaker:**Martin Hairer (EPFL)

**Date:**Mi 20.9.2023, 09:00

**Room:**Hörsaal 12.02, Univ. Graz (Heizhaus)

**Abstract:**

We report on recent progress on the problem of building a stochastic process

that admits the hypothetical Yang-Mills measure as its invariant measure.

One interesting feature of our construction is that it preserves

gauge-covariance in the limit even though it is broken by our UV

regularisation. This is based on joint work with Ajay Chandra, Ilya

Chevyrev, and Hao Shen.

#### Plenarvortrag im Rahmen der ÖMG-Tagung

**Title:**Unit and distinct distances in typical norms

**Speaker:**Lisa Sauermann (Univ. Bonn)

**Date:**Mi 20.9.2023 10:30

**Room:**HS 12.02, Univ Graz (Heizhaus)

**Abstract:**

Given $n$ points in the plane, how many pairs among these points can have distance exactly $1$? More formally, what is the maximum possible number of unit distances among a set of $n$ points in the plane? This problem is a very famous and still largely open problem, called the Erd\H{o}s unit distance problem. One can also study this problem for other norms on $\mathbb{R}^2$ (or more generally on $\mathbb{R}^d$ for any dimension $d$) that are different from the Euclidean norm. This direction has been suggested in the 1980s by Ulam and Erd\H{o}s and attracted a lot of attention over the years. We give an almost tight answer to this question for almost all norms on $\mathbb{R}^d$ (for any given $d$). Furthermore, for almost all norms on $\mathbb{R}^d$, we prove an asymptotically tight bound for a related problem, the so-called Erd\H{o}s distinct distances problem. Our proofs combine combinatorial and geometric ideas with algebraic and topological tools.

Joint work with Noga Alon and Matija Buci\'{c}.

#### Vortrag im Rahmen der ÖMG Tagung

**Title:**Implicit control for L´evy-type dividend-impulse problem

**Speaker:**Zbigniew Palmowski (Wroclaw University of Science and Technology)

**Date:**19.09.2023, 16 bis 16:30 Uhr

**Room:**SR 11.06 (Uni Graz)

**Abstract:**

tba

#### Plenarvortrag im Rahmen der ÖMG Tagung

**Title:**Matrix Distributions and Insurance Risk Models

**Speaker:**Hansjörg Albrecher (University of Lausanne)

**Date:**19.09.2023, 09:00 bis 10:00 Uhr

**Room:**HS 12.01, Univ. Graz

**Abstract:**

n this talk, some recent developments on matrix distributions and their connection

to absorption times of inhomogeneous Markov processes are discussed. We show how and why

such constructions are natural tools for the modelling of insurance risks, for both non-life and

life insurance applications. A particular emphasis will be given on the higher-dimensional

case. We also illustrate how certain extensions to the non-Markovian case involve fractional

calculus and lead to matrix Mittag-Leffler distributions, which turn out to be a flexible and

parsimonious class for the modelling of large but rare insurance loss events.

#### Kolloquium

**Title:**Statistical Modeling and Data Science

**Speaker:**()

**Date:**21.09.; 27.09. und 29.09.2023

**Room:**SR für Statistik (NT03098), Kopernikusgasse 24, 3. OG.

**Abstract:**

Im Rahmen eines Kolloquiums im Fachgebiet ``Statistical Modeling and Data Science'' finden folgende Vorträge statt: \newline

21.09., 09:00 - 10:00 Uhr, Michael Scholz: ``Interpretable Local Machine Learning for Huge and Distributed Data''}\newline \newline

27.09., 09:00 - 10:00 Uhr, Jana Lasser: ``From Alternative conceptions of honesty to alternative facts in communications by U.S. politicians''} \newline \newline

27.09., 11:00 - 12:00 Uhr, Alejandra Avalos-Pacheco: ``Integrative Large-Scale Bayesian Learning: From Factor Analysis to Graphical Models''} \newline \newline

29.09., 09:00 - 10:00 Uhr, Dennis Schroers: ``Functional Data Analysis for Term Structure Models''} \newline \newline

29.09., 11:00 - 12:00 Uhr, Matthias Neumann: ``Parametric random set models for virtual testing of electrode materials''}

\newline

#### Österreichische Mathematikertagung 2023

**Title:**

**Speaker:**()

**Date:**18.9.-22.9.2023

**Room:**KFU Graz

**Abstract:**

{\bf Hauptvortragende:} [2mm]

Hansjörg Albrecher (Univ. Lausanne)

Mathias Beiglböck (Univ. Wien)

Martin Hairer (EPFL)

Robert Scheichl (Univ. Heidelberg)

Oliver Roche-Newton (Univ. Linz)

Sandra Müller (TU Wien)

Angkana Rüland (Univ. Heidelberg)

Lisa Sauermann (MIT)

Damaris Schindler (Univ. Göttingen)

Karin Schnass (Univ. Innsbruck)

Elisabeth Werner (CWRU, Cleveland)

Vortrag zum Förderungspreis 2023[2mm]

{\bf Öffentlicher Vortrag:} Philipp Grohs[2mm]

{\bf Minisymposien:}

{\it Additive Combinatorics} (A. Geroldinger, W. A. Schmid)

{\it Computational Topology and Geometry} (M. Kerber, O. Aichholzer, B. Vogtenhuber)

{\it Stochastic Mass Transport} (G. Pammer, M. Beiglböck)

{\it Inverse Problems in Imaging} (M. Holler, K. Bredies)

{\it Versicherungs- und Finanzmathematik} (S. Thonhauser)

{\it Lehrerinnen- und Lehrertag} (C. Dorner, C. Krause)

{\it High-Dimensional Approximation} (M. Ullrich, D. Krieg)

{\it Probabilistic Methods in Convexity} (J. Prochno, E. Werner)

{\it Set Theory} (V. Fischer, S. Müller)

{\it Continuous Optimization Methods} (R. Bot, C. Clason)

{\it Diophantine analysis and equidistribution} (C. Aistleitner, J. Thuswaldner)[2mm]

{\bf Registrierung:} {\tt https://oemg-tagung-2023.at/}

#### Plenarvortrag im Rahmen der ÖMG-Tagung

**Title:**Convex bodies -- barely floating

**Speaker:**Elisabeth Werner (Case Western Reserve University)

**Date:**Fr 22.9.2023, 09:00

**Room:**HS 12.02, Univ Graz (Heizhaus)

**Abstract:**

In analogy to the classical surface area, a notion of affine surface area -- invariant under affine transformations -- has been defined.

The isoperimetric inequality states that the usual surface area is minimized for a Euclidean ball. Affine isoperimetric inequality states that

affine surface area is maximized for ellipsoids.

Due to this inequality and its many other remarkable properties, the affine surface area

finds applications in many areas of mathematics and applied mathematics. This has led to intense research in recent years and numerous

new directions have been developed.

We will discuss some of them and we will show how affine surface area is related to a geometric object, that is interesting in its own right,

the floating body.

