Watse Sybesma, Háskóli Íslands

Carlos Argaez Garcia, October 27, 2020

Math Phys colloquium

Speakers: Watse Sybesma, University of Iceland

Title: The formation of black holes and the supermassive object at the center of our galaxy.

Room:  Via Zoom. Link to be sent.
Time: Friday 30th October, 12:00am

Abstract:

This year’s Nobel Prize in Physics is all about black holes with a narrative that covers both the theoretical and observational point of view. On the theoretical side: Roger Penrose, who showed that the theory of general relativity allows the formation of black holes. On the observational side: Reinhard Genzel and Andrea Ghez, who provided evidence that an invisible and extremely heavy object governs the orbits of stars at the center of our galaxy. A supermassive black hole is the only currently known explanation. In this talk an introductory overview is provided. Watse Sybesma (born in 1989 in the Netherlands) obtained his PhD degree at the university of Utrecht in the Netherlands, where he worked on black holes related topics. Afterwards, he obtained a grant from the Icelandic research fund (Rannís) to further pursue this line of research at the mathematical physics group of the university of Iceland as a postdoctoral researcher.

Adam Timar, University of Iceland

Carlos Argaez Garcia, October 23, 2020

Math colloquium

Speakers: Adam Timar, University of Iceland

Title: Uniform Spanning Forests of infinite graphs.

Room:  Via Zoom. Link to be sent.
Time: Friday 30th October, 10:00am

Abstract:

Consider a spanning tree of a given finite graph, chosen uniformly at random. The so-defined Uniform Spanning Tree (UST) has long been an object of interest, since it is intimately connected to harmonic functions on the graph and random walks.
Given an infinite graph G, such as a cubic lattice, one can take its exhaustion by finite graphs G_n and consider a suitably defined limit of the UST of G_n. The limiting measure is called the Uniform Spanning Forest of G. The resulting random object has had crucial importance in statistical physics on 2 dimensional lattices, but also turned out to be interesting when G is a Cayley graphs of a group. Among other uses, it allows for a simple probabilistic interpretation of an intricate geometric parameter.
The talk will be addressed to a general math audience. We will give a brief overview of the above topics, and present some recent results that are joint work with Gábor Pete.

Lukas Schneiderbauer

Valentina Giangreco, September 24, 2020

PhD defense, Friday September 25 at 14.00, in Veröld, room 023

Semi-Classical Black Hole Holography 

The defense will be streamed: https://www.youtube.com/user/HIvarp/live

Dissertation title: Semi-Classical Black Hole Holography 

Opponents: Dr. Veronika Hubeny, Professor of Physics at UC Davis,USA
Dr. Mukund Rangamany, Professor of Physics at UC Davis, USA

Advisor: Dr. Lárus Thorlacius, Professor at the Faculty of Physical Sciences, University of Iceland

Doctoral committee: Dr. Bo Sundborg, Professor at the Department of Physics at Stockholm University, Sweden
Dr. Valentina Giangreco M. Puletti, Professor at the Faculty of Physical Sciences, University of Iceland
Dr. Þórður Jónsson, Professor at the Faculty of Physical Sciences, University of Iceland

Chair of Ceremony: Dr. Einar Örn Sveinbjörnsson, Professor and the Head of the Faculty of Physical Sciences, University of Iceland

Abstract:
This thesis discusses two aspects of semi-classical black holes.
First, a recently improved semi-classical formula for the entanglement entropy of
black hole radiation is examined. This entropy is an indicator of information loss and determines whether black hole evaporation is an information preserving process or destroys quantum information. Assuming information conservation, Page expressed the entanglement entropy as a function of time, which is referred to as the “Page curve.” Using the improved formula for evaporating black hole solutions of a gravitational model introduced by Callan, Giddings, Harvey and Strominger (CGHS) and modified by Russo, Susskind and Thorlacius (RST), we find that the entanglement entropy follows the Page curve and thus is consistent with unitary evolution.
Second, the notion of quantum complexity is explored in the context of black holes. The quantum complexity of a quantum state measures how many “simple operations” are needed to create that state. Susskind conjectured that the quantum complexity of a black hole state corresponds to a certain volume inside the black hole. A modified conjecture equates the quantum complexity with the gravitational action evaluated for a certain region of spacetime which intersects the black hole interior. We test the complexity conjectures for semi-classical black hole solutions in the CGHS/RST model and find that both conjectures yield the expected behavior.

Kristján Jónasson, University of Iceland.

Carlos Argaez Garcia, September 11, 2020

Math colloquium

Speakers: Kristján Jónasson, University of Iceland.

