### Talks in 2017

#### Habilitationsvortrag (Lehrprobe)

**Title:**The Tarski-Seidenberg Principle

**Speaker:**Dr. Christopher Frei (Univ. of Manchester)

**Date:**Freitag, 15. 12. 2017, 14:00 Uhr

**Room:**SR Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24, 2.Stock

**Abstract:**

#### Doktoratskolleg Discrete Mathematics

**Title:**

**Speaker:**Discrete Mathematics Day 2017 ()

**Date:**Thursday, 14.12.2017, 10:30-16:40

**Room:**Hörsaal BE01, Steyregasse 30, EG

**Abstract:**

10:30 opening

10:40-11:30 main talk 1: Christopher Frei (Manchester)

11:30-10:40 Math.Video

10:45-12:15 PhD talk 1: JunSeok Oh (KFU Graz)

12:15-12:25 Math.Video

12:25-13:30 Lunch buffet

13:45-14:15 PhD talk 2: Shu-Qin Zhang (MU Leoben)

14:15-14:25 Math.Video

14:30-15:00 PhD talk 3: Irene de Parada (TU Graz)

15:00-15:10 Math.Video

15:10-15:40 Coffee break

15:40-16:30 main talk 2: Silke Rolles (TU München)

16:30-16:40 Math.Video

A more detailed programme will follow.

#### Strukturtheorie-Seminar

**Title:**Reinforced random walk

**Speaker:**Michael Kalab (TU Graz)

**Date:**Donnerstag, 7.12.2017, 11 Uhr c.t.

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

**Abstract:**

In this master-seminar, linearly reinforced random walks are explained and some results are presented.

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Tamari-like intervals and planar maps

**Speaker:**Wenjie Fang (TU Graz)

**Date:**Dienstag 5.12.2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

Tamari lattice is a partial order defined on objects counted by Catalan numbers, such as binary trees and Dyck paths. It is a well-studied object in combinatorics, with several generalizations. In this talk, we will see how intervals in these Tamari-like lattices are related to planar maps. More precisely, we discovered a bijection between non-separable planar maps and intervals in the generalized Tamari lattice, which naturally extends to a bijection between bridgeless planar maps and intervals in the original Tamari lattice. We will also discuss the consequences of these bijections in enumerative and structural studies of the related objects.

#### Seminar Applied Analysis and Computational Mathematics

**Title:**Eigenvalue bounds for the magnetic Laplacians and Schroedinger operators

**Speaker:**Diana Barseghyan (University of Ostrava)

**Date:**14.12.2017, 14:00

**Room:**Seminarraum STEG006, Steyrergasse 30, Erdgeschoss

**Abstract:**

We are going to derive spectral estimates for several classes of magnetic Laplacians. They include the magnetic Laplacian on three-dimensional regions with Dirichlet boundary conditions as well as the magnetic Laplacian defined in $\mathbb{R}^3$ with the local change of the magnetic field. We establish two-dimensional Berezin-Li-Yau and Lieb-Thirring-type bounds in the presence of constant magnetic fields and, using them, get three-dimensional estimates for the eigenvalue moments of the corresponding magnetic Laplacians. Also we derive separately the Lieb-Thirring bounds for the magnetic Schroeodinger operators defined on two dimensional circle with radially symmetric magnetic field and electric potentials.

#### FWF START Seminar (Mini-Colloquium)

**Title:**Inhomogeneous Diophantine Approximation with Restricted Denominators

**Speaker:**Agamemnon Zafeiropoulos (TU Graz)

**Date:**4.12.2017, 16:00

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

**Abstract:**

We formulate and prove a Khintchine-type law for inhomogeneous Diophantine approximation. The denominators form a lacunary sequence of integers, while the size of the set of well-approximable numbers is given with respect to a probability measure with Fourier coefficients of a prescribed logarithmic decay rate.

(Remark: Agamemnon Zafeiropoulos is a new member of the Institute of Analysis and Number Theory, who started here as a Postdoc researcher in November 2017.)

#### FWF START Seminar (Mini-Colloquium)

**Title:**Joint universality for dependent L-functions

**Speaker:**Lukasz Pankowski (Adam Mickiewicz University Poznan)

**Date:**4.12.2017, 15:15

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

**Abstract:**

We prove that, for arbitrary Dirichlet $L$-functions $L(s;\chi_1),\ldots, L(s;\chi_n)$ (including the case when $\chi_j$ is equivalent to $\chi_l$ for $j\ne k$), suitable shifts of type $L(s+i\alpha_jt^{a_j}\log^{b_j}t;\chi_j)$ can simultaneously approximate any given analytic functions on a simply connected compact subset of the right open half of the critical strip, provided the pairs $(a_j,b_j)$ are distinct and satisfy certain conditions. Moreover, we consider a discrete analogue of this problem where $t$ runs over the set of positive integers.

#### Reminder: Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**The Partition Adjacency Matrix realization problem

**Speaker:**L\'aszl\'o Sz\'ekely (University of South Carolina)

**Date:**Freitag 1. Dezember, 14:00 Kaffeepause 13:30 (Steyrergasse 30, 2. Stock, Mathematik)

**Room:**Hörsaal BE01, Steyrergasse 30, Erdgeschoss

**Abstract:**

On Facebook, people with high number of connections tend to be connected more likely than randomness would suggest, while in biological networks vertices with high number of connections tend to be connected less likely than randomness would suggest. In terms of network science, the first network is assortative, while the second is disassortative.

Degrees (number of connections) do not tell if a network is assortative or disassortative. The Joint Degree Matrix (JDM) of a network (graph) counts number of edges between the sets of degree $i$ and degree $j$ vertices, for any $i,j$. The JDM realization problem asks whether a graph exists with prescribed number of connections (degree) at the vertices, and with prescribed number of edges between the sets of degree $i$ and degree $j$ vertices, for any $i,j$. The JDM realization problem is well understood. The usual measure for assortativity, the assortativity coefficient, the Pearson correlation coefficient of degree between pairs of linked nodes, is computable from the JDM.

A further generalization of the JDM is the following.

Given a set $W$ and numbers $d(w)$ associated with $w\in W$, and a $W_i:i\in I$ partition of $W$, with numbers $c(W_i,W_j)$ associated with unordered pairs of partition classes, the Partition Adjacency Matrix (PAM) realization problem asks whether there is a simple graph $G$ on the vertex set $W$ with degrees $d_G(w)=d(w)$ for $w\in W$, with exactly $c(W_i,W_j)$ edges with endpoints in $W_i$ and $W_j$; and the PAM construction problem asks for such a graph, if they exist. (These problems are conjectured to be NP-hard.) The bipartite version of these problems are more restricted: $I=I_1\cup I_2$ and $c(W_i,W_j)=0$ whenever $i,j\in I_1$ or $i,j\in I_2$.

We provide algebraic Monte-Carlo algorithms for the bipartite Partition Adjacency Matrix realization and construction problems, which run in polynomial time, say, when $|I|$ is fixed. When the algorithms provide a positive answer, they are always correct, and when the truth is positive, the algorithms fail to report it with small probability.

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Tangle Crossing Numbers and theTanglegram Kuratowski Theorem

**Speaker:**\'Eva Czabarka (University of South Carolina)

**Date:**Freitag 1. Dezember, 15:00

**Room:**Hörsaal BE01, Steyrergasse 30, Erdgeschoss

**Abstract:**

A tanglegram of size $n$ is a triplet $(L,R,M)$ where $L$ and $R$ are rooted binary trees with $n$ leaves each, and $M$ is a perfect matching between the two sets of leaves. Two tanglegrams $(L_1,R_1,M_1)$ and $(L_2,R_2,M_2)$ are the same if there is a pair of tree-isomorphisms $(\phi,\psi)$ mapping $L_1$ to $L_2$ and $R_1$ to $R_2$ such that matched pairs of leaves get paired to matched pairs of leaves. Tanglegrams are used in phylogenetics, where for example they can represent the phylogenetic trees of parasites and hosts, where the matching gives which parasite infects which host.