#### Seminarvortrag

**Title:**On unified frameworks for optimal control and least squares problems

**Speaker:**Dipl.-Ing. Richard Löscher, BSc (TU Graz)

**Date:**Dienstag, 8.8.2023, 10:00 Uhr

**Room:**TU Graz, Steyrergasse 30, Seminarraum AE06 (STEG050), EG

**Abstract:**

Abstract:

The aim of this talk is twofold.

Firstly, it focuses on the analysis of distributed optimal control problems constrained by partial differential equations (PDEs).

Under some natural assumptions on the involved differential operators, it is shown that this class of problems

admits a certain structure and can be analyzed in an abstract framework. This structure carries over to every conforming discrete setting, where

optimal approximation results are gained, depending on the regularity of the desired target and the cost parameter. Various

model problems, including a space-time optimal control problem for the wave equation, are shown to fit into this framework and numerical examples

will be given that support the theory.

Secondly, the stable solution of (ill-posed) problems of PDEs is discussed when using a least squares approach. Again, this will be

done in an abstract framework. Under merely the same assumptions as in the case of optimal control problems, a full analysis of the

continuous and the discrete setting are carried out, revealing that the approach provides a reliable error estimator under a

standard saturation assumption. Again, various applications of the framework are discussed and numerical examples are given.

#### Seminarvortrag

**Title:**Regular Approximation and T-Compatibility

**Speaker:**Dr. Martin Halla (Universität Göttingen)

**Date:**Montag, 7.8.2023, 10.30 Uhr

**Room:**TU Graz, Steyrergasse 30, Seminarraum AE06 (STEG050), EG

**Abstract:**

Regular Approximation and T-Compatibility

In the 1970s Stummel initiated the study of ``discrete approximation schemes'', which is a very generic framework to conduct the convergence analysis of numerical methods for e.g. source problems, holomorphic eigenvalue problems, nonlinear problems, etc.. In this context a most important property is the regularity of approximations. However, until recently this framework was only applied to weakly coercive problems for which the regularity of Galerkin approximations is easy to obtain.

In the last two decades the notion of weak T-coercivity became popular to study the well-posedness (Fredholmness) of all kinds of partial differential equations, e.g. Maxwells equations, interior transmission eigenvalue problems and dispersive transmission problems.

In this talk I present a technique to mimic the weak T-coercivity analysis on the discrete level to obtain the regularity and hence convergence of approximations. I present the application to Maxwells equations, mixed systems and the equations of stellar oscillations.

#### Optimization Seminar

**Title:**Quantum guesswork: a combinatorial instance of quantum hypothesis testing

**Speaker:**Michele Dall'Arno (Dep. of Computer Science and Engineering, Toyohashi University of Technology)

**Date:**4.8.2023, 10:00

**Room:**Seminar room AE06, Steyrergasse 30, ground floor

**Abstract:**

{

We consider a game-theoretical scenario in quantum information theory involving two parties, say Alice and Bob. At each round, Alice chooses a

quantum state from a given ensemble, known to both parties, and sends it to Bob. Bob is allowed to perform any quantum operation on the state and

to query Alice multiple times, one state at a time, until he correctly guesses the state. The game is repeated many times, and Bob's aim is to

minimize the average number of queries needed. This problem, known as quantum guesswork, can be reframed as an instance of quantum hypothesis

testing, and has therefore long been conjectured not to admit analytical solutions except for the cases in which the hypothesis testing problem

is solvable, that is, for binary and symmetric ensembles.

Here, we disprove such a belief by deriving conditions under which the guesswork problem (by definition, a continuous optimization) can be recast

as a combinatorial problem, and therefore can be solved analytically by exhaustive search. We further show that such conditions are verified by

any qubit ensemble, thus conclusively settling the problem in dimension two, and we show that in that case the guesswork is recast as an instance

of the quadratic assignment problem (QAP). In particular, the problem consists of assigning a given set of weights to a given set of

three-dimensional real vectors (the Bloch vectors of the states of the ensemble) in order to maximize the norm of the baricenter, thus

generalizing the (maximization version of the) turbine balancing problem to the three-dimensional, not-necessarily-symmetric case.

By showing that the corresponding Gram matrix is benevolent, we explicitly solve the QAP for a class of qubit ensembles whose image in the Bloch

sphere is an anti-prism, including uniform anti-prisms such as symmetric, informationally complete (SIC) and mutually unbiased basis (MUBs)

ensembles. For qubit ensembles whose image is centrally symmetric, we show that the size of the exhaustive search, as well as its complexity, can

be quadratically reduced. We exploit this fact to devise and implement a parallel branch and bound algorithm in the C programming language that

computes the guesswork of qubit ensembles of up to 30 states within hours.}

#### Vortrag

**Title:**Über geometrische Eigenschaften von Banachräumen

**Speaker:**Bernhard Aigner ()

**Date:**2.8.2023, 12:30 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

Vorgestellt werden ausgewählte geometrische Eigenschaften von Banachräumen, im Besonderen die klassischen Konvexitätseigenschaften strikte, lokal gleichmäßige und gleichmäßige Konvexität mit ausgewählten Beispielen und Anwendungen. Ferner wird ein kurzer Einblick in das junge Begriffsfeld der sogenannten Durchmesser zwei Eigenschaften gegeben, die als "maximale" Kontrastbegriffe zu Regularitätseigenschaften wie der Radon-Nikodym-Eigenschaft geschaffen wurden.

#### Combinatorics Seminar: Friday 7th July 15:00

**Title:**Almost every matroid has a rank-3 wheel or rank-3 whirl as minor

**Speaker:**Jorn van der Pol (University of Waterloo)

**Date:**7.7.2023, 15:00

**Room:**Online meeting (Webex)

**Abstract:**

Many conjectures -- but few results -- exist for the statistical properties of large "random" matroids. For example, the question of which matroids appear as a minor of almost every matroid has been settled for only a few matroids. I will present recent progress in this direction: almost every matroid has at least one of two particular matroids, the rank-3 wheel $M(K_4)$ or the rank-3 whirl $W^3$, as a minor.

At the heart of the argument lies a counting version of the Ruzsa--Szemerédi (6,3)-theorem on 3-uniform hypergraphs, which is then generalised in several ways to obtain the main result.

For this talk, no knowledge about matroids is assumed.

--------------------

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m49b4af86f873e7d36187a851c7fd52e4}

\]

Meeting number:

2734 674 7217

--------------------

#### Joint Seminar in Statistics and Econometrics

**Title:**Valid Inference Based on Multivariate Data: Statistical Toolboxes Designed to Answer Global and Local Questions

**Speaker:**Prof. Dr. ARNE BATHKE (University of Salzburg)

**Date:**6. Juli 2023, 17:00 h

**Room:**SR f. Statistik (NT03098), Kopernikusgasse 24/3.OG

**Abstract:**

When there are several endpoints (response variables) and different predictors, one typically wants to find out which predictors are relevant, and for which endpoints. We present two rather general approaches towards inference for multivariate data, one accommodating binary, ordinal, and metric endpoints, and the other allowing for a factorial design structure. The first approach uses rank-based statistics and an F-approximation of the sampling distribution, the second employs asymptotically valid resampling techniques (bootstrap). We also try to address the question of how well the proposed methods actually accomplish their goals, and how to use the respective toolboxes that have been developed.