Title: Maximum likelihood estimation of multivariate normal parameters when values are missing.

Room:  Via Zoom. Link to be sent.
Time: Friday 18th September, 10:00am

Abstract:

I have been working on a program to estimate the covariance matrix of a multivariate normal distribution in the presence of missing values via maximum likelihood. Many programs offer to do this by computing pairwise covariances (giving a potentially non-positive-definite matrix). There is a package in R (mvnmle) to do the ML-computation, but it is inefficient on several counts. Matlab’s statistical toolbox has a function mvnmle, and its financial toolbox has ecmnmle which are both quite fast, but they lack flexibility, for example to incorporate REML to eliminate bias, to use regularization (when many values are missing), or to reduce the number of parameters by incorporating some variance structure.

This work is in progress and still unpublished but preliminary results are promising. In the talk I shall tell you a little about the program and the underlying algorithms.

Bobby Cheng, University of Sussex, UK

Carlos Argaez Garcia, September 11, 2020

Math colloquium

Speakers: Bobby Cheng, University of Sussex, UK

Title: Quantum Resonances in Relativistic Systems.

Room:  Via Zoom. Link to be sent.
Time: Friday 9th October, 10:00am

Abstract:

Significant amounts of research have been completed on mathematical quantum resonances in the non-relativistic setting. However success in generalizing these results to the relativistic setting have been limited. In this talk I will describe the work undertaken to study resonances of the Dirac operator, perturbed by an electric potential with certain ‘nice’ properties, and establish two key trace formulae.

Peter Giesl, University of Sussex, UK

Carlos Argaez Garcia, September 11, 2020

Math colloquium

Speakers: Peter Giesl, University of Sussex, UK

Title: Existence and construction of a contraction metric as solution of a matrix-valued PDE.

Room:  Via Zoom. Link to be sent.
Time: Friday 2nd October, 10:00am

 

Abstract:

A contraction metric is a Riemannian metric, with respect to which the distance between adjacent solutions of an ordinary differential equation (ODE) decreases.

A contraction metric can be used to prove existence and uniqueness of an equilibrium of an autonomous ODE and determine a subset of its basin of attraction without requiring information about its location. Moreover, a contraction metric is robust to small perturbations of the system. 

We will prove a converse theorem, showing the existence of a contraction metric for an equilibrium by characterising it as a matrix-valued solution of a certain linear partial differential equation (PDE). This leads to a construction method by numerically solving the matrix-valued PDE using mesh-free collocation. We use and present a recent extension of mesh-free collocation of scalar-valued functions, solving linear PDEs, to matrix-valued ones. Finally, we briefly discuss a method to verify that the computed metric satisfies the conditions of a contraction metric.

This is partly work with Holger Wendland, Bayreuth as well as Sigurdur Hafstein and Iman Mehrabinezhad, Iceland.

Joachim Toft , Department of mathematics, Faculty of Technology, Linnæus University, Växjö, Sweden

Carlos Argaez Garcia, September 11, 2020

Speakers: Joachim Toft , Department of mathematics, Faculty of Technology, Linnæus University, Växjö, Sweden

Title: Analytic pseudo-differential calculus via the Bargmann transform.

Room:  Via Zoom. Link to be sent.
Time: Friday 25th September, 10:00am

Abstract:

The Bargmann transform is a transform which maps Fourier-invariant function spaces and their duals to certain spaces of formal power series expansions, which sometimes are convenient classes of analytic functions.

In the 70th, Berezin used the Bargmann transform to translate problems in operator theory into a pseudo-differential calculi, where the involved symbols are analytic functions, and the corresponding operators map suitable classes of entire functions into other classes of entire functions.

Recently, some investigations on certain Fourier invariant subspaces of the Schwartz space and their dual (distribution) spaces have been performed by the author. These spaces are called Pilipovi ́c spaces, and are defined by imposing suitable boundaries on the Hermite coefficients of the involved functions or distributions. The family of Pilipovi ́c spaces contains all Fourier invariant Gelfand- Shilov spaces as well as other spaces which are strictly smaller than any Fourier invariant non-trivial Gelfand-Shilov space. In the same way, the family of Pilipovi ́c distribution spaces contains spaces which are strictly larger than any Fourier invariant Gelfand-Shilov distribution space.

In the talk we show that the Bargmann images of Pilipovi ́c spaces and their distribution spaces are convenient classes of analytic functions or power series expansions which are suitable when investigating analytic pseudo-differential operators (i. e. Berezin or Wick operators).