A tanglegram layout (i.e. the way tanglegrams are usually drawn) is as follows: draw the two rooted binary trees in the plane with straight lines and without crossing edges such that the leaves of $L$ are on the line $x=0$ and $L$ is drawn in the semi-plane $x\le 0$, the leaves of $R$ are drawn on the line $x=1$ and $R$ is drawn in the semi-plane $x\ge 1$, and the edges of the matching are drawn with straight line. The crossing number of a layout is the number of unordered pairs of matching edges that cross and the tangle crossing number of a tanglegram is the minimum crossing number over all of its layouts. The tangle crossing number is related to a number of biologically important quantities, e.g. the number of times parasites switched hosts. I will present some results about the tangle crossing number, including a Kuratowski type theorem.

#### Zahlentheoretisches KolloquiumACHTUNG - Die Beginnzeit des Vortrages hat sich geändert!

**Title:**Metric discrepancy results for geometric progressions with small ratios 3/2, 4/3, etc.

**Speaker:**Prof. Dr. Katusi Fukuyama (Kobe University, Japan)

**Date:**Dienstag, 28. 11. 2017, 12:00 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24/II

**Abstract:**

#### Sturkturtheorie-Seminar

**Title:**The connective constant

**Speaker:**Christian Lindorfer (TU Graz)

**Date:**Donnerstag, 23.11.2017, 11 Uhr c.t.

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

**Abstract:**

In this master seminar, self-avoiding walks on infinite graphs are discussed,

with focus on Cayley graphs and quasi-transitive graphs.

The connective constant is the exponential growth rate of the number of self-avoiding walks of length n. Its computation for lattices is a difficult problem coming from Statistical Physics. In the talk, some basic properties, recent results, and computations are presented.

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**On the number of arithmetic progressions in sparse random sets

**Speaker:**Christoph Koch (University of Warwick)

**Date:**Dienstag 21.11.2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

We study arithmetic progressions $\{a,a+b,a+2b,\dots,a+(\ell-1) b\}$, with $\ell\ge 3$, in random subsets of the initial segment of natural numbers

[$n$]$:=\{1,2,\dots, n\}$. Given $p\in[0,1]$ we denote by [$n$]$_p$ the random subset of [$n$] which includes every number with probability $p$, independently of one another. The focus lies on sparse random subsets, i.e.\ when $p=p(n)=o(1)$ with respect to $n\to\infty$.

Let $X_\ell$ denote the number of distinct arithmetic progressions of length $\ell$ which are contained in [$n$]$_p$. We determine the limiting distribution for $X_\ell$ not only for fixed $\ell\ge 3$ but also when $\ell=\ell(n)\to\infty$ sufficiently slowly. Moreover, we prove a central limit theorem for the joint distribution of the pair $(X_{\ell},X_{\ell'})$ for a wide range of $p$. Our proofs are based on the method of moments and combinatorial arguments, such as an algorithmic enumeration of collections of arithmetic progressions.

These results are joint work with Y.~Barhoumi-Andr\'eani and H.~Liu (Warwick).

#### Zahlentheoretisches Kolloquium

**Title:**Sum of elements locating along horizontal rays in Pascal pyramid

**Speaker:**Prof. Dr. László Szalay (Univ. Sopron)

**Date:**Freitag, 17. 11. 2017, 14:00 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24/II

**Abstract:**

Abstract.

After surveying the related results in Pascal triangle we turn

our attention to Pascal pyramid to consider horizontal rays. The newest

result desribes linearly recurrent sequences with rational function coefficients as the sum of elements located along some specific rays. In this way we obtain, for instance, the central Delannoy numbers. The application of the theorem proves many recurrence relations conjectured in The On-Line Encyclopedia of Integer Sequences of Sloane.

#### Strukturtheorie-Seminar

**Title:**Decision Problems and Automaton Structures

**Speaker:**Jan Philipp Wächter (Univ. Stuttgart)

**Date:**Monday, 13.11.2017, 11:15

**Room:**Seminarraum Analysis-Zahlentheorie, Kopernikusgasse 24/II

**Abstract:**

Traditionally, algebraic structures are presented by stating generators and relations between words over these generators. There are, however, alternatives to this way of presentation. One of these is the use of automata. Although this approach does not work for every group, the class of groups admitting automaton presentations, the so-called automaton groups, have received quite some attention in research since many groups answering important questions in group theory (such as the Milnor Problem and the Burnside Problem) turn out to be automaton groups. Starting with groups, this interest seems to extend more and

more also to automaton semigroups as it often turns out to be much easier to obtain undecidability results for automaton semigroups than it is for automaton groups.

In this talk, we are going to introduce automaton semigroups and groups, and look into known results as well as open problems concerning decision problems in this area.

#### Festkolloquium

**Title:**Festkolloquium zum Anlass des 50. Geburtstages von Herrn Univ.-Prof. Dr. Olaf Steinbach

**Speaker:**()

**Date:**Freitag, 10.11.2017, 14:00 Uhr

**Room:**Hörsaal BE01, Steyrergasse 30, EG, TU Graz

**Abstract:**

\hskip 5pt 14:00 Wolfgang L. Wendland (Universität Stuttgart)

\hskip 1.1cm On Neumann's series and the double layer potential

14:45 Martin Neumüller (Johannes Kepler Universität Linz)

\hskip 1.1cm On Space Time Methods

15:30 Kaffeepause

16:00 Matthias Taus (MIT)

\hskip 1.1cm Fast and accurate methods for wave propagation

16:45 Sergej Rjasanow (Universität des Saarlandes)

\hskip 1.1cm Alternative effective numerical methods for partial differential

equations:

\hskip 1.1cm Differences and Bridges

#### Sturkturtheorie-Seminar

**Title:**Bachelor thesis: lamplighter random walks on finite graphs

**Speaker:**Eva Hainzl (TU Graz)

**Date:**Donnerstag, 9.11.2017, 11 Uhr c.t.

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

**Abstract:**

In this report on the bachelor thesis, we present results on the convergence

to stationarity of lamplighter random walks on some finite graphs.

(Due to a master thesis defense, the talk might start with a small delay.)

#### Seminar Angewandte Analysis und Numerische Mathematik

**Title:**Eigenvalues of Robin Laplacian with strong attractive parameter

**Speaker:**Dr. Nicolas Popoff (Université de Bordeaux)

**Date:**21.11.2017, 10:00 Uhr

**Room:**A 306

**Abstract:**

I will present recent results on the asymptotics in singular limits of low-lying eigenvalues of self-adjoint operators defined in corner domains. As a model case, I will present the Robin Laplacian with a large Dirichlet parameter.

Firstly, I will give results for the regular case. The asymptotics is given through an effective semi-classical Hamiltonian, defined on the boundary, involving the mean curvature. We deduce from these results a Faber-Krahn inequality for the regular case, rising the question of optimizing the mean curvature of an open set of fixed volume.

Secondly, I will focus on the analysis in n-dimensional corner domains, in which the the singularities of the boundary modify the asymptotics. I will present the recursive class of corner domains and associated singular chains. The asymptotics of the first eigenvalues is obtained through a minimization process over the tangent geometries, and a multi-scale analysis provides an estimate of the remainder.

#### Vortrag im IST Seminar

**Title:**Lower Bounds for Searching Robots, some Faulty

**Speaker:**Emo Welzl (Department of Computer Science, ETH Zürich)

**Date:**23.10. 2017, 16:15

**Room:**Seminarraum IST, Inffeldgasse 16b, 2.Stock

**Abstract:**

We consider the following generalization of the classical ``cow path problem''. Suppose we are sending out $k$ robots from $0$ to search the real line at constant speed (with turns) to find a target at an unknown location; $f$ of the robots are faulty (of so-called crash type), meaning that they fail to report the target although visiting its location. The goal is to find the target in time at most $\lambda |d|$, if the target is located at $d$, $|d|\ge 1$, for $\lambda$ as small as possible. We show that it cannot be achieved for $\lambda < 2\frac{(1+\rho)^{1+\rho}}{\rho^\rho} + 1$ where

$\rho := 2\frac{f+1}{k}-1$, which is tight due to earlier work. This also gives some better than previously known lower bounds for so-called Byzantine-type faulty robots (that may, deceitfully, actually wrongly report a target).