#### Zahlentheoretisches Kolloquium

**Title:**The regular singular inverse problem in differential Galois theory

**Speaker:**Thomas Serafini (TU Graz)

**Date:**30.06.2023, 14:00 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie, Kopernikusgasse 24, 2.OG

**Abstract:**

The inverse problem in differential Galois theory consists of asking

which algebraic groups over a field k can be realized as Galois groups

of differential equations over a differential field K with constants k.

If k is algebraically closed of characteristic zero and K = k(z), the

answer has been known since the work of J.Hartmann in 2005, and the

answer is that every algebraic group can be realized.

Over \mathbb{C}(z), a stronger result has been known for a longer time: every

algebraic group can be realized as the differential Galois group of a

regular singular differential equation.

We intend to give an overview of the theory, and explain how a recent

result of R.Feng and M.Wibmer (2022) lets us extend the result known

over \mathbb{C} to arbitrary algebraically closed fields.

#### Combinatorics Seminar

**Title:**Universality for degenerate graphs

**Speaker:**Peter Allen (London School of Economics)

**Date:**Friday 30th June 12:30

**Room:**Online meeting (Webex)

**Abstract:**

A graph $\Gamma$ is universal for a family $\mathcal{F}$ of graphs if for each $F \in \mathcal{F}$ there is a copy of $F \in \Gamma$ (not necessarily induced, and the copies are not necessarily disjoint).

Alon and Capalbo considered the case that $\mathcal{F}$ is the family of $n$-vertex graphs with maximum degree $k$, and showed that there is a universal graph for this family with $O\left(n^{2-2/k}\right)$ edges, which is sharp. Alon asked what the answer is if one replaces 'maximum degree' with 'degeneracy'.

We give a probabilistic construction of a universal graph for the family of $n$-vertex $D$-degenerate graphs with $\tilde{O}\left(n^{2-1/D}\right)$ edges, which is optimal up to the polylog. In this talk, I will describe the construction and give most of the details of the proof of its universality.

This is joint work with Julia Boettcher and Anita Liebenau.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Zahlentheoretisches Kolloquium

**Title:**Arithmetic of cubic number fields: Jacobi–Perron, Pythagoras, and indecomposables

**Speaker:**Magdaléna Tinková (Czech Technical University and TU Graz)

**Date:**23.06.2023, 14:00

**Room:**Seminarraum Analysis-Zahlentheorie

**Abstract:**

Additively indecomposable integers are a useful tool in the study of universal quadratic forms or the Pythagoras number in totally real number fields. In the case of real quadratic fields, they can be derived from the continued fraction of concrete quadratic irrationalities. Thus, it is natural to ask whether there exists analogous relation between indecomposable integers and multidimensional continued fractions. In this talk, we will focus on the Jacobi–Perron algorithm and discuss elements originating from expansions of concrete vectors for several families of cubic fields. This is joint work with Vítězslav Kala and Ester Sgallová.

#### Combinatorics Seminar

**Title:**The asymptotic number of tournament score sequences

**Speaker:**Brett Kolesnik (Oxford University)

**Date:**Friday 23rd June 12:30

**Room:**Online meeting (Webex)

**Abstract:**

A tournament is an orientation of the complete graph. The score sequence lists the out-degrees in non-decreasing order. Works by Erdős and Moser, Winston and Kleitman, and Kim and Pittel bounded the number of score sequences. In this talk, we locate the precise asymptotics. These agree numerically with those conjectured by Takács. We will discuss connections to geometry, renewal theory, infinitely divisible probability distributions, the Erdős–Ginzburg–Ziv numbers, and random lattice walks.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Geometry Seminar

**Title:**Column-convex \{0,1\}-matrices, consecutive coordinate polytopes and flow polytopes

**Speaker:**GONZALEZ DE LEON Rafael Santiago (Loyola University Chicago)

**Date:**21.06.2023, 13:45 Uhr

**Room:**Seminarraum 2, Kopernikusgasse 24, 8010 Graz

**Abstract:**

We study normalized volumes of a family of polytopes associated with column-convex {0,1}-matrices. Such a family is a generalization of the family of consecutive coordinate polytopes, studied by Ayyer, Josuat-Vergès, and Ramassamy, which in turn generalizes a family of polytopes originally proposed by Stanley in EC1. We prove that a polytope associated with a column-convex {0,1}-matrix is integrally equivalent to a certain flow polytope. We use the recently developed machinery in the theory of volumes and lattice point enumeration of flow polytopes to find various expressions for the volume of these polytopes, providing new proofs and extending results of Ayyer, Josuat-Vergès, and Ramassamy. This is joint work with Chris Hanusa, Alejandro Morales, and Martha Yip.

#### Mathematisches Kolloquium

**Title:**Kolloquium aus Anlass des 60. Geburtstagesvon ao.-Univ.-Prof. Dr. Sophie Frisch

**Speaker:**()

**Date:**16.06.2023

**Room:**HS BE01, Steyrergasse 30, EG

**Abstract:**

10 Uhr: Prof. Dr. Carmelo Antonio FINOCCHIARO (University of Catania)

Titel: A Topological Insight on the Ideal Theory of Rings of Integer-Valued Polynomials

11 Uhr: Prof. Dr. Giulio PERUGINELLI (University of Padova)

Titel: The Ubiquity of Integer-Valued Polynomials: Polynomial Dedekind Domains

Gemeinsames Mittagessen

14 Uhr: Univ.-Prof. Dr. Daniel SMERTNIG (Universität Graz)

Titel: From Bass Rings to Monoids of Graph Agglomerations

15 Uhr: Dr. Roswitha RISSNER (Universität Klagenfurt)

Titel: Powers of Irreducibles in Rings of Integer-Valued Polynomials

#### Vortrag

**Title:**Non-local relativistic delta shell interactions

**Speaker:**Lukas HERIBAN (CTU Prague)

**Date:**Donnerstag, 15.6.2023, 12:45 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

The recent development of boundary triples theory allowed us to study self-adjointness of the Dirac operator perturbed by the singular potential $D_l^A=D+A\delta_\Sigma$, where $A$ is arbitrary hermitian matrix and $\delta_\Sigma$ stands for the single-layer distribution supported on closed non-self-intersecting Lipschitz-smooth curve or surface $\Sigma$ in $\mathbb{R}^n$, $n\in\{2,3\}$. It was proved that the operator $D_l^A$ corresponds to a self-adjoint extension of a symmetric operator $D_0\varphi = D\varphi$, $\varphi \in H^1_0(\mathbb{R}^n\setminus \Sigma)$. Contrary to the general belief, by studying seemingly similar formal operator $D^A_{nl} = D+A|\delta_\Sigma\rangle \langle \delta_\Sigma|$ we will be able to define completely new self-adjoint extensions of the operator $D_0$. Let us mention that this is not the case in one dimension, where operators $D^A_l$ and $D^A_{nl}$ are one and the same. Finally, in case of $C^2$ curve resp. surface we will be able to construct regular approximations and prove the norm-resolvent convergence to the operator $D_{nl}^A$ without renormalization.