We deduce continuity properties for such pseudo-differential operators when the symbols and target functions possess certain (weighted) Lebesgue estimates. We also show that the counter image with respect to the Bargmann transform of these results generalise some continuity results for (real) pseudo-differential operators with symbols in modulation spaces, when acting on other modulation space.

The talk is based on collaborations with Nenad Teofanov and Patrik Wahlberg, and parts of the content of the talk is available at:

N. Teofanov, J. Toft Pseudo-differential calculus in a Bargmann setting, Ann. Acad. Sci. Fenn. Math. 45 (2020), 227–257.

Yadollah Zare, Galatasaray University

Carlos Argaez Garcia, June 9, 2020

Math colloquium

Speakers: Yadollah Zare, Galatasaray University

Title: Center conditions: pull-back of differential equations.

Abstract:

The space of polynomial differential equations of a fixed degree with a center singularity has many irreducible components. We prove that pull-back differential equations form an irreducible component of such a space. This method is inspired by Ilyashenko and Movasati’s method. The main concepts are the Picard-Lefschetz theory of a polynomial in two variables with complex coefficients, the Dynkin diagram of the polynomial and the iterated integral.

Room:  V-158, VR-II
Time: Friday 12th June, 10:00am

Notice that is recommended to keep two meters apart from other attendants.

Meistarafyrirlestrar á næstunni

Benedikt Magnússon, May 29, 2020
28. maí14:00Tölfræði 
Statistics
Þórarinn JónmundssonLíkön og aðferðir til að meta lærdóm: greining árangur nemenda í stærðfræðigreininguModels and methods to evaluate learning: a case study of students enrolled in mathematical analysis
2. júní11:00Hagnýt Tölfræði
Applied Statistics
Þórey HeiðarsdóttirGreining með slembiþáttalíkani á þróun blóðþrýstings og gönguvegalengdar í tveggja ára langtímarannsóknUsing mixed models to analyse progression of blood pressure and walking distance in a two year longitudinal study 
2. júní14:00Hagnýt Tölfræði
Applied Statistics
Ólafur Jón JónssonGreining á niðurstöðum kennslukannana Háskóla Íslands 2013-17Analysis of results from student evaluation of teaching surveys in the University of Iceland 2013 – 2017
3. júní13:00Tölfræði
Statistics
Sindri Emmanúel AntonssonÁhættureiknar fyrir sykursýki aðlagaðir að íslensku þýðiAdapting diabetes risk scores to an Icelandic population
3. júní11:00Stærðfræði
Mathematics
Bergur SnorrasonRudin-Carleson theoremsRudin-Carleson setningar
3. júní13:00Stærðfræði
Mathematics
Hjörtur BjörnssonCovering Spaces for Domains in the Complex PlaneÞekjurúm fyrir svæði í tvinntalnasléttunni
3. júní13:30Stærðfræði
Mathematics
Hulda Hvönn KristinsdóttirThe art of counting – Textbook in enumerative combinatorics for upper secondary schoolsListin að telja – Kennslurit í talningar- og fléttufræði fyrir framhaldsskóla

María Óskarsdóttir, Háskólinn í Reykjavík

Carlos Argaez Garcia, March 5, 2020

Speaker: María Óskarsdóttir, University of Reykjavík

Title: Ranking nodes relative to influence with the Personalized PageRank algorithm applied to fraud detection and credit risk measurement

Room: VRII-258
Time: Thursday 12th March, 10:50hrs

Abstract:

Various phenomena in both the physical and the digital world can be represented with networks, that is, entities that are connected in some way, for example communication, computer, financial and social networks. A central theme in the analysis of networks is finding the most important nodes in a network. The PageRank algorithm was developed to rank webpages in search engines, to find the most important webpages on the internet, but has been applied in numerous others applications. The ranking can be personalized so that nodes which are important relative (or close) to a predefined set of nodes are ranked higher. This approach has been used to identify certain behavior in networks where there is a strong social effect, for example fraud and churn. In this presentation we show how the personalized PageRank algorithm can be extended for two specific types of networks. First, we look at a bipartite network which consists of claims and the involved parties, i.e. policyholders and brokers, with the goal of finding fraudulent insurance claims. Then we consider multiplex networks, in which each node can be connected to another node by more than one type of edge, such as two different networks connecting the same individuals. They arise naturally in lending, as two borrowers can be connected by geographical location, economic activity, and many other relationships. We present a methodology to leverage multiplex networks by a novel multiplex Personalized PageRank algorithm, which we subsequently apply to credit risk assessment.