(Joint work with Andrey Kupavskii.)

#### Festkolloquium

**Title:**Festkolloquium aus Anlass des 60.Geburtstages von Prof.Dr.Robert Tichy

**Speaker:**()

**Date:**19. - 20. Oktober 2017

**Room:**HS BE01, Steyrergasse 30/EG, TU Graz

**Abstract:**

Vortragende:[5mm]

Donnerstag, 19.10.2017

09:00-09:30: Eröffnung durch VR Bischof und Dekan Ernst

09:30-10:00: Harald Niederreiter

Donald Knuth’s problem and Robert Tichy’s solution

10:00-10:45 János Pintz

Some conjectures of Erdös and Turán on consecutive

primegaps

10:45-11:15 Kaffeepause

11:15-12:00 Kálmán Györy

S-parts of values of binary forms and decomposable forms

Mittagspause

14:15-15:00 Yuri Bilu

Effective bounds for singular units

15:00- 15:45 Pietro Corvaja

The Hilbert Property for algebraic varieties

15:45-16:15 Kaffeepause

16:15-17:00 Clemens Fuchs

Diophantine triples and linear recurrences of Pisot type

17:15-18:00 A. V.[5mm]

Freitag, 20.10.2017

09:00-09:45 Klaus Schmidt

Entropy and periodic points of algebraic actions of discrete groups

09:45-10:30 Vitaly Bergelson

Ramsey Theory at the Junction of Additive and Multiplicative Combinatorics

10:30-11:00 Kaffeepause

11:00-11:45 István Berkes

On the uniform theory of lacunary series

Mittagspause

14:00-14:45 Michael Drmota

Digital Expansions and Uniform Distribution

14:45-15:15 Kaffeepause

15:15-16:00 Gerhard Larcher

On Weyl Products and Irregularities of Distribution[5mm]

Aktuelle Informationen unter:

http://www.math.tugraz.at/Tichy60}

#### Zahlentheoretisches Kolloquium

**Title:**On the representation of k-free integers by binary forms

**Speaker:**Dr. Stanley Yao Xiao (University of Oxford)

**Date:**6. 10. 2017, 14:15

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008)

**Abstract:**

Let $F$ be a binary form of degree $r \geq 3$, integer coefficients, on-zero discriminant, and such that the largest irreducible factor of $F$ has degree $d$. For a positive number $Z$ and a positive integer $k \geq 2$ put R_{F,k}(Z)$ for the number of $k$-free integers in the interval $[-Z,Z]$ which is representable by $F$. We shall give an asymptotic formula for $R_{F,k}(Z)$ when $k > \min\{7d/18, \lceil d/2 \rceil - 2\}$. This is joint work with C.L. Stewart.

#### FWF START Seminar

**Title:**Distribution of zeros of the derivatives of the Riemann zeta function and Dirichlet L-functions

**Speaker:**Ade Irma Suriajaya (RIKEN Tokyo)

**Date:**27.9.2017, 13:30

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

**Abstract:**

Speiser in 1935 showed that the Riemann hypothesis is equivalent to the first derivative of the Riemann zeta function having no zeros on the left-half of the critical strip. This result shows that the distribution of zeros of the Riemann zeta function is related to that of its derivatives. The number of zeros and the distribution of the real part of non-real zeros of the derivatives of the Riemann zeta function have been investigated by Berndt, Levinson, Montgomery, and Akatsuka. Berndt, Levinson, and Montgomery investigated the general case, meanwhile Akatsuka gave sharper estimates under the truth of the Riemann hypothesis. This result is further improved by Ge. In the first half of this talk, we introduce these results and generalize the result of Akatsuka to higher-order derivatives of the Riemann zeta function.

Analogous to the case of the Riemann zeta function, the number of zeros and many other properties of zeros of the derivatives of Dirichlet L-functions associated with primitive Dirichlet characters were studied by Yildirim. In the second-half of this talk, we improve some results shown by Yildirim for the first derivative and show some new results. We also introduce two improved estimates on the distribution of zeros obtained under the truth of the generalized Riemann hypothesis. We also extend the result of Ge to these Dirichlet L-functions when the associated modulo is not small. Finally, we introduce an equivalence condition analogous to that of Speiser’s for the generalized Riemann hypothesis, stated in terms of the distribution of zeros of the first derivative of Dirichlet L-functions associated with primitive Dirichlet characters.

After the talk there will be coffee & cake.

#### Zahlentheoretisches Kolloquium

**Title:**Improving Burgess via Polya-Vinogradov

**Speaker:**Dr. Leo Goldmakher (Williams College)

**Date:**Freitag, 15. 9. 2017, 14:15 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24/II

**Abstract:**

For a typical character $\pmod p$, a classical result of Polya and Vinogradov

implies cancellation for character sums longer than $p^{1/2}$. Burgess' bound

allows us to go further, implying cancellation for character sums longer

than $p^{1/4}$. But in practice, one often needs to bound shorter character

sums, and no such bounds are known for a general character. I will describe

recent work (joint with Elijah Fromm) in which we show that even a mild

improvement of the Polya-Vinogradov inequality would imply cancellation in

character sums as short as $p^{0.00001}$, thus significantly improving the

Burgess bound.

#### ICG Visual Computing Seminar

**Title:**Discrete Geodesic Paths in the Space of Images

**Speaker:**Martin Rumpf (Univ. Bonn)

**Date:**11.7.2017, 13:00h

**Room:**Seminarraum ICG, Inffeldgasse 16, 2.OG

**Abstract:**

The space of images will be considered as a Riemannian manifold, where the

underlying Riemannian metric simultaneously measures the cost of image

transport and intensity variation, introduced by Trouve and Younes as the

metamorphosis model.

A robust and effective variational time discretization of geodesics paths

will proposed and a variational scheme for a time discrete exponential map

will investigated.

The approach requires the definition of a discrete path energy consisting of

a sum of consecutive image matching functionals over a set of image

intensity maps and pairwise matching deformations.

The talk will present existence and convergence results and discuss

applications in image morphing and image animation.

#### Seminar Angewandte Analysis und Numerische Mathematik

**Title:**A shape optimization problem for the relativistic $\delta$-shell interaction in ${\mathbb R}^3$

**Speaker:**Dr. Albert Mas (Universitat de Barcelona)

**Date:**6.7.2017, 15:00 Uhr

**Room:**AE02

**Abstract:**

We will investigate spectral properties of $H+V_\lambda$, where $H=-i\alpha\cdot\nabla+m\beta$ is the free Dirac operator in ${\mathbb R}^3$, $m>0$ denotes the mass and $V_\lambda$ is an electrostatic shell potential (which depends on a parameter $\lambda\in{\mathbb R}$) located on the boundary of a smooth domain in ${\mathbb R}^3$. I will present an isoperimetric-type inequality for the admissible range of $\lambda$s for which the coupling $H+V_\lambda$ generates pure point spectrum in $(-m,m)$. That the ball is the unique optimizer of this inequality will also be discussed. This is a joint work with N. Arrizabalaga and L. Vega.

#### Seminar Angewandte Analysis und Numerische Mathematik

**Title:**Self-adjoint operators of the type div sgn grad

**Speaker:**Dr. Konstantin Pankrashkin (Universite Paris-Sud & ENSTA ParisTech)

**Date:**6.7.2017, 14:00 Uhr

**Room:**AE02

**Abstract:**

Being motivated by the study of negative-index metamaterials, we will discuss the definition and the spectral properties of the operators given by the differential expressions $\text{div}\,h\,\text{grad}$ in a bounded domain $U$ with a function $h$ which is equal to $1$ on a part of $U$ and to a constant $b<0$ on the rest of $U$. We will see how the properties of such operators depend on the parameter $b$ and the geometry of $U$. In particular, for the critical case $b=-1$ one can have a non-empty essential spectrum, and our results extend the constructions of Behrndt and Krejcirik for symmetric rectangles (2014) to arbitrary smooth geometries. The proof features an interplay between the machinery of boundary triples and the microlocal analysis. Based on a joint work with Claudio Cacciapuoti and Andrea Posilicano (University of Insubria).