#### Seminarvortrag

**Title:**Echoes of the Klein paradox in approximations of the relativistic point interactions

**Speaker:**Matej TUSEK (CTU Prague)

**Date:**Donnerstag, 15.6.2023, 12:15 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

Abstract:

When the one-dimensional relativistic point interactions are approximated by scaled regular potentials, the coupling constants have to be renormalized in the limit. This effect was observed by P. Šeba in 1989 for the first time in the paper called ”Klein’s Paradox and the Relativistic Point Interaction” (Lett. Math.Phys. 18). However, the paper does not contain any explicit explanation of a relation between the Klein paradox and the renormalization of the coupling constants. The aim of this short talk is to trace such a relation.

#### Zahlentheoretisches Kolloquium

**Title:**Energy, Discrepancy, and Polarization of Greedy Sequences on the Sphere

**Speaker:**Dr. Ryan Matzke (Vanderbilt University)

**Date:**07.06.2023, 14:15 Uhr

**Room:**SR AE06, Steyrergasse 30, EG

**Abstract:**

Greedy algorithms are surprisingly effective in a wide range of optimization problems, though have only recently been considered as a possible way to find point configurations with low discrepancy or energy.

In this talk, we will discuss the performance of a greedy algorithm on the sphere to minimize the Riesz energies

$$ E_s(\{ z_1, ..., z_N\}) = \sum_{i \neq j} \frac{1}{s} \|x-y\|^{-s}$$

and the quadratic spherical cap discrepancy

$$ D_2(\{ z_1, ..., z_N\}) = \int_{-1}^{1} \int_{\mathbb{S}^d} \Big| \frac{\# ( C(x,t) \cap \{ z_1, ..., z_N\} )}{N} - \sigma(C(x,t)) \Big|^2 d\sigma(x) dt.$$

As an intermediate step, we study the maximal Riesz polarization on the sphere,

$$ \mathcal{P}_s( \mathbb{S}^d , N) = \sup_{\omega_N \subset \mathbb{S}^{d}, |\omega_N| = N} \; \inf_{x \in \mathbb{S}^d} \sum_{y \in \omega_N} \frac{1}{s} \|x - y \|^{-s}.$$

In an analogue of the connection between energy and best packings, as $s \rightarrow \infty$, optimal polarization configurations become best coverings. We show that that the first and second order asymptotics of optimal Riesz polarization nicely parallel those of energy minimization, and greedily generated point sets are nearly optimal for polarization.

The research in this presentation is joint work with Dmitriy Bilyk (University of Minnesota), Michelle Mastrianni (University of Minnesota), and Stefan Steinerberger (University of Washington).

#### Geometry Seminar

**Title:**Generalized Modules of Diagonal Harmonics, and Triangular Combinatorics

**Speaker:**Bergeron François (Université du Québec à Montréal)

**Date:**07.06.2023, 13:45 Uhr

**Room:**Seminarraum 2, Kopernikusgasse 24/IV

**Abstract:**

The last decade has seen an ever growing research interest in the study of generalizations of spaces of “diagonal harmonics” for the symmetric group. Indeed, this bring into play natural interactions between Algebraic Combinatorics, Symmetric Function Theory, Representation Theory, Algebraic Geometry, Knot Theory, and Theoretical Physics. On the combinatorial side, the subject has evolved from the special case of “Classical Catalan Combinatorics” to the current all inclusive notion of “Triangular Catalan Combinatorics”, successively going through the contexts of “Rational Catalan Combinatorics” and then “Rectangular Catalan Combinatorics”, in increasing level of generality. Each of these has raised new questions in other fields. Just to illustrate, the rational case relates to calculation of the Khovanov-Rozansky homology of torus knots, whilst the rectangular case extends this to torus links.

We will describe interesting aspects of the combinatorics of the new triangular context, showing that it makes natural many relevant notions that previously appeared to be rather mysterious. We will also see that this context has nice closure properties that where lacking in previous ones. We will conclude with many open natural questions that remain to be explored.

#### Combinatorics Seminar

**Title:**Partitioning cubic graphs into isomorphic linear forests

**Speaker:**Gal Kronenberg (Oxford University)

**Date:**Friday 2nd June 12:30

**Room:**Online meeting (Webex)

**Abstract:**

The linear arboricity of a graph $G$, denoted by $\textrm{la}(G)$, is the minimum number of edge-disjoint linear forests (i.e. collections of disjoint paths) in $G$ whose union is all the edges of $G$. It is known that the linear arboricity of every cubic graph is $2$. In 1987 Wormald conjectured that every cubic graph with even number of edges, can be partitioned such that the two parts are isomorphic linear forests.

This is known to hold for Jeager graphs and for some further classes of cubic graphs (see, Bermond-Fouquet-Habib-Peroche, Wormald, Jackson-Wormald, Fouquet-Thuillier-Vanherpe-Wojda). In this talk, we will present a proof of Wormald's conjecture for all large connected cubic graphs.

This is joint work with Shoham Letzter, Alexey Pokrovskiy, and Liana Yepremyan.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Combinatorics Seminar

**Title:**Colouring hypergraphs with bounded codegree

**Speaker:**Dong-Yeap Kang (Birmingham University)

**Date:**Friday 26th May 12:30

**Room:**Online meeting (Webex)

**Abstract:**

A hypergraph is linear} if every pair of the vertices is contained in at most one edge. In 1972, Erd\H{o}s, Faber, and Lov\'{a}sz conjectured that every linear hypergraph on $n$ vertices with maximum degree at most $n$ has chromatic index at most $n$.

In this talk, we discuss a proof of the conjecture for all large $n$ and some generalisations. This is joint work with Deryk Osthus, Daniela Kühn, Abhishek Methuku, and Tom Kelly.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Zahlentheoretisches Kolloquium

**Title:**Second best approximations and the Lagrange spectrum

**Speaker:**Dmitriy Gayfulin (TU Graz)

**Date:**23.5.2023, 15:00

**Room:**HS A (Kopernikusgasse 24, 1st floor)

**Abstract:**

Given an irrational number $\alpha$ consider its irrationality measure function

$$

\psi_{\alpha}(t)=\min\limits_{1\le q\le t, q\in\mathbb{Z}}\|q\alpha\|.

$$

The set of all values of

$$

\lambda(\alpha)=\bigl(\limsup\limits_{t\to\infty} t\psi_{\alpha}(t)\bigr)^{-1},

$$

where $\alpha $ runs through the set $\mathbb{R}\setminus\mathbb{Q}$ is called the Lagrange spectrum $\mathbb{L}$. Denote by $\mathcal{Q}=\{q_1,q_2,\ldots,q_n,\ldots\}$ the set of denominators of the convergents to $\alpha$. One can consider another irrationality measure function

$$

\psi^{[2]}_{\alpha}(t)=\min\limits_{1\le q\le t, q\in\mathbb{Z},q\not\in\mathcal{Q}}\|q\alpha\|

$$ connected with the properties of so-called second best approximations. Or, in other words, approximations by rational numbers, whose denominators are not the denominators of the convergents to $\alpha$. Replacing the function $\psi_{\alpha}$ in the definition of $\mathbb{L}$ by $\psi^{[2]}_{\alpha}$, one can get a set $\mathbb{L}_2$ which is called the ''second'' Lagrange spectrum. In my talk I give the complete structure of the discrete part of $\mathbb{L}_2$.

