Tue, 11 Oct 2022

14:30 - 15:00
L3

Fooled by optimality

Nick Trefethen
(University of Oxford)
Abstract

An occupational hazard of mathematicians is the investigation of objects that are "optimal" in a mathematically precise sense, yet may be far from optimal in practice. This talk will discuss an extreme example of this effect: Gauss-Hermite quadrature on the real line. For large numbers of quadrature points, Gauss-Hermite quadrature is a very poor method of integration, much less efficient than simply truncating the interval and applying Gauss-Legendre quadrature or the periodic trapezoidal rule. We will present a theorem quantifying this difference and explain where the standard notion of optimality has failed.

Mon, 15 Mar 2021

15:45 - 16:45
Virtual

Unknot recognition in quasi-polynomial time

Marc Lackenby
(University of Oxford)
Abstract

I will outline a new algorithm for unknot recognition that runs in quasi-polynomial time. The input is a diagram of a knot with n crossings, and the running time is n^{O(log n)}. The algorithm uses hierarchies, normal surfaces and Heegaard splittings.

Mon, 01 Mar 2021

15:45 - 16:45
Virtual

Quasi-isometric rigidity of generic cyclic HNN extensions of free groups

Sam Shepherd
(University of Oxford)
Abstract

Studying quasi-isometries between groups is a major theme in geometric group theory. Of particular interest are the situations where the existence of a quasi-isometry between two groups implies that the groups are equivalent in a stronger algebraic sense, such as being commensurable. I will survey some results of this type, and then talk about recent work with Daniel Woodhouse where we prove quasi-isometric rigidity for certain graphs of virtually free groups, which include "generic" cyclic HNN extensions of free groups.

Mon, 01 Feb 2021

15:45 - 16:45
Virtual

Introduction to Hierarchically Hyperbolic Groups

Davide Spriano
(University of Oxford)
Abstract

Hierarchically Hyperbolic Groups (HHGs) were introduced by Behrstock—Hagen—Sisto to provide a common framework to study several groups of interest in geometric group theory, and have been an object of great interest in the area ever since. The goal of the talk is to provide an introduction to the theory of HHGs and discuss the advantages of the unified approach that they provide. If time permits, we will conclude with applications to growth and asymptotic cones of groups.

Mon, 25 Jan 2021

15:45 - 16:45
Virtual

The Friedl-Tillmann polytope

Dawid Kielak
(University of Oxford)
Abstract

I will introduce the Friedl-Tillmann polytope for one-relator groups, and then discuss how it can be generalised to the Friedl-Lück polytope, how it connects to the Thurston polytope, and how we can view it as a convenient source of intuition and ideas.

Thu, 16 Jun 2022

16:00 - 17:00
L4

Ax-Schanuel and exceptional integrability

Jonathan Pila
(University of Oxford)
Abstract

In joint work with Jacob Tsimerman we study when the primitive
of a given algebraic function can be constructed using primitives
from some given finite set of algebraic functions, their inverses,
algebraic functions, and composition. When the given finite set is just {1/x}
this is the classical problem of "elementary integrability".
We establish some results, including a decision procedure for this problem.

Fri, 20 May 2022

14:00 - 15:00
TBA

p-adic Dehn twists

Nadav Gropper
(University of Oxford)
Fri, 13 May 2022

14:00 - 15:00
N3.12

Representations of Galois groups

Håvard Damm-Johnsen
(University of Oxford)
Abstract

We can learn a lot about an integral domain by studying the Galois group of its fraction field. These groups are generally quite complicated and hard to understand, but their representations, so-called Galois representations, contain more easily accessible information. These also play the lead in many important theorems and conjectures of modern maths, such as the Modularity theorem and the Langlands programme. In this talk we give a quick introduction to Galois representations, motivated by lots of examples aimed at a general algebraist audience, and talk about some open problems.

Wed, 01 Jun 2022

16:00 - 17:00
L5

Existence of branched coverings of surfaces

Filippo Baroni
(University of Oxford)
Abstract

A branched covering between two surfaces looks like a regular covering map except for finitely many branching points, where some non-trivial ramification may occur. Informally speaking, the existence problem asks whether we can find a branched covering with prescribed behaviour around its branching points.

A variety of techniques have historically been employed to tackle this problem, ranging from studying representations of surface groups into symmetric groups to drawing "dessins d'enfant" on the covering surface. After introducing these techniques and explaining how they can be applied to the existence problem, I will briefly present a conjecture unexpectedly relating branched coverings and prime numbers.
 

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