## Thomas Selig (12/10/2018)

Valentina Giangreco, október 2, 2018

Málstofa í stærðfræði

### Titill: The Abelian sandpile model on permutation graphs

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 12. Óktober kl. 11.40

### Ágrip:

A permutation graph is a graph whose edges are given by the inversions of a permutation. The Abelian sandpile model (ASM) is a Markov chain on the set of so-called configurations of a graph. Of particular interest are the recurrent configurations, i.e. those that appear infinitely often in the long-time running of the model. We exhibit a bijection between the set of recurrent configurations for the ASM on permutation graphs and the set of tiered trees, introduced by Duggan et al. This provides a new bijective proof of a famous result linking the level polynomial of the ASM to the ubiquitous Tutte polynomial. We also show a link between the minimal recurrent configurations and the set of complete non-ambiguous binary trees, introduced by Aval et al.

## Tobias Zingg (04/10/2018)

Valentina Giangreco, október 2, 2018

Math Phys seminar

### Titill: plasmons and physical density-density response from holography

Staðsetning: Aðalbygging A-069

Tími: Thursday October 4, at 1.20 -2.20 pm

### Ágrip:

The holographic correspondence can be extended to identify plasmonic excitations in strongly correlated systems. These modes have relevance in the growing field of plasmonics, and play also a crucial role in relating properties of holographic ‘screened’ density-density response response functions to ‘physical’ density-density response functions, with the latter being the quantity actually related to experimental observation.

## Þorsteinn Jónsson (02/07/18)

Anders Claesson, júní 29, 2018

Málstofa í stærðfræði

### Titill: Hnikaaðferðir til að læra dreififöll gagna

Staðsetning: V-147 (VR-II)
Tími: Mánudagur 2. júlí kl. 10:30

### Ágrip:

Á þessari málstofu mun ég kynna safn aðferða sem að leyfir okkur að skilgreina tölfræðileg líkön sem lýsa líkindadreifingum sem búa til gögn.
Til þessa kynnum við tauganet sem hraða, en ótrúlega áhrifaríka leið til að leysa hnikaverkefni fyrir vel valið kostnaðarfall.
Ég mun ræða tvær mismunandi aðferðir til þess að skilgreina þetta kostnaðarfall ásamt því að sýna áhugaverðar niðurstöður.

## Thomas Selig (27/6/18)

Sigurður Örn Stefánsson, júní 25, 2018

Málstofa í stærðfræði

### Titill: EW-tableaux, permutations and recurrent configurations of the sandpile model on Ferrers graphs.

Staðsetning: VRII, V-147
Tími: Miðvikudagur 27. júní kl. 10:30

### Ágrip:

The Abelian sandpile model (ASM) is a dynamic process on a graph. More specifically, it is a Markov chain on the set of configurations on that graph. Of particular interest are the recurrent configurations, i.e. those that appear infinitely often in the long-time running of the model. We study the ASM on Ferrers graphs, a class of bipartite graphs in one-to-one correspondence with Ferrers diagrams. We show that minimal recurrent configurations are in one-to-one correspondence with a set of certain 0/1 fillings of the Ferrers diagrams introduced by Ehrenborg and van Willigensburg. We refer to these fillings as EW-tableaux, and establish a bijection between the set of EW-tableaux of a given Ferrers diagram and a set of permutations whose descent bottoms are given by the shape of the Ferrers diagram. This induces a bijection between these permutations and minimal recurrent configurations of the ASM. We enrich this bijection to encode all recurrent configurations, via a decoration of the corresponding permutation. We also show that the set of recurrent configurations over all Ferrers graphs of a given size are in bijection with the set of alternating trees of that size.

## Delphin Sénizergues (11/06/18)

Sigurður Örn Stefánsson, júní 11, 2018

Málstofa í stærðfræði

### Titill: Random metric spaces constructed using a gluing procedure

Staðsetning: VRII-V147
Tími: Mánudagur 18. júní kl. 10:50

### Ágrip:

I will introduce a model of random trees which are constructed by iteratively gluing an infinite number of segments of given length onto each other. This model can be generalized to a gluing of „blocks“ that are more complex than segments. We are interested in the metric properties of the limiting metric space, mainly its Hausdorff dimension. We will show that its Hausdorff dimension depends in a non-trivial (and surprising !) manner on the different scaling parameters of the model and the dimension of the blocks.