#### Vortrag

**Title:**Regularization and finite element error estimates for constrained optimal control problems with energy regularization

**Speaker:**Richard Löscher (Institut für Angewandte Mathematik, TU Graz)

**Date:**Freitag, 12.5.2023, 9:00 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

#### Vorstellungsvortrag im Rahmen eines Habilitationsverfahrens

**Title:**Rational Points Near Hyper-Surfaces (Erinnerung)

**Speaker:**Niclas Technau (TU Graz)

**Date:**12.05.2023, 14:00 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie, Kopernikusgasse 24, 2.OG

**Abstract:**

How dense are the rational points close to a compact and smooth hyper-surface $M$ in $R^n$?

In fact, consider all rationals whose (common) denominator is at most $Q$ and which are $\delta$-close to $M$.

We discuss this quantitative question without requiring any knowledge of number theory at all, using only basic Fourier analysis instead.

One knows that as long as the Gaussian curvature of $M$ is uniformly bounded from below, the number of such rational points

is of size $\delta Q^n$ when $\delta > Q^{\varepsilon -1}$ --- as one expects from a simple volume-informed random model.

In this talk, we consider a rich class of hyper-surfaces M where the Gaussian curvature

vanishes at one point to a very high order, and we establish a sharp counting theorem.

Indeed, the aforementioned random model is now inadequate.Hence, we propose a refined random model.

This is based on joint work with Rajula Srivastava (U. Bonn).

#### Combinatorics Seminar

**Title:**Configurations of boxes

**Speaker:**Istvan Tomon (Umeå University)

**Date:**Friday 12th May 12:30

**Room:**Online meeting (Webex)

**Abstract:**

Configurations of axis-parallel boxes in $\mathbb{R}^d$ are extensively studied in combinatorial geometry. Despite their innocent appearance, there are many old problems involving their structure that are still not well understood. I will talk about a construction, which shows that starting from dimension $d\geq 3$, configurations of boxes may be more complex than people conjectured.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Vortrag im Rahmen des Seminars Applied Analysis and Computational Mathematics

**Title:**Optimal control of the heat equation in anisotropic Sobolev spaces

**Speaker:**Manuel Nestler (TU Graz)

**Date:**Donnerstag, 11.5.2023, 15:00 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

#### Vortrag im Rahmen des Seminars Applied Analysis and Computational Mathematics

**Title:**A variational formulation and finite element approximations for intial value problems of some ordinary integro-differential equations

**Speaker:**Alexander Mikl (TU Graz)

**Date:**Donnerstag, 11.5.2023, 14:00 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

#### Optimization Seminar

**Title:**User-Centered Diagnosis for Over-Constrained Problems

**Speaker:**Alexander Felfernig (Institut für Softwaretechnologie, TU Graz)

**Date:**9.5. 2023, 16:15

**Room:**Online meeting (Webex)

**Abstract:**

The ``no solution could be found dilemma'' is omnipresent in various application contexts. If a user specifies a set of requirements which

cannot be completely supported, the question arises, which of those requirements should be adapted. In this presentation, I will summarize some

approaches to deal with such situations in a personalized fashion.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m08eab8cd8af8502205a8cf177eebc3c7}

\]

Meeting number: 2730 761 9040

Password: Bp2tsHfiC24

#### Vortrag

**Title:**ÄNDERUNG - Point evaluation in Paley—Wiener spaces

**Speaker:**Prof. Kristian Seip (Norwegian University of Science and Technology)

**Date:**05.05.2023, 14:30 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie, Kopernikusgasse 24, 2.OG

**Abstract:**

We will look at the following problem on time—frequency localization: What is the norm of the point evaluation functional in the classical Paley—Wiener $L^p$ spaces for $0<p<\infty$? The central challenge in this problem is how to go beyond the “power trick” which allows you to relate an estimate for a given $p$ to that for $kp$ for a positive integer $k$. I will discuss some results and multiple conjectures around this problem. The talk is based on recent joint work with Ole Fredrik Brevig, Andrés Chirre, and Joaquim Ortega-Cerdà (to appear in J. Anal. Math., see

https://arxiv.org/abs/2210.13922).

#### Vorstellungsvortrag im Rahmen eines Habilitationsverfahrens

**Title:**The complexity landscape of multi-stage robust optimization

**Speaker:**Stefan Lendl (Universität Graz und s2 data & algorithms)

**Date:**5.5. 2023, 11:00

**Room:**Hörsaal F, Kopernikusgasse 24, 3. Stock

**Abstract:**

We consider multi-stage robust optimization problems like recoverable robust optimization and two-stage robust optimization. Recoverable robust

optimization is a multi-stage approach, in which it is possible to adjust a first-stage solution after the uncertain cost scenario is revealed.

Two-stage robust optimization can be seen as games between a decision maker and an adversary. After the decision maker fixes part of the solution,

the adversary chooses a scenario from a specified uncertainty set. Afterwards, the decision maker can react to this scenario by completing the

partial first-stage solution to a full solution.

In this talk we focus on various hardness results for multi-stage robust optimization problems and efficient algorithms that can be obtained for

specific variants and different types of uncertainty sets. We will discuss under which conditions these types of problems become hard for higher

levels of the polynomial hierarchy. We will also discuss the complexity of multi-stage robust optimization for (representative) selection in more

detail.

#### Vortrag im Rahmen des Seminars Operatortheorie

**Title:**Some recent developments in Hardy-type inequalities

**Speaker:**Fritz Gesztesy (Baylor University, Waco, TX, USA)

**Date:**4.5.2023, 12:15 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

Abstract:

We intend to give an overview of some recent results on (optimal) Birman-Hardy-Rellich-type inequalities. In particular, we will discuss power-weighted inequalities and some of their logarithmic refinements in all space dimensions $n \geq 1$. In dimension one we will present a new perturbative Hardy-type inequality which sheds new light on the origin of Hardy inequalities.

Hardy-type inequalities are a basic tool in the spectral theory of differential operators and we intend to briefly illustrate their relevance.

This is based on various joint work with L. Littlejohn, I. Michael, R. Nichols, and M. M. H. Pang.