#### Zahlentheoretisches Kolloquium

**Title:**Prescribing the binary digits of squarefree numbers and quadratic residues

**Speaker:**Rainer Dietmann (Royal Holloway, London)

**Date:**3.7.2017, 15 ct

**Room:**Seminarraum Analysis und Zahlentheorie, (NT02008), Kopernikusgasse 24/II

**Abstract:**

Abstract: In joint work with C. Elsholtz and I. Shparlinski we study the equidistribution of multiplicatively defined sets such as squarefree numbers or

quadratic non-residues in sets which are defined in an additive way, for example sumsets, Hilbert cubes or

sets having digit restrictions. In particular, we show that if one fixes any proportion of less than 2/5 of

the digits of all numbers of a given binary bit length, then the remaining set still has the asymptotically

expected number of squarefree integers.

#### Strukturtheorie-Seminar

**Title:**ON A QUESTION OF YU. MUKHIN

**Speaker:**Prof. Wolfgang Herfort (TU Wien)

**Date:**Monday, 3.7.2017, 14 s.t. (!!!)

**Room:**Seminarraum Analysis und Zahlentheorie, (NT02008), Kopernikusgasse 24/II

**Abstract:**

Yu. N. Mukhin asked in 1984 in the Kourovka Notebook (9.32) to classify all locally compact groups in which for any two closed subgroups X and Y their set theoretic product XY is a closed subgroup.

In joint work with K. H. Hofmann and F. G. Russo the class of “near abelian” groups has been introduced and extensively discussed. As a byresult we can offer a complete answer to Mukhin’s question.

In this talk I will highlight the concepts and present the classification result.

#### Zahlentheoretisches Kolloquium

**Title:**Smooth numbers with digital restrictions

**Speaker:**Dr. Walid Wannes (TU Graz)

**Date:**Freitag, 30. 6. 2017, 15:00

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24/II

**Abstract:**

Abstract:

An integer $n$ is said to be $y-$smooth if its largest prime factor

$P(n)$ is less than $y$. As usual, we denote by $S(x,y)$ the set of

$y-$smooth numbers up to $x$,

$$ S(x,y)= \{ 1\leq n\leq x, P(n)\leq y\}.$$

In this talk, we provide an asymptotic formula for the number of

integers $n$ in $S(x,y)$ such that $S_{q}(n) \equiv l \mod m$ for $l \in

\mathbb{Z}$ and $m \geq2$, where $S_q(n)$ denotes the sum of the digits

in base $q$ of the integer $n$. Also, we show that the sequence $\big(

\alpha S_{q}(n)\big)_{n \in S(x,y)}$ is uniformly distributed modulo 1

if and only if $\alpha $ is irrational.

Furthermore, we study the number of ordered pairs $(a,b) \in A\times B$

such that $P(a+b)\leq y$ and $S_{q}(a+b) \equiv l \mod m$, $(l \in

\mathbb{Z}$ and $m \geq2)$, for a given sets of integers $A$ and $B$.

Finally, we discuss sums of the form

$$ \sum_{n \in S(x,y)\atop{S_{q}(n) \equiv l \mod m}} f(n-1), $$

where $f$ is a multiplicative function, $l \in \mathbb{Z}$ and $m \geq2$.

#### Zahlentheoretisches KolloquiumACHTUNG - Zeit und Ort des Vortrages haben sich geändert!

**Title:**An invitation to spectral spaces

**Speaker:**Dr. Carmelo Finocchiaro (Univ. Roma Tre)

**Date:**Freitag, 30. 6. 2017, 14:00 s.t.

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24/II

**Abstract:**

Prime ideals and prime spectra of rings play a central role both in

Commutative Ring Theory and Algebraic Geometry, being them foundation of

Scheme Theory, for instance. Order properties and topological properties

of spaces of prime ideals have been useful to characterize some classes of

rings. From the topological point of view, since the 60s it has been of

interest to put in evidence conditions that a topological space must

satisfy in order to be homeomorphic to the prime spectrum of some

(commutative) ring (with 1). This was the main subject of the PhD thesis

of M. Hochster, where he proved that the topological spaces that are

homeomorphic to the prime spectrum of a ring - also called *spectral
spaces* - are precisely the spaces $X$ satisfying the following axioms:

\begin{enumerate}

\item $X$ is compact;

\item $X$ admits a basis of open and compact subspaces that is

closed under finite intersections.

\item $X$ is

*sober*, that is, every irreducible closed

subspace of $X$ has a unique generic point.

\end{enumerate}

While for some class of spectral spaces, like Riemann-Zariski spaces (see

\cite{fi-fo-lo}), a class of rings realizing Hochster corrispondence was

explicitly found, for several other spaces naturally arising in

Commutative Ring Theory it is non trivial to understand if they are

spectral because, in particular, it can be not so easy to verify condition

(3) of Hochster's characterization.

A goal of this survey talk is to present some new perspective about the

study of spectral spaces and, in particular, a criterion, based on

ultrafilters, to decide if a topological space is spectral (see

\cite{fi}). Some recent new examples will be discussed.

\begin{thebibliography}{99}

\bibitem{fi} C. A. Finocchiaro, Spectral spaces and ultrafilters.

*Comm. Algebra*

**42**2014, no. 4, 1496--1508.

\bibitem{fi-fo-lo} C. A. Finocchiaro, M. Fontana, K. A. Loper, The

constructible topology on spaces of valuation domains.

*Trans. Amer.*

Math. Soc.

Math. Soc.

**365**2013, no. 12, 6199--6216.

\end{thebibliography}

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Supersaturation Problem for the Bowtie

**Speaker:**Tamás Makai (TU Graz)

**Date:**Dienstag 27.6.2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

The Tur\'an function $ex(n,F)$ denotes the maximal number of edges in an $F$-free graph on $n$ vertices. However once the number of edges in a graph on $n$ vertices exceeds $ex(n,F)$, many copies of $F$ appear. We study the function $h_F(n,q)$, the minimal number of copies of $F$ in a graph on $n$ vertices with $ex(n,F)+q$ edges. The value of $h_F(n,q)$ has been extensively studied when $F$ is colour critical. In this paper we consider a simple non-colour-critical graph, namely the bowtie and establish bounds on $h_F(n,q)$ when $q=o(n^2)$.

This is joint work with Mihyun Kang and Oleg Pikhurko.

**Title:**Kolloquium aus Finanz- und Versicherungsmathematik

**Speaker:**()

**Date:**26. Juni bis 3. Juli 2017

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

**Abstract:**

Sehr geehrte Damen und Herren,

im Rahmen der Besetzung einer §99 Professoren-Laufbahnstelle aus Finanz- und Versicherungsmathematik am Institut für Statistik finden sechs Bewerbungsvorträge statt, zu denen wir Sie herzlich einladen.

Montag, 26.6.2017

9:00-9:30 Paul Krühner (TU Wien): Density bounds for Ito processes and their applications in acturial and mathematical finance,

10:30-11:00 Julia Eisenberg (Univ. Graz): The effects of exchange rates on some actuarial risk measures,

12:00-12:30 Michaela Szölgyenyi (WU Wien): Numerical methods for SDEs in finance and insurance mathematics

Mittwoch, 28.6.2017

9:00-9:30 Peter Hieber (Univ. Ulm): Retirement products: Recent developments,

10:30-11:00 Stefan Thonhauser (TU Graz): On the solution of optimization problems in insurance

Montag, 3.7.2017

10:30-11:00 Christa Cuchiero (Univ. Wien): Polynomial processes in stochastic portfolio theory

#### FWF START Seminar

**Title:**Counting constrained almost primes

**Speaker:**Sumaia Saad Eddin (Graduate School of Mathematics, Nagoya University, Japan)

**Date:**14.6.2017, 11:00

**Room:**Seminarraum 1 (Geometrie), Kopernikusgasse 24, 4.Obergeschoß

**Abstract:**

A natural number is called $k$-almost prime if it has exactly $k$ prime factors, counted with multiplicity. In this talk I consider the asymptotic counting of such numbers (mostly with $k\leq 3$) with additional constraints put on the prime factors. E.g., if we take $k=2$ and write $n=pq$ with $p$ and $q$ primes of similar size we obtain the so called RSA-integers that play an important role in cryptography. We also consider some examples with $k=3$ inspired by cryptography and the study of coefficients of cyclotomic polynomials. The talk is based on two papers with coauthors P. Moree (accepted in 2016), and Florian Luca, Robert Osburn and Alisa Sedunova (2017+).

