## 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.

## Daniel Fernandez Moreno (07/07/17)

Anders Claesson, júlí 4, 2017

Málstofa í stærðfræði

### Titill: Entanglement entropy at non-equilibrium in holography

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Föstudagur 7. júlí kl. 13:20

### Ágrip:

In recent years, holographic models have proved to be successful at studying far-from-equilibrium physics. This provides a new approach to studying quantum quenches in strongly coupled systems. In this talk, based on ArXiv:1705.04696, I will focus on the local quench-like time evolution obtained by joining two 1+1 dimensional heat baths at different temperatures. I will present results for the entanglement entropy of different entangling regions obtained by adapting the time-dependent Hubeny-Rangamani-Takayanagi prescription. The interest of this study relies on the presence of emergent collective behavior, which can provide insight into the interplay between quantum effects and out of equilibrium physics.

## Christopher Kellett (06/07/17)

Anders Claesson, júlí 4, 2017

Málstofa í stærðfræði

### Titill: Back and Forth in Lyapunov’s Second Method: Non-Uniform Subtleties

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Fimmtudagur 6. júlí kl. 13:20

### Ágrip:

Lyapunov’s second or direct method provides an easy-to-check sufficient condition for stability properties of equilibria. The converse question – given a stability property, does there exist an appropriate Lyapunov function? – has been fundamental in differentiating and classifying different stability properties, particularly with regards to „uniform“ stability.
In this talk, I will review the usual textbook definitions for Lyapunov functions for time-varying systems and describe where they are deficient. Some interesting new sufficient (and probably necessary) conditions pop up along the way.

## Peter J. Olver (09/06/17)

Anders Claesson, júní 7, 2017

Málstofa í stærðfræði

### Titill: Equivalence, Invariants, Puzzles, and Cancer

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Föstudagur 9. júní kl. 16:00

### Ágrip:

A fundamental issue in computer vision is recognizing when two objects in an image are the „same“. The underlying mathematical apparatus for studying such equivalence problems is transformation group (or, more generally, groupoid) theory. Cartan’s solution to the equivalence and symmetry problem for submanifolds relies on the associated geometric invariants, through what is now known as the differential invariant signature. Furthermore, the new equivariant approach to the method of moving frames provides a systematic and algorithmic approach that can be applied to very general Lie group and even Lie pseudo-group actions. The talk will conclude with recent applications to automated assembly of broken objects, such as jigsaw puzzles, and cancer detection.

## Örn Arnaldsson (06/06/17)

Anders Claesson, júní 2, 2017

PhD thesis defense rehearsal

### Titill: Involutive moving frames

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Þriðjudagur 6. júní kl. 10:50

### Ágrip:

Cartan’s equivalence method and the method of the equivariant moving frames are the two best known methods for solving equivalence problems in differential geometry, differential equations, calculus of variations and control theory. My thesis demonstrates how the two methods are really two sides of the same coin and combines them in a powerful hybrid method that increases the computational efficiency of both of its progenitors. This novel viewpoint provides effortless proofs to some previously hard-to-prove results, such as the Lie-Tresse theorem. Furthermore, we obtain proof of a long standing conjecture of Cartan on the termination of his equivalence method.

## Valentina Giangreco (26/05/17)

Anders Claesson, maí 24, 2017

Málstofa í stærðfræði

### Titill: Non-analyticity of holographic Rényi entropy in Lovelock gravity

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Föstudagur 26. maí kl. 13:20

### Ágrip:

In the first part of my talk I will introduce the definition of Rényi entropy, and some basic concepts of the so-called holographic principle (AdS/CFT). The second part of the talk is devoted to the analysis of certain black hole instabilities in gravity theories with higher derivative corrections (Lovelock gravity theories) and their implications for the holographic Rényi entropy.

## Jason Smith (21/04/17)

Anders Claesson, apríl 18, 2017

Málstofa í stærðfræði

### Titill: Poset Fibrations and Their Applications to Pattern Posets

Staðsetning: Tg-227 (Tæknigarður, 2. hæð)
Tími: Föstudagur 21. apríl kl. 13:30

### Ágrip:

A poset fibration is a rank and order preserving surjective map between posets. It was shown by Quillen that many properties of posets can be maintained across a fibration, we introduce some of these results. Pattern occurrence has been studied on a wide range of combinatorial objects, and using the notion of pattern containment we can define a poset on these objects. Many such pattern posets have been studied in the literature and the results on these posets often have a similar theme. Using poset fibrations we introduce some results that show why such similarities arise between different pattern posets.