**Title:**Combinatorics Day

**Speaker:**()

**Date:**Friday 28th April

**Room:**

**Abstract:**

09:30 - 10:15 - {\bf Mauricio Collares} (TU Graz) ``Set-colouring Ramsey numbers''}

(SR 1 Geometry Institute, Kopernikusgasse 24)

\vspace{.5cm}

10:30 - 11:00 Coffee Break

(Geometry Institute, Kopernikusgasse 24)

\vspace{.5cm}

11:00 - 11:45 - {\bf Sahar Diskin} (Tel Aviv University) ``The Erdős-Rényi component phenomenon: component sizes in percolation on high-dimensional product graphs''} (Part of the Advanced Topics seminar)

(SR 2 Geometry Institute, Kopernikusgasse 24)

\vspace{.5cm}

12:00 - 14:00 - Lunch Break

\vspace{.5cm}

14:00 - 15:00 - Rigorosum of Tuan Anh Do

(AE06, Steyrergasse 30)

\vspace{.5cm}

16:00 - 16:30 - Coffee Break

(2nd Floor, Steyrergasse 30)

\vspace{.5cm}

16:30 - 17:30 - {\bf Michael Krivelevich} (Tel Aviv University) ``Fast construction on a restricted budget''}

(BE01, Steyrergasse 30)

\vspace{.5cm}

Abstracts can be found at

https://www.math.tugraz.at/comb/workshops/Workshop2023Abstracts.pdf

#### Strukturtheorie Seminar

**Title:**Essential minimality of ratio-limit boundary for random walks

**Speaker:**Dr. Adam Dor-On (Haifa University, Israel)

**Date:**Tuesday, 25 April, 13:15

**Room:**Seminar room A306, Steyrergasse 30, 3rd floor

**Abstract:**

The ratio-limit boundary $\partial_R \Gamma$ for a random walk on a group $\Gamma$ is defined as the remainder obtained after compactifying $\Gamma$ with respect to ratio-limit kernels $H(x,y) = \lim_n P^n(x,y)/P^n(e,y)$, where $P^n(x,y)$ is the probability to pass from $x$ to $y$ in $n$ steps. These limits were shown to exist for various classes of groups, normally by establishing a local limit theorem which measures the asymptotic behavior of $P^n(x,y)$ as $n \rightarrow \infty$. In increasing level of generality, works of Woess, Lalley, Gouezel and Dussaule established such local limit theorems for certain symmetric random walks on relatively hyperbolic groups. Techniques developed in these works then allow us to study $\partial_R \Gamma$. For instance, when $\Gamma$ is hyperbolic, Woess was

able to show that $\partial_R \Gamma$ is the Gromov boundary of $\Gamma$ .

In this talk we will explain how for a large class of random walks on relatively hyperbolic groups, the ratio-limit boundary is essentially minimal. That is, there is a unique minimal closed $\Gamma$ -invariant subspace of $\partial_R \Gamma$ . This result is motivated by applications in operator algebras, and indeed, by using it we are able to show the existence of a co-universal equivariant quotient of Toeplitz C*-algebras for a large

class of random walks on relatively hyperbolic groups. The talk will focus

mostly on geometry, topology, and dynamics, and if time permits I will

explain some operator algebraic aspects and motivation.

This talk is based on joint work with Matthieu Dussaule and Ilya Gekhtman.

#### Combinatorics Seminar

**Title:**On the evolution of triangle-free graphs in the ordered regime

**Speaker:**Matthew Jenssen (King's College London.)

**Date:**Friday 21st April 12:30

**Room:**Online meeting (Webex)

**Abstract:**

Erdos, Kleitman and Rothschild proved that the number of triangle-free graphs on $n$ vertices is asymptotic to the number of bipartite graphs; or in other words, a typical triangle-free graph is bipartite. Osthus, Promel and Taraz proved a sparse analogue of this result: if $m > (\sqrt{3}/4 +\epsilon) n^{3/2} \sqrt{\log n}$, a typical triangle-free graph with $m$ edges is bipartite (and this no longer holds below this threshold).

What do typical triangle-free graphs at sparser densities look like and how many of them are there? We consider what we call the ordered regime, where typical triangle-free graphs are not bipartite but have a dense max-cut. In this regime we prove asymptotic formulas for the number of triangle-free graphs and give a precise probabilistic description of their structure. This leads to further results such as determining the threshold at which typical triangle-free graphs are $q$-colourable for $q \geq 3$, determining the threshold for the emergence of a giant component in the complement of a max-cut, and many others.

This is joint work with Will Perkins and Aditya Potukuchi.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Combinatorics Seminar

**Title:**An exponential improvement for diagonal Ramsey

**Speaker:**Julian Sahasrabudhe (University of Cambridge)

**Date:**Friday 24th March 12:30

**Room:**Online meeting (Webex)

**Abstract:**

Let $R(k)$ be the $k$th diagonal Ramsey number: the smallest $n$ for which every red/blue edge-colouring of the complete graph on $n$ vertices contains a monochromatic complete graph of size $k$. I will discuss the proof that $R(k) < (4-c)^k$, for some absolute constant $c>0$. This is the first exponential improvement over the bound of Erdős and Szekeres, proved in 1935.

This is based on joint work with Marcelo Campos, Simon Griffiths and Rob Morris.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Combinatorics Seminar

**Title:**Ramsey numbers upon vertex deletion

**Speaker:**Yuval Wigderson (Tel Aviv University)

**Date:**Friday 17th March 12:30

**Room:**Online meeting (Webex)

**Abstract:**

The Ramsey number $r(G)$ of a graph $G$ is the minimum $N$ so that every red/blue edge-coloring of $K_N$ contains a monochromatic copy of $G$. Conlon, Fox, and Sudakov conjectured that if we delete a single vertex from $G$, then the Ramsey number can decrease by at most a constant factor. Though very natural (and true in a variety of special cases), this conjecture turns out to be false in general. In this talk, I'll explain how one disproves this conjecture, as well as the connections this problem has to a number of other questions in Ramsey theory.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m3162bb7e6bef850e659f657a18095a1c}

\]

Meeting number: 2733 453 3442

Password: bSDVGJDp976

#### Combinatorics Seminar

**Title:**The cop number of random hypergraphs

**Speaker:**Dominik Schmid (TU Graz)

**Date:**Friday 3rd March 12:30

**Room:**AE06 Steyrergasse 30, EG

**Abstract:**

The vertex-persuit game Cops and Robbers} is usually played on a graph, in which a group of cops attempt to catch a robber moving along the edges of the graph. The cop number} of a graph is the minimum number of cops required to win the game.

An important conjecture in this area due to Meyniel states that the cop number of a connected graph is $O(\sqrt{n})$. In 2016, Pra\l at and Wormald showed that this conjecture holds with high probability for connected random graphs. Moreoever, \L uczak and Pra\l at found that on a log-scale the cop number shows a surprising zigzag} behaviour in dense random graphs. In this paper, we consider the game of cops and robbers on a hypergraph, where the players move along hyperedges instead of edges. We conjecture that the cop number of a connected $k$-uniform hypergraph is $O\left(\sqrt{\frac{n}{k}}\right)$ and show that this conjecture holds with high probability up to $\log$-factors for the random binomial $k$-uniform hypergraph $G^k(n,p)$ for a broad range of parameters $p$ and $k$. As opposed to the case of $G(n,p)$, on a log-scale our upper bound on the cop number arises as the minimum of two complementary zigzag curves.