#### Strukturtheorie-Seminar

**Title:**Positive Definite Functions on Coxeter Groups and Noncommutative Sidon Sets

**Speaker:**Marek Bożejko (Instytut Matematyczny Polskiej Akademii Nauk)

**Date:**14.06.2017, 15 Uhr c.t.

**Room:**Seminarraum 2 Geometrie (Kopernikusgasse 24, 4.Stock)

**Abstract:**

#### Kolloquiumsvortrag

**Title:**Games and learning: examples from an expanding interface

**Speaker:**Christos H. Papadimitriou (Computer Science Division, University of California at Berkeley)

**Date:**9.6.2017, 15:15

**Room:**HS i11, Inffeldgasse 16b, 1. Kellergeschoss

**Abstract:**

Learning and games comprise some of the most sophisticated behaviors

of agents, and examples of situations combining the two keep coming

up. I will focus on three recent instances in my research: How can

you learn successfully if your data can be of varying quality and is

provided by strategic agents who would rather work less on improving

the data quality? If the points to be classified are strategic, what kind of classifier can best anticipate their gaming of the system? Finally, recent results suggest that learning dynamics in games typically fail to converge to the Nash equilibrium. Is gym this bad news for learning dynamics? Or perhaps for the Nash

equilibrium?

#### Strukturtheorie-Seminar

**Title:**Polynomial Convolutions in Max-Plus Algebra

**Speaker:**Dr. Amnon Rosenmann (TU Graz)

**Date:**Thursday, 8 June 2017, 11:00 c.t.

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

**Abstract:**

Recently Marcus, Spielman & Srivastava (2015) and Marcus (2016) studied polynomial convolutions and Hadamard products that are inspired by free probability. These convolutions capture the expected characteristic polynomials of random matrices. We explore analogues of these convolutions in the setting of Max-Plus Algebra. In this setting, the max-permanent replaces the determinant and the maximum is the analogue of the expected value. Our results resemble those of Marcus et al. Moreover, whereas in the classical setting Marcus et al provide bounds on the roots of the convolution of polynomials, we get exact description of the roots of the convolution of characteristic polynomials in the Max-Plus setting. A brief introduction to operations in Max-Plus Algebra will be given.

This is a joint work with Franz Lehner and Aljosa Peperko.

#### Festvortrag im Rahmen der abgeschlossenen Habilitation

**Title:**Risk modeling and optimization

**Speaker:**Stefan Thonhauser (TU Graz)

**Date:**Mittwoch 07.06.2017, 17:00

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

In this talk I will give some overview on risk theoretic modeling and recent developments.

Classical approaches try to describe insurance risks by means of a stochastic process and quantify them by a risk functional,

typically a short-fall probability or a function depending on a possible deficit. Since these approaches are based on a static parameter choice,

they are oversimplifying in many situations and are not able to cover many realistic phenomena. We will put a focus on controllable extensions

of classical models, which lead to stochastic optimization problems, and touch some model alternatives.

For example such extensions comprise investments in a financial market, risk control via reinsurance or shareholder participation.

Using concrete examples we can discuss different solution procedures and related mathematical questions.

Finally, some relations to other fields of applied mathematics are highlighted.

#### Zahlentheoretisches Kolloquium

**Title:**A babystep-giantstep method for faster deterministic integer factorization

**Speaker:**Markus Hittmeir (Universität Salzburg)

**Date:**Freitag, 2. 6. 2017, 13 Uhr

**Room:**SR Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24, 2.Stock

**Abstract:**

Abstract.}

In 1977, Volker Strassen presented a deterministic and rigorous algorithm for solving the problem to compute the prime factorization of natural numbers $N$. His method is based on fast polynomial arithmetic techniques and runs in time $\widetilde{O}(N^{1/4})$, which has been state of the art for the last forty years. In this talk, we discuss the core ideas for improving the bound by a superpolynomial factor. The runtime complexity of our algorithm is of the form

\[

\widetilde{O}\left(N^{1/4}\exp(-C\log N/\log\log N)\right).

\]

#### Strukturtheorie-Seminar

**Title:**Linear representations of non-commutative rational functions, free probability theory, and large random matrices

**Speaker:**Tobias Mai (Universität des Saarlandes)

**Date:**01.06.2017, 11:00c.t.

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

The concept of linear representations (aka realizations or linearizations) provides some very powerful tool to deal with non-commutative rational functions, namely elements in the universal field of fractions for the ring of non-commutative polynomials (in finitely many variables). While these methods are mostly of purely algebraic origin, they are also nicely compatible with the analytic machinery of (operator-valued) free probability. This theory and the underlying notion of free independence were invented around 1985 by D. Voiculescu, originally for operator-algebraic purposes. It can be seen as a highly non-commutative analogue of classical probability theory and has deep connections to many other fields of mathematics, especially to random matrix theory. In my talk, I will explain how this fascinating interplay leads to explicit algorithms for the computation of distributions and Brown measures, respectively, of evaluations of non-commutative rational functions in freely independent random variables. As we will see, this can be used to determine the asymptotic eigenvalue distribution of certain random matrix models. Furthermore, some concrete examples will show that these algorithms are easily accessible for numerical computations. This is based on joint works with S. Belinschi, J. W. Helton, and R. Speicher.

#### Mathematisches Kolloquium ACHTUNG - Beginnzeit wurde geändert!

**Title:**Parallelism between the growth of the known Mersenne primes and the development of informatics

**Speaker:**Prof. Dr. Attila Pethő (Universität Debrecen, dzt. TU Graz)

**Date:**Mittwoch, 31. 5. 2017

**Room:**15:30: Kaffee, Sozialraum f. Analysis u. Zahlentheorie, Kopernikusg.24/II16:00: Vortrag, Seminarraum f. Statistik (NT03098) Kopernikusg.24/III

**Abstract:**

#### FWF START Seminar

**Title:**Waring-Goldbach Problem with Sparse Subsets of Primes

**Speaker:**Yildirim Akbal (TED University, Ankara, Turkey)

**Date:**18.5.2017, 14:00

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

**Abstract:**

Classical Waring--Goldbach problem concerns representability of all large integers satisfying a certain local condition as sums of fixed number of $k$-th powers

of prime numbers where $k \geq 1$. For instance Goldbach's conjecture states that

every even number $\geq 4$ can be expressed as a sum of two primes. Denote by

$H(k)$ the least integer $s$ such that every sufficiently large positive integer satisfying the aforementioned local condition may be expressed as a sum of

$s$ $k$-th powers of primes. Following the pioneering work of Vinogradov (1937) (which

yields $H(1) \leq 3$), Hua (1938-1959) showed that $H(k) \leq 2^k+1$. He then reduced

his bound to $H(k) \leq 4k \log k(1 + o(1))$ for every large $k$. In this talk, we shall

look at Waring--Goldbach problem with primes chosen from Piatetski Shapiro

sequences; sequences of the form $\left\{ \lfloor n^c \rfloor \right\}_{n=1}^\infty$ where $c > 1$. Such sequences are

known to contain infinitely many primes when $1 < c < 1.18$.