## Tony Guttmann (28/05/18)

Anders Claesson, maí 25, 2018

Málstofa í stærðfræði

### Titill: On the number of Av(1324) permutations

Staðsetning: V-147 (VR-II)
Tími: Mánudagur 28. maí kl. 10:50

### Ágrip:

We give an improved algorithm for counting the number of 1324-avoiding permutations, resulting in 14 further terms of the generating function, which is now known to length 50.
We re-analyse the generating function and find compelling evidence that unlike other classical length-4 pattern-avoiding permutations, the generating function does not have a simple power-law singularity, but rather, the number of 1324-avoiding permutations of length n behaves as $$B\cdot \mu^n \cdot \mu_1^{\sqrt{n}} \cdot n^g$$.
We estimate $$\mu = 11.600 \pm 0.003.$$ The presence of the stretched exponential term $$\mu_1^{\sqrt{n}}$$ is an unexpected feature of the conjectured solution, but we show that such a term is present in a number of other combinatorial problems.
(A.J. Guttmann with A.R. Conway and P. Zinn-Justin)

## Kevin John Torres Grosvenor (22/05/18)

Anders Claesson, maí 17, 2018

Math Phys seminar

### Titill: Nonrelativistic Naturalness and the quest for Emergent Lorentz Symmetry

Staðsetning: V-147 (VR-II)
Tími: Þriðjudagur 22. maí kl. 10:30

### Ágrip:

I will discuss our proposal for a nonrelativistic solution to the Higgs mass hierarchy problem, its dependence on the emergence of various shift symmetries, and the role it might play in the search for a nonrelativistic theory of particle physics in which Lorentz symmetry emerges at „low“ energies.

## Michael Melgaard (13/04/18)

Anders Claesson, apríl 10, 2018

Málstofa í stærðfræði

### Titill: Rigorous mathematical results on nonlinear PDEs arising in Quantum Chemistry

Staðsetning: V-147 (VR-II)
Tími: Föstudagur 13. apríl kl. 13:30

### Ágrip:

An introduction to electronic structure models is given and rigorous results are discussed on the existence of solutions (ground states and excited states) to weakly coupled, semi-linear elliptic PDEs with nonlocal operators arising in Hartree-Fock, Kohn-Sham and multiconfigurative many-particle models in quantum chemistry, in particular for systems with relativistic effects and external magnetic fields.

## Matteo Baggioli (09/04/18)

Anders Claesson, apríl 5, 2018

Math Phys seminar

### Titill: Holographic quantum phase transitions: time to get dirty!

Staðsetning: HB-5 (Háskólabíó)
Tími: Mánudagur 9. apríl kl. 10:50

### Ágrip:

We study the effects of quenched one-dimensional disorder on the holographic Weyl semimetal quantum phase transition (QPT). We observe the smearing of the sharp QPT linked to the appearance of rare regions at the horizon where the local order parameter is non-zero. We discuss the role of the disorder correlation and we compare our results to the weakly coupled expectations from condensed matter theory and simulations. We analyze also the interplay of finite temperature and disorder and we find preliminary indications for the presence of log-oscillatory structures in the order parameter.

## Anthony Thomas Lyons (26/03/18)

Anders Claesson, mars 22, 2018

Málstofa í stærðfræði

### Titill: The dressing method for the Camassa-Holm equation

Staðsetning: HB-5 (Háskólabíó)
Tími: Mánudagur 26. mars kl. 10:50

### Ágrip:

The Camassa-Holm equation is a nonlinear shallow water model which has been the focus of a great deal of mathematical research in hydrodynamics for the past two decades. This interest is in part due to the versatility of the system, being relevant as a fluid model possessing solutions which display wave-breaking along with global solutions in the form of soliton, peakon and cuspon solutions.
The inverse scattering transform has been successfully implemented to construct numerous global solutions of this system, and in this talk we present a recently developed variation of this method for the Camassa-Holm equation, known as the dressing method. This efficient implementation allows one to integrate several nonlinear hydrodynamical models, and in particular we shall outline the details of this new dressing method and use it to construct the one and two-soliton solutions of the Camassa-Holm equation.