## Málstofa í stærðfræði

Anders Claesson, febrúar 15, 2018

### Titill: Facially restricted graph colouring’s

Staðsetning: HB-5 (Háskólabíó)
Tími: Mánudagur 19. febrúar kl. 10:50

### Ágrip:

Arguably one of the best known theorems from combinatorics is the four colour theorem, stating that every planar graph can be coloured using at most four colours such that no edge connects two vertices of the same colour. In this talk I will discus variants on this theorem in particular list colouring’s and facial restriction’s on the colouring. In this, I present the method of discharging in Graph Theory, used to finally prove the four colour theorem nearly 140 years after it was first stated, which has been used to prove theorems elsewhere in Mathematics.

## Postdoctoral position in mathematics at the University of Iceland

Sigurður Örn Stefánsson, febrúar 2, 2018

Applications are invited for a postdoctoral position at the University of Iceland financed by The Icelandic Research Fund. The research project is called:

„Scaling limits of random enriched trees“

and is in the field of probabilistic combinatorics. The project includes studying scaling limits of random graphs, statistical mechanical models on random planar maps and related subjects. The application deadline is March 12, however applications will continue to be accepted until the position is filled.

We are looking for a candidate who has completed a PhD within the last 5 years or is close to defending a PhD thesis. Her/his specialization and interests should be in this area.

Applications should be sent directly by e-mail to sigurdur[at]hi.is, including a CV, list of publications or an abstract of a planned PhD thesis, a research statement and names and e-mail addresses of two referees, who have agreed to provide recommendation.

The appointment is temporary for two years from the 1st of August 2018, or otherwise according to agreement, with a possibility of an extension of one year. All applications will be answered.

Ass. Prof. Sigurdur Orn Stefansson (e-mail: sigurdur[at]hi.is)

## PhD position in mathematics at the University of Iceland

Sigurður Örn Stefánsson, febrúar 2, 2018

Applications are invited for a three year PhD position in mathematics at the University of Iceland with a starting date in Fall 2018. The position is funded by a grant from the Icelandic Research Fund.

The successful candidate will work in the area of probabilistic combinatorics with emphasis on scaling limits of random graphs, statistical mechanical models on random planar maps and related subjects. A master degree, or equivalent, in mathematics is required. The application deadline is March 12, however applications will continue to be accepted until the position is filled.

Applications should be sent directly by e-mail to sigurdur[at]hi.is, including a CV, transcripts from undergraduate and master studies, a short description of research interests and names and e-mail addresses of two referees, who have agreed to provide recommendation.

Ass. Prof. Sigurdur Orn Stefansson (e-mail: sigurdur[at]hi.is)

## Hjörtur Björnsson (27/11/17)

Anders Claesson, nóvember 23, 2017

Málstofa í stærðfræði

### Titill: Lyapunov functions for almost sure exponential stability

Staðsetning: VRII-158
Tími: Mánudagur 27. nóvember kl. 15:00

### Ágrip:

We present a generalization of results obtained by X. Mao in his book „Stochastic Differential Equations and Applications“ (2008). When studying what Mao calls „almost sure exponential stability“, essentially a negative upper bound on the almost sure Lyapunov exponents, he works with Lyapunov functions that are twice continuously differentiable in the spatial variable and continuously differentiable in time. Mao gives sufficient conditions in terms of such a Lyapunov function for a solution of a stochastic differential equation to be almost surely exponentially stable. Further, he gives sufficient conditions of a similar kind for the solution to be almost surely exponentially unstable. Unfortunately this class of Lyapunov functions is too restrictive. Indeed, R. Khasminskii showed in his book „Stochastic Stability of Differential Equations“ (1979/2012) that even for an autonomous stochastic differential equation with constant coefficients, of which the solution is stochastically stable and such that the deterministic part has an unstable equilibrium, there cannot exists a Lyapunov function that is differentiable at the origin. These restrictions are inherited by Mao’s Lyapunov functions. We therefore consider Lyapunov functions that are not necessarily differentiable at the origin and we show that the sufficiency conditions Mao proves can be generalized to Lyapunov functions of this form.

## Sigurður Freyr Hafstein (13/11/17)

Anders Claesson, nóvember 9, 2017

Málstofa í stærðfræði

### Titill: Dynamical Systems and Lyapunov functions

Staðsetning: VRII-158
Tími: Mánudagur 13. nóvember kl. 15:00

### Ágrip:

We discuss dynamical systems and the theory of Lyapunov functions and complete Lyapunov functions. Further, we discuss several different numerical methods for the computation of Lyapunov functions and the corresponding estimation of basins of attraction.