#### FWF START Seminar

**Title:**Inhomogeneous Diophantine Approximation with Restricted Denominators

**Speaker:**Agamemnon Zafeiropoulos (University of York, UK)

**Date:**16.5.2017, 15:00

**Room:**Seminarraum A206, Steyrergasse 30, 2. Stock

**Abstract:**

We formulate and prove a Khintchine-type law for inhomogeneous Diophantine approximation. The denominators form a lacunary sequence, and the probability measure which gives the size of the set of well approximable numbers has Fourier coefficients with a prescribed logarithmic decay rate.

#### FWF START Seminar

**Title:**On the digits of primes

**Speaker:**Gautier Hanna (Aix-Marseille University, France)

**Date:**16.5.2017, 14:00

**Room:**Seminarraum A206, Steyrergasse 30, 2. Stock

**Abstract:**

In this talk, I will explain how we can extend the previous work of Mauduit and Rivat about the Rudin--Shapiro sequence for all sequences which count blocks of digits. At first I will consider the case when the length of the block is constant, and then the case when it is a non decreasing function. If time permits, I will present briefly new results (work in progress with Olivier Robert) about the orthogonality between the Möbius function, and some functions related to polynomials along binary digits (like the Rudin--Shapiro sequence).

#### Strukturtheorie-Seminar

**Title:**Ricci curvature for Markov chains via dynamic optimal transport

**Speaker:**Dr. Jan Maas (Institute of Science and Technology Austria - ISTA)

**Date:**Donnerstag, 11.5.2017, 11 Uhr c.t.

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

In the past decade there has been a lot of progress in analysis on metric measure spaces based on ideas from optimal transport. We discuss how some of these ideas can be developed for Markov chains on discrete spaces, using a discrete analogue of the Kantorovich-Wasserstein metric. In particular we present a discrete notion of Ricci curvature based on geodesic convexity of the entropy, which allows us to obtain discrete functional inequalities, such as spectral gap and logarithmic Sobolev inequalities. We also discuss recent applications to interacting particle systems. This is based on joint works with Matthias Erbar (Bonn), Prasad Tetali (Georgia Tech), and Max Fathi (Toulouse).

#### Vortrag im Seminar Kombinatorik und Optimierung

**Title:**Hilbert's Nullstellensatz and Linear Algebra: An Algorithm for Determining Combinatorial Infeasibility

**Speaker:**Susan Margulies (United States Naval Academy / University of Klagenfurt)

**Date:**9.5. 2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Parterre

**Abstract:**

Unlike systems of linear equations, systems of multivariate polynomial

equations over the complex numbers or finite fields can be compactly used to

model combinatorial problems. In this way, a problem is feasible (e.g. a

graph is 3-colorable, Hamiltonian, etc.) if and only if a given system of

polynomial equations has a solution. Via Hilbert's Nullstellensatz, we

generate a sequence of large-scale, sparse linear algebra computations from

these non-linear models to describe an algorithm for solving the underlying

combinatorial problem. As a byproduct of this algorithm, we produce algebraic

certificates of the non-existence of a solution (i.e., non-3-colorability,

non-Hamiltonicity, or non-existence of an independent set of size k).

In this talk, we present theoretical and experimental results on the size of

these sequences, and the complexity of the Hilbert's Nullstellensatz

algebraic certificates. For non-3-colorability over a finite field, we

utilize this method to successfully solve graph problem instances having

thousands of nodes and tens of thousands of edges. We also describe methods

of optimizing this method, such as finding alternative forms of the

Nullstellensatz, adding carefully-constructed polynomials to the system,

branching and exploiting symmetry.

#### Probevorlesung im Rahmen des Habilitationsverfahrens

**Title:**Die Methode der asymptotischen Entwicklung am Beispiel des senkrechten Wurfs mit Luftwiderstand

**Speaker:**Dr. Günther Of (TU Graz)

**Date:**8.5.2017, 15:00 Uhr

**Room:**AE02

**Abstract:**

Die Vernachlässigung von Termen mit kleinen Parametern in den Differentialgleichun\-gen physikalischer Modelle führt häufig auf stark vereinfachte Modelle und grobe Näherungen der tatsächlichen Lösungen. Die asymptotische Entwicklung bedient sich einer Reihenentwicklung der Lösung und des Originalmodells, um präzisere Approximationen zu ermöglichen.

Die Technik der asymptotischen Entwicklung wird anhand eines Modellbeispiels eingeführt. Dazu wird der senkrechte Wurf mit Luftwiderstand betrachtet, wobei das Stokessche Gesetz zur Beschreibung des Luftwiderstands bei kleinen Geschwindig\-kei\-ten dienen soll. Neben der Lösung des Modells ohne Luftwiderstand wird eine Näherung unter Berücksichtigung der ersten Entwicklungsterme bestimmt. Diese beiden Näherungen werden mit der exakten Lösung für verschiedene Parameter verglichen, insbesondere im Hinblick auf die Wurfhöhe und die Flugdauer.

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Random simplicial complexes: a survey

**Speaker:**Nicola del Giudice (TU Graz)

**Date:**Dienstag 2.5.2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

After their introduction in 1947 by Erd\H{o}s, random graphs had a great impact on discrete mathematics and computer science. Since graphs are one-dimensional simplicial complexes, it is natural to develop an analogous theory for $k$-dimensional random simplicial complexes, for all $k \geq 1$.

In 2006, Linial and Meshulam introduced a first model for the random $2-$dimensional simplicial complex, and since then many scientists focused their attention on this subject, laying the foundations of the application of the probabilistic method in topology.

In this talk I would like to explain the L-M model and the notion of `homological connectivity'. Then, I would like to survey some of the work done in recent years on random simplicial complexes, presenting a different natural model and new results obtained working by analogy with the $G(n,p)$ theory.

**Title:**Schrödinger Operators and Boundary Value Problems

**Speaker:**()

**Date:**24.5. - 28.5.2017

**Room:**Seminar room MBI, Kopernikusgasse 24, 3rd floor

**Abstract:**

\vskip 0.5 cm\noindent

MO 10:00 M. Holzmann, {\it Magnetic Schrödinger operators with electric $\delta$-potentials}

MO 11:00 D. Barseghyan, {\it A regular analogue of the Smilansky model}

MO 14:00 V. Lotoreichik, {\it Optimization of the lowest eigenvalue induced by \hskip 4.95cm singular interactions}

TU 10:00 P. Exner, {\it Leaky graphs and Robin billiards: some open problems}

TU 11:00 M. Khalile, {\it Robin Laplacian on infinite sectors and applications

\hskip 3.9cm

to polygons}

WE 14:00 A. Khrabustovskyi, {\it Periodic Schrödinger operators with $\delta'$-potentials}

WE 15:00 P. Schlosser, {\it A Lieb-Thirring inequality for Schrödinger operators with

\hskip 4.3cm $\delta$-potentials supported on a hyperplane}

TH 10:00 S. Stadler, {\it Eigenvalue inequalities for partial differential operators}

TH 11:00 J. Behrndt, {\it The Birman-Schwinger principle and embedded eigenvalues}

$ $

#### Vortrag im Seminar Kombinatorik und Optimierung

**Title:**An Iterative Time-Bucket Refinement Algorithm for a Resource-Constrained Project Scheduling Problem

**Speaker:**Günther Raidl (TU Wien)

**Date:**21.3. 2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Parterre

**Abstract:**

We consider a resource-constrained project scheduling problem

originating in particle therapy for cancer treatment, in which the

scheduling has to be done in high resolution. Traditional mixed integer

linear programming techniques such as time-indexed formulations or

discrete-event formulations are known to have severe limitations in such

cases, i.e., growing too fast or having weak linear programming relaxations.

We suggest a relaxation based on partitioning time into so-called

time-buckets. This relaxation is iteratively solved and serves as basis for

deriving feasible solutions using heuristics. Based on these primal and dual

bounds the time-buckets are successively refined. Combining these parts we

obtain an algorithm that provides good approximate solutions soon and

eventually converges to an optimal solution. Diverse strategies for doing

the time-bucket refinement are investigated. The approach shows excellent

performance in comparison to the traditional formulations and a GRASP

metaheuristic.