#### Joint Seminar in Statistics and Econometrics

**Title:**Inference in PCA under weak identifiability

**Speaker:**Prof. Davy Paindaveine (Université libre de Bruxelles)

**Date:**2. März 2023, 17:00 h

**Room:**SR f. Statistik (NT03098), Kopernikusgasse 24/III

**Abstract:**

We consider inference on principal directions in non-standard asymptotic scenarios where principal directions are unidentifiable in the limit. To fix ideas, we first tackle the problem of testing the null hypothesis $H_0 : \theta_1=\theta^0_1$ against the alternative $H_1 : \theta_1 \neq \theta_1^0$, where $\theta_1$ is the ”first” eigenvector of the underlying covariance matrix and $\theta_1^0$ is a fixed unit $p$-vector. In the classical setup where eigenvalues $\lambda_1>\lambda_2 \geq \ldots \geq \lambda_p$ are fixed, the likelihood ratio test (LRT) and the Le Cam optimal test for this problem are asymptotically equivalent under the null hypothesis, hence also under sequences of contiguous alternatives. We show that this equivalence does not survive asymptotic scenarios where $\lambda_{n1}/\lambda_{n2} = 1 + O(r_n)$ with $r_n = O(1/\sqrt{n})$. For such scenarios, the Le Cam optimal test still asymptotically meets the nominal level constraint, whereas the LRT becomes extremely liberal. Consequently, the former test should be favored over the latter one whenever the two largest sample eigenvalues are close to each other. By relying on the Le Cam theory of asymptotic experiments, we study in the aforementioned asymptotic scenarios the non-null and optimality properties of the Le Cam optimal test and show that the null robustness of this test is not obtained at the expense of efficiency. Our asymptotic investigation is extensive in the sense that it allows $r_n$ to converge to zero at an arbitrary rate. While these results relate to robustness to weak identifiability, we also introduce sign tests that combine such non-standard robustness with more classical robustness to outliers and/or heavy tails. Finally, we tackle the corresponding point estimation problem by considering in this framework the asymptotic behaviour of the celebrated scatter estimator from Tyler (1987).

#### Zahlentheoretisches Kolloquium

**Title:**Asymptotics and limit theorems for horocycle ergodic integrals à la Ratner

**Speaker:**Dr. Davide Ravotti (Universität Wien)

**Date:**28.02.2023, 14:00 Uhr

**Room:**Seminarraum Analysis und Zahlentheorie

**Abstract:**

In this talk I will present a simple method, inspired by the works of Ratner and Burger, to study ergodic integrals for the classical horocycle flow. The asymptotic expansion we can prove following this approach is a strengthening of the result by Flaminio and Forni in two ways: the coefficients in the expansion are shown to be Hölder continuous with respect to the base point and the term corresponding to the functions in the kernel of the Casimir operator is explicitly described.

Furthermore, we recover the spatial limit theorems by Bufetov and Forni and the temporal limit theorem by Dolgopyat and Sarig (this latter proof is by E. Corso).

#### Gastvortrag

**Title:**Continuous and discrete dynamic topology

**Speaker:**Jānis Lazovskis (Riga Technical University, Riga Business School)

**Date:**22.02.2022, 10:30 Uhr

**Room:**Seminarraum 2, Kopernikusgasse 24/IV

**Abstract:**

I will present two approaches to the common theme of a topological space changing over time. The first approach is a way to understand algebraic changes in the homology of a simplicial complex of a point cloud, as its points move around in Euclidean space. The second approach is about binary dynamics of a fixed point cloud, where edges are determined by a biologically-motivated reconstruction of a mammalian brain. The dynamics in both cases are arbitrary, but the second approach focuses on recovering long-term patterns with topology at the local scale. This is joint work with Barbara Giunti, Ran Levi, and others.

#### Zahlentheoretisches Kolloquium

**Title:**Recent work on van der Waerden’s conjecture

**Speaker:**Prof. Dr. Rainer Dietmann (Royal Holloway, University of London)

**Date:**15.02.2023, 12:00 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie, Kopernikusgasse 24, 2.OG

**Abstract:**

Recently, there was a lot of activity regarding an old conjecture of van der Waerden, culminating in its solution by Bhargava, and including joint work by Sam Chow and myself on which I want to report in this talk:

We showed that the number of irreducible monic integer polynomials of degree n, with coefficients in absolute value bounded by H, which have Galois group different from S_n and A_n, is of order of magnitude O(H^{n-1.017}), providing that n is at least 3 and is different from 7,8,10. Apart from the alternating group and excluding degrees 7,8,10, this establishes the aforementioned conjecture to the effect that irreducible non-S_n polynomials are significantly less frequent than reducible polynomials.

The method is quite flexible and can also be applied to other problems.

#### Zahlentheoretisches Kolloquium

**Title:**Values of binary partition function represented by a sum of three squares

**Speaker:**Dr. Maciej Ulas (Jagiellonian University, Krakow)

**Date:**07.02.2023, 11:00 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie, Kopernikusgasse 24, 2.OG

**Abstract:**

Let $m$ be a positive integer and $b_{m}(n)$ be the number of partitions of $n$ with parts being powers of 2, where each part can take $m$ colors. We show that if $m=2^{k}-1$, then there exists the natural density of integers $n$ such that $b_{m}(n)$ can not be represented as a sum of three squares and it is equal to $1/12$ for $k=1, 2$ and $1/6$ for $k\geq 3$. In particular, for $m=1$ the equation $b_{1}(n)=x^2+y^2+z^2$ has a solution in integers if and only if $n$ is not of the form $2^{2k+2}(8s+2t_{s}+3)+i$ for $i=0, 1$ and $k, s$ are non-negative integers, and where $t_{n}$ is the $n$th term in the Prouhet-Thue-Morse sequence. A similar characterization is obtained for the solutions in $n$ of the equation $b_{2^k-1}(n)=x^2+y^2+z^2$. This talk is based on a joint work with Bartosz Sobolewski (Jagiellonian University).

#### Vortrag im Rahmen des Seminars Applied Analysis and Computational Mathematics

**Title:**An adaptive least squares boundary element method for elliptic boundary value problems

**Speaker:**Univ.-Prof. Dr. Olaf Steinbach (Institut für Angewandte Mathematik, TU Graz)

**Date:**Montag, 6.2.2023, 10:00 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

#### Vortrag im Rahmen des Seminars Applied Analysis and Computational Mathematics

**Title:**Stability of space-time boundary element methods for 1D wave problems

**Speaker:**Daniel Hoonhout (TU Delft)

**Date:**Montag, 6.2.2023, 9:00 Uhr

**Room:**Seminarraum AE02, Steyrergasse 30, EG

**Abstract:**

**Title:**Graz-ISTA Number Theory Days

**Speaker:**()

**Date:**Thursday 2 February 2023, 13:30 - 17:15

**Room:**HS BE01

**Abstract:**

13:30 - 14:30: Efthymios Sofos (Glasgow):}

Averages of arithmetic functions over values of multivariable polynomials

15:00 - 16:00: Jakob Glas (ISTA):}

Rational points on del Pezzo surfaces over function fields

16:15 - 17:15: Shuntaro Yamagishi (ISTA):}

On Poonen's question regarding polynomial representing all natural numbers

Seminar webpage for abstracts and further information:

https://sites.google.com/view/gntd/upcoming-seminars

#### Optimization Seminar

**Title:**Group Assignment Problem on Intervals (Last problem from Gerhard)

**Speaker:**Walter Unger (Fachbereich Informatik, RWTH Aachen)

**Date:**1.2.2023, 10:30

**Room:**Seminar room AE06, Steyrergasse 30, ground floor

**Abstract:**

The talk considers the following problem which got suggested to the speaker

as last problem by the late Gerhard Woeginger who passed away on April 1, 2022.