## Sigurður Örn Stefánsson (30/10/17)

Anders Claesson, október 26, 2017

Málstofa í stærðfræði

### Titill: The phase structure of random outerplanar maps

Staðsetning: VRII-158
Tími: Mánudagur 30. október kl. 15:00

### Ágrip:

An outerplanar map is a drawing of a planar graph in the sphere which has the property that there is a face in the map such that all the vertices lie on the boundary of that face. We study the phase diagram of random outerplanar maps sampled according to non-negative weights that are assigned to each face of a map. We prove that for certain choices of weights the map looks like a rescaled version of its boundary when its number of vertices tends to infinity. The outerplanar maps are then shown to converge in the Gromov-Hausdorff sense towards the α-stable looptree introduced by Curien and Kortchemski (2014), with the parameter α depending on the specific weight-sequence. This allows us to describe the transition of the asymptotic geometric shape from a deterministic circle to the Brownian tree.

Based on arXiv:1710.04460 with Benedikt Stufler.

## Bjarki Ágúst Guðmundsson (18/09/17)

Anders Claesson, september 13, 2017

Málstofa í stærðfræði

### Titill: Enumerating permutations sortable by k passes through a pop-stack

Staðsetning: VRII-158
Tími: Mánudagur 18. september kl. 15:00

### Ágrip:

In an exercise in the first volume of his famous series of books, Knuth considered sorting permutations by passing them through a stack. He noted that, out of the $$n!$$ permutations on $$n$$ elements, $$C_n$$ of them can be sorted by a single pass through a stack, where $$C_n$$ is the $$n$$-th Catalan number. Many variations of this exercise have since been considered, including allowing multiple passes through the stack and using different data structures. West classified the permutations that are sortable by 2 passes through a stack, and a formula for the enumeration was later proved by Zeilberger. The permutations sortable by 3 passes through a stack, however, have yet to be enumerated. We consider a variation of this exercise using pop-stacks. For any fixed $$k$$, we give an algorithm to derive a generating function for the permutations sortable by $$k$$ passes through a pop-stack. Recently the generating function for $$k=2$$ was given by Pudwell and Smith (the case $$k=1$$ being trivial). Running our algorithm on a computer cluster we derive the generating functions for $$k$$ at most 6. We also show that, for any $$k$$, the generating function is rational.

## Nuno Romao (15/09/17)

Anders Claesson, september 11, 2017

Málstofa í stærðfræði

### Titill: Vortex moduli and the physics of gauged sigma-models

Staðsetning: Naustið, Endurmenntun
Tími: Föstudagur 15. september kl. 13:30

### Ágrip:

Vortices appear as static and stable solutions in field theories known as gauged sigma-models; these are defined on surfaces and can have both linear and nonlinear targets. I will give an overview of recent results concerning the underlying moduli spaces (parametrizing all vortex configurations up to gauge equivalence with fixed topology) and explain their physical significance. My talk will focus on the case where the target is a two-sphere with circle action; in this simple nonlinear model, many important questions can be answered at least in particular examples.

## Uwe Leck and Ian Roberts (01/09/17)

Anders Claesson, ágúst 29, 2017

Málstofa í stærðfræði

### Titill: Extremal problems for finite sets related to antichains

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Föstudagur 1. september kl. 13:30

### Ágrip:

This talk requires little more than mathematical maturity as it relates to problems concerning finite sets. The basic ideas are simple, and the problems are easy to state, but the problems range from simple to very hard.
An antichain (or Sperner family) in the Boolean lattice $$B_n$$ is a collection $$A$$ of subsets of $$[n] = \{1,2,…,n\}$$ such that no set in $$A$$ is a subset of another. By Sperners famous theorem, the largest possible cardinality of an antichain in $$B_n$$ is $${n \choose\lfloor n/2\rfloor}$$. Antichains are fundamental in extremal set theory; but also with applications in other areas such as Search Theory.
We will discuss several extremal problems and results involving antichains such as: minimizing the union-closure of uniform antichains of a given size; finding the number of different antichains in $$B_n$$; determining the possible cardinalities of maximal antichains; and others.
Some of the problems are solved and some provide tantalising unsolved problems.

## Marston Conder (13/07/17)

Anders Claesson, júlí 12, 2017

Málstofa í stærðfræði

### Titill: Experimental Algebra and Combinatorics

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Fimmtudagur 13. júlí kl. 11:00

### Ágrip:

Some 40 years after the computer-based proof of the 4-Colour Theorem by Appel and Haken, there is still a degree of healthy skepticism about the use of computers to prove nice theorems in mathematics. But there is a distinction between proofs that are highly dependent on computation (verifiable or otherwise), and the use of computer-based experimentation to analyse and construct examples, to produce data that might exhibit patterns from which conjectures can be drawn and tested, or to investigate a range of possible scenarios — subsequently leading to theorems that can be proved by hand.
In this talk I will describe a range of instances of experimental computations involving finite and infinite groups that have led to unexpected but theoretically provable discoveries about discrete objects possessing a high degree of symmetry. These include discoveries about the genus spectra of particular classes of regular maps on surfaces, the smallest regular and chiral polytopes, and various kinds of edge-transitive graphs. Such examples highlight the value of experimental computation, and the surprising outcomes it can often produce.