#### Advanced Topics in Discrete Mathematics

**Title:**Many collinear k-tuples with no k+1 collinear points

**Speaker:**Milos Stojakovic (University of Novi Sad)

**Date:**Friday 17.3.2017, 11:00-11:45 Coffee Break from 10:30

**Room:**Seminarraum 2 Geometrie, 4. Stock, Kopernikusgasse 24

**Abstract:**

Paul Erd\H{o}s asked the following question in the 60's: How many collinear k-tuples can a planar $n$-point set contain, if it contains no $k+1$ points on a line (where $k>3$ is fixed)?

We will present an elementary construction that significantly improves the previously known lower bound for this value.

#### FESTVORTRAG

**Title:**Periods & billard on the ellipsoid

**Speaker:**Prof. Dr. Gisbert Wüstholz (ETH Zürich)

**Date:**Donnerstag, 16. 3. 2017, 15 Uhr

**Room:**Seminarraum Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24/II

**Abstract:**

#### Vortragsreihe im Seminar für Kombinatorik und Optimierung

**Title:**Introduction to Positional Games

**Speaker:**Milos Stojakovic (University of Novi Sad)

**Date:**Dienstag 14.3.2017, 14:15-17:00; Freitag 17.3.2017, 14:15-16:00; Dienstag 21.3.2017, 15:30-17:00

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

Positional Game Theory provides a solid mathematical footing for a variety of two-player games of perfect information, usually played on discrete objects, with a number of applications in other branches of mathematics and computer science. The field is just a few decades old, and it has experienced a considerable growth in recent years. Our goal is to introduce some basic concepts and notions, followed by several recent results and open problems.

The prerequisites include just undergraduate knowledge of discrete mathematics and probability, so the lectures could be of interest for people with a wide range of backgrounds and different levels of seniority.

#### FWF START Seminar

**Title:**The L2 discrepancy of Davenport's symmetrized lattice

**Speaker:**Bence Borda (Budapest)

**Date:**14.3.2017, 13.15

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

**Abstract:**

In 1956 Davenport constructed a symmetrized lattice of 2n points with an irrational slope A in the unit square. He proved that if A is badly approximable by rational numbers, then the L2 discrepancy of this point set is at most constant times the square root of log n. I recently showed that the L2 discrepancy is in fact asymptotically equivalent to an explicitly computable constant times the square root of log n in the special case when A is a quadratic irrational. Moreover, this explicit constant factor has surprising connections to the arithmetic of the quadratic field Q(A). In fact, when A is the golden ratio Davenport's symmetrized lattice has a smaller L2 discrepancy than any other construction of a finite point set in the unit square known to this date.

#### Gastvortrag

**Title:**Topological Analysis in Information Spaces

**Speaker:**Hubert Wagner (IST Austria)

**Date:**10.03.2017, 09:00-10:00

**Room:**Seminarraum 2 Geometrie (Kopernikusgasse 24, 4.Stock)

**Abstract:**

Understanding high dimensional data remains a challenging problem.

Computational topology, in an effort dubbed Topological Data Analysis (TDA), promises to simplify, characterize and compare such data. However, standard TDA focuses on Euclidean spaces, while many types of high-dimensional data naturally live in non-Euclidean ones. Spaces derived from text, speech, image... data are best characterized by non-metric dissimilarities, many of which are inspired by information-theoretical concepts. Such spaces will be called information spaces.

I will present the theoretical foundations of topological analysis in information spaces. First, intuition behind basic computational topology methods is given. Then, various dissimilarity measures are defined along with information theoretical and geometric interpretation. Finally, I will show how the framework of TDA can be extended to information spaces. In particular, I will explain to what extent existing software packages can be adapted to this new setting.

No previous knowledge about (computational) topology or information theory is required. This is joint work with Herbert Edelsbrunner and Ziga Virk.

#### Zahlentheoretisches Kolloquium

**Title:**Markov chains, generalized continued fractions, and Pringsheim-type convergence criteria

**Speaker:**Dr. Hendrik Baumann (TU Clausthal)

**Date:**Freitag, 10.3.2017, 14:00 Uhr

**Room:**SR Analysis-Zahlentheorie (NT02008), Kopernikusgasse 24, 2.Stock

**Abstract:**

In many applications, invariant measures for Markov chains have to be calculated. Most often, this has to be done in an algorithmic way. For Markov chains with a special transition structure (quasi-birth-death processes), efficient algorithms can be written in terms of matrix-valued continued fractions.

In the first part of this talk, we will discuss details concerning this relationship, and in particular, we will see that the convergence of the continued fractions which occur in the context of Markov chains is strongly connected to Pringsheim-type convergence criteria for continued fractions.

In the Markov chain context, the continued fractions and its approximants

have a probabilistic interpretation. In the second part of the talk, we

will consider Markov chains with more general transition structures. Based

on the probabilistic interpretation, we will propose a definition for

generalized continued fractions. Although the primary intention of this

construction is the algorithmic treatment of generally structured Markov

chains, it turns out that this definition enables us to

* define $\mathcal{R}$-valued generalized continued fractions

where $\mathcal{R}$is an arbitrary Banach algebra,

* identify a huge variety of generalizations of continued

fractions found in the literature as special cases,

* prove Pringsheim-type criteria in the same way as for

non-generalized continued fractions.

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Deletion of oldest edges in a preferential attachment graph

**Speaker:**Tony Johansson (Uppsala University)

**Date:**Dienstag 7.3.2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

We consider a variation on the Barab\'asi-Albert random graph process with fixed parameters $m\in \mathbb{N}$ and $1/2 < p < 1$. With probability $p$ a vertex is added along with $m$ edges, randomly chosen proportional to vertex degrees. With probability $1 - p$, the oldest vertex still holding its original $m$ edges loses those edges. It is shown that the degree of any vertex either is zero or follows a geometric distribution. If $p$ is above a certain threshold, this leads to a power law for the degree sequence, while a smaller $p$ gives exponential tails.

It is also shown that the graph contains a unique giant component with high probability if and only if $m \ge 2$.

#### FWF START Seminar

**Title:**Influence of Measure on Oscillations of Error Terms

**Speaker:**Kamalakshya Mahatab (NTNU Trondheim)

**Date:**7.3.2017, 13:15

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

**Abstract:**

We shall discuss the influence of Omega bounds on Lebesgue measure of certain sets on oscillation of error terms appearing in various asymptotic formulas.

#### Seminar Angewandte Analysis und Numerische Mathematik

**Title:**Discrete spectrum of interactions concentrated near conical surfaces

**Speaker:**Dr. Thomas Ourmieres-Bonafos (Universite Paris-Sud)

**Date:**28.3.2017 14:00 Uhr

**Room:**AE02

**Abstract:**

We study the spectrum of two kinds of operators involving a conical geometry: the Dirichlet Laplacian in conical layers and Schrödinger operators with attractive $\delta$-interactions supported by infinite cones. Under the assumption that the cones have smooth cross-sections, we prove that such operators have infinitely many eigenvalues accumulating below the threshold of the essential spectrum and we express the accumulation rate in terms of the eigenvalues of an auxiliary one-dimensional operator with a curvature-induced potential.

This is joint work with Konstantin Pankrashkin.

#### Strukturtheorie-Seminar

**Title:**Multipartite rational functions: the universal skew field of fractions of a tensor product of free algebras

**Speaker:**Jurij Volčič (University of Auckland)

**Date:**Dienstag 28.02.2017, 10:15

**Room:**Seminarraum AE02 (STEG006), Steyrergasse 30, EG

**Abstract:**

A commutative ring embeds into a field if and only if it has no zero divisors; moreover, in this case it admits a unique field of fractions. On the other hand, the problem of localization of noncommutative rings and embeddings into skew fields (that is, division rings) is much more complex. For example, there exists noncommutative rings without zero divisors that do not admit embeddings into a skew field, and rings with several non-isomorphic ``skew fields of fractions''. This lead Paul Moritz Cohn to introduce the notion of the universal skew field of fractions to the general theory of skew fields in the 70's. However, almost all known examples of rings admitting universal skew fields of fractions belong to a relatively narrow family of Sylvester domains. One of the exceptions is the tensor product of free algebras. With the help of matrix evaluations we will construct the skew field of multipartite rational functions, which turns out to be the desired universal skew field of fractions. We will also explain its role in the difference-differential calculus in free analysis.