There are $n$ people who like to take a walk with a group of some size.

Each person has its own interval $[a_i,b_i]$, which restricts its own preferred group size.

Is there an assignment for all people to groups of the preferred size?

We will show that this problem can be solved in polynomial time and

discuss some variants.

#### Optimization Seminar

**Title:**Mathematical Modeling and Optimization in Energy Systems

**Speaker:**Sonja Wogrin ( Institut für Elektrizitätswirtschaft und Energieinnovation, TU Graz)

**Date:**30.1.2023, 16:15

**Room:**Seminar room AE06, Steyrergasse 30, ground floor

**Abstract:**

In Europe, we have embarked on the journey towards net-zero power systems and want to reach full decarbonization by

2050 (European Commission ``A clean planet for all''). Austria wants to reach carbon neutrality in the power system

already by 2030 (Erneuerbaren Ausbau Gesetz) and the decarbonization of the entire energy system by 2040, which

will require coupling the power, transport, heat and gas sectors. In this talk we discuss how mathematical

modeling, optimization and game theory are key ingredients to achieving such ambitious climate goals by tackling

relevant challenges in energy economics. In particular, complex systems such as the electric power system are

governed by physical laws and technical limitations that often lead to non-convex, mixed-integer, large-scale

optimization models. Moreover, electric energy is traded through liberalized electricity markets -- competitive

environments that can be assessed via game theory and that can lead to interesting hierarchical equilibrium

problems that call for bilevel programming. This talk emphasizes the significant contributions that mathematicians

can make to tackle the climate crisis.

Bio:

Since Summer 2021 Sonja Wogrin has been the head of the Institute of Electricity Economics and Energy Innovation. She studied mathematics at TU Graz and obtained her diploma degree in 2008 and then moved to Spain where she obtained her Ph.D. at the Instituto de Investigación Tecnológica (IIT) of the Universidad Pontificia Comillas in Madrid in 2013. She worked at the IIT from 2009 to 2021. At TU Graz Sonja Wogrin is involved in many interdisciplinary research projects and is the initiator of the planned TU Graz research center ``Energy Economics & Energy Analytics'' in which contributions from all areas of mathematics are very welcome.

#### Combinatorics Seminar

**Title:**On the enumeration of planar maps with tight boundaries

**Speaker:**Grégory Miermont (ENS Lyon)

**Date:**Friday 27th January 12:15

**Room:**Online meeting (Webex)

**Abstract:**

We consider the enumeration problem of bipartie and quasi-bipartite planar maps with internal faces, counted with Boltzmann weights, and external faces with prescribed degrees. the generating series for these objects solves a simple functional equation discovered by Eynard and generalized by Collet and Fusy, who also proposed a bijective derivation based on the Bouttier-Di Francesco-Guitter bijection. One observation is that the functional equation becomes even simpler if one asks that the external face boundaries be tight, in the sense that their lengths are minimal in their respective free homotopy class, in the surface obtained by removing one point from every external face. We give bijective interpretations, based on geometric decompositions, for this simpler formula in two particular cases: when there are three external faces (pairs of pants), and when there are no internal faces (tight maps). I will discuss implications of these for the statistics of large planar maps, as well as for the quasi-polynomiality for the number of plane tight maps observed by Norbury. This is based on joint work with Jérémie Bouttier and Emmanuel Guitter.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m44797227fd680cc7956ebb840b6f033a}

\]

Meeting number: 2730 500 3129

Password: vQydpg372D4

#### Combinatorics Seminar

**Title:**A Random Hall-Paige Conjecture

**Speaker:**Alexey Pokrovskiy (University College London)

**Date:**Friday 20th January 12:15

**Room:**Online meeting (Webex)

**Abstract:**

A rainbow subgraph in an edge-coloured graph is one in which all edges have different colours. This talk will be about finding rainbow subgraphs in colourings of graphs that come from groups. An old question of this type was asked by Hall and Paige. Their question was equivalent to the following ``Let $G$ be a group of order $n$ and consider an edge-coloured $K_{n,n}$, whose parts are each a copy of $G$ and with the edge $\{x,y\}$ coloured by the group element $xy$. For which groups $G$, does this coloured $K_{n,n}$ contain a perfect rainbow matching?'' This question is equivalent to asking ``which groups G contain a complete mapping'' and also ``which multiplication tables of groups contain transversals''. Hall and Paige conjectured that the answer is ``all groups in which the product of all the elements is in the commutator subgroup of $G$''. They proved that this is a necessary condition, so the main part of the conjecture is to prove that that rainbow matchings exist under their condition.

The Hall-Paige Conjecture was confirmed in 2009 by Wilcox, Evans, and Bray with a proof using the classification of finite simple groups. Recently, Eberhard, Manners, and Mrazovic found an alternative proof of the conjecture for sufficiently large groups using ideas from analytic number theory. Their proof gives a very precise estimate on the number of complete mappings that each group has.

In this talk, a third proof of the conjecture will be presented using a different set of techniques, this time coming from probabilistic combinatorics. This proof only works for sufficiently large groups, but generalizes the conjecture in a new direction. Specifically we not only characterize when the edge coloured $K_{n,n}$ contains a perfect rainbow matching, but also when random subgraphs of it contain a perfect rainbow matching. This extension has a number of applications, such as to problems of Snevily, Cichacz, Tannenbaum, Evans.

This is joint work with Alp Muyesser.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m44797227fd680cc7956ebb840b6f033a}

\]

Meeting number: 2730 500 3129

Password: vQydpg372D4

#### Combinatorics Seminar

**Title:**Ringel's conjecture on tree-packing

**Speaker:**Katherine Staden (Open University)

**Date:**Friday 13th January 12:15

**Room:**Online meeting (Webex)

**Abstract:**

When can (the edge-set of) a graph $G$ be decomposed into copies of a given graph $H$? This question goes all the way back to Euler; despite this, the setting where the number of vertices in $G$ and $H$ are comparable is not yet well-understood. I will talk about the resolution of a conjecture of Ringel from 1963 where $G$ is the complete graph on $2n+1$ vertices and $H$ is any given tree with $n$ edges. This is joint work with Peter Keevash; the conjecture was independently resolved by Montgomery, Pokrovskiy and Sudakov.

Meeting link:

\[

\text{https://tugraz.webex.com/tugraz/j.php?MTID=m44797227fd680cc7956ebb840b6f033a}

\]

Meeting number: 2730 500 3129

Password: vQydpg372D4

#### Strukturtheorie-Seminar

**Title:**Reducible automata and poly-context-free groups

**Speaker:**Prof. Daniele D'Angeli (Univ. Cusano, Rom)

**Date:**Freitag, 13.1.2023, 14 Uhr c.t

**Room:**SR AE06, Steyrergasse 30, EG

**Abstract:**

In this talk I will describe a class of automaton groups,

generated by so-called reducible automata, which are shown to be

direct limit of finite index subgroups of the direct product of free

groups. This result partially supports a conjecture by T. Brough

regarding the general structure of groups with poly-context-free word

problem, i.e., whose word problem is the intersection of finitely

many context-free languages.

Joint work with M.Cavaleri, A.Donno, and E.Rodaro