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Random Factor Graph Models: The Replica Symmetric Phase

**Speaker:**Tobias Kapetanopoulos (Goethe-Universität Frankfurt)

**Date:**Dienstag 21.2.2017, 15:00

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

We proved the absence of extensive long-range correlations throughout the replica symmetric phase}, i.e. below the condensation threshold, for a wide class of random factor graph models, including the p-spin Potts antiferromagnet, random k-NAESAT, random k-XORSAT (for even k), etc. This is done by using Janson’s technique of Small Subgraph Conditioning} to nail down the precise limiting distribution of the free energy in this phase. As an application we show that in the replica symmetric phase the random graph model is statistically indistinguishable from the so-called ``planted model''. This result allows us to verify a general conjecture about the reconstruction phase transition in random factor graph models, which deals with the extent of point-to-set correlations. Additionally, we derive a version of the well-known Kesten-Stigum} bound for general factor graph models.

Joint work with Amin Coja-Oghlan, Charilaos Efthymiou, Nor Jaafari and Mihyun Kang

#### Vortrag im Seminar für Kombinatorik und Optimierung

**Title:**Core Forging by Warning Propagation

**Speaker:**Kathrin Skubch (Goethe-Universität Frankfurt)

**Date:**Dienstag 21.2.2017, 14:15

**Room:**Seminarraum AE06, Steyrergasse 30, Erdgeschoss

**Abstract:**

The $k$-core of a graph is the largest subgraph of minimum degree $k$. It can be determined algorithmically by the peeling process that removes an arbitrary vertex of degree less than $k$ while there is one. In this paper we study the $k$-core of the Erd\H{o}s-R\'enyi random graph $G(n,m)$ for $m=\frac{dn}2$ with some fixed positive constant $d>0$.

We propose a new approach to the $k$-core problem in random graphs. More precisely, we devise a randomised algorithm that produces graphs with a $k$-core of a given order and size.

This algorithm is based on an enhanced ``configuration model'' that explicitly designates which vertices will wind up in the core. As it turns out, the necessary structure to construct such configuration model can be set out by way of Warning Propagation, a message passing scheme that plays an important role in physics work on random constraint satisfaction problems.

This is joint work with Amin Coja-Oghlan, Oliver Cooley and Mihyun Kang.

#### Strukturtheorie-Seminar

**Title:**Direct product of automorphism groups of digraphs

**Speaker:**Lukasz Wojakowski (Uniwersitet Wrocławski)

**Date:**Donnerstag 16.02.2017, 10:00 c.t.

**Room:**AE02 (STEG006), Steyrergasse 30, EG

**Abstract:**

The problem of representability of a permutation group $A$ as the full automorphism group of a digraph $G = (V, E)$ was first studied for regular permutation groups by many authors, the solution of the problem for undirected graphs was first completed by Godsil in 1979. For digraphs, L. Babai in 1980 proved that, except for the groups $S_2^2$, $S_2^3$ , $S_2^4$, $C_3^2$ and the eight element quaternion group $Q$, each regular permutation group is the automorphism group of a digraph. Later on, the direct product of automorphism groups of graphs was studied by Grech. It was shown that, except for an infinite family of groups $S_n \times S_n$, $n\ge $2, and three other groups $D_4 \times S_2$, $D_4\times D_4$, and $S_4 \times S_2 \times S_2$, the direct product of automorphism groups of two graphs is, itself, an automorphism group of a graph. We study the direct product of automorphism groups of digraphs. We show that, except for the infinite family of permutation groups $S_n \times S_n , n \ge 2$ and four other permutation groups $D_4 \times S_2$, $D_4 \times D_4$, $S_4 \times S_2 \times S_2$, and $C_3 \times C_3$, the direct product of automorphism groups of two digraphs is itself the automorphism group of a digraph.

#### Vortrag im Seminar Kombinatorik und Optimierung

**Title:**The Traveling Salesperson Problem with Forbidden Neighborhoods on Regular 2D and 3D Grids

**Speaker:**Philipp Hungerländer (Universität Klagenfurt)

**Date:**15.2. 2017, 10:15

**Room:**Seminarraum AE06, Steyrergasse 30, Parterre

**Abstract:**

We suggest and examine an extension of the Traveling Salesperson Problem (TSP)

motivated by an application in mechanical engineering. The TSP with forbidden neighborhoods (TSPFN) with radius $r$ is asking for a shortest Hamiltonian cycle of a given graph G, where vertices traversed successively have a distance larger than $r$.

The TSPFN is motivated by an application in mechanical engineering, more precisely in laser beam melting. This technology is used for building complex workpieces in several layers, similar to 3D printing. For each layer new material has to be heated up at several points. The question is

now how to choose the order of the points to be treated in each layer such that internal stresses are low. Furthermore, one is interested in low cycle times of the workpieces. One idea is to look for

short paths between the points or more precisely between the segments in each layer that do not connect segments that are too close so that the heat quantity in each region is not too high in short

periods. In particular in the instances resulting from this application the layers are rectangular non-regular grids.

In this work we start with the consideration of regular grids, i.e. adjacent vertices in the same row

or column all have the same distance from each other. First we suggest a linear integer

programming formulation of TSPFN. Then we examine TSPFN with $r=0$, $r=1$ and $r= \sqrt 2$. We

determine the length and structure of optimal solutions and show that these problems can be

solved in linear time. After discussing optimal TSPFN tours in the plane we briefly consider the

three dimensional case and determine optimal TSPFN tours for $r=0$ and $r=1$ on regular 3D grids.

(Joint work with Anja Fischer und Anna Jellen)

#### Strukturtheorie-Seminar

**Title:**Free infinite divisibility of $R$-diagonal elements.

**Speaker:**Kamil Szpojankowski (Politechnika Warszawska)

**Date:**Dienstag 14.02.2017, 14:15

**Room:**AE02 (STEG006), Steyrergasse 30, EG

**Abstract:**

#### Gastvortrag

**Title:**Topological Data Analysis through Homology and Discrete Morse Theory

**Speaker:**Ulderico Fugacci (Universitaet Kaiserslautern)

**Date:**13.02.2017, 10:30

**Room:**Seminarraum 2 Geometrie (Kopernikusgasse 24, 4.Stock)

**Abstract:**

In almost all areas of modern applied sciences, data analysis is facing the challenge of extracting useful and relevant features from even larger, high-dimensional and noisy data. Topological Data Analysis (TDA) is a new discipline, spanning algebraic topology and computational geometry, aimed to address these needs. One of the claims in TDA is that data has shape and the shape matters. In a nutshell, TDA gives a general framework to analyze data from this new point of view allowing the retrieval of geometrical but coordinate-free information.

Two of the most relevant tools in TDA consist of homology and discrete Morse theory. Specifically, homology and its recent development, called persistent homology, provide the topological information of a shape including connectivity and the classification of loops, handles, and voids within the space. Discrete Morse theory, on the other hand, is a powerful tool to handle shapes by providing a morphology- and homology-consistent model of the space to be analyzed.

In the talk, we will introduce these two tools focusing our attention on their mutual connections and on the possibility to exploit them for efficiently retrieving the core information from large-size and high-dimensional data.

#### Vortrag im Seminar Kombinatorik und Optimierung

**Title:**Motion planning and random graphs

**Speaker:**Michael Kerber (Institut für Geometrie, TU Graz)

**Date:**7.2.2017, 10:15

**Room:**Seminarraum AE06, Steyrergasse 30, Parterre

**Abstract:**

I will give an introduction to Motion Planning, a branch of computational

robotics with many intersections to computational geometry. Recently, the

study of random graphs became an extremely popular topic in this context. I

will give an overview of the seminal paper ``Sampling-based Algorithms for

Optimal Motion Planning'' by Karaman and Frazzoli (2011) and explain the

connection in some detail.