Title:

Abstract: Let $\mathcal{P}$ be a partition of a finite set $X$. We say that a transformation $f:X\to X$ stabilises the partition $\mathcal{P}$ if for all $P\in \mathcal{P}$ there exists $Q\in \mathcal{P}$ such that $Pf\subseteq Q$. Let $T(X,\mathcal{P})$ denote the semigroup of all full transformations of $X$ that preserve the partition $\mathcal{P}$.

In 2005 Pei Huisheng found an upper bound for the minimum size of the generating sets of $T(X,\mathcal{P})$, when $\mathcal{P}$ is a partition in which all of its parts have the same size. In addition, Pei Huisheng conjectured that his bound was exact. In 2009, Araújo and Schneider used representation theory to solve Pei Huisheng's conjecture.

A more general task is to find the minimum size of the generating sets of $T(X,\mathcal{P})$, when $\mathcal{P}$ is an arbitrary partition. In this talk we presents the solution of this problem and discuss some of the proof techniques, which range from representation theory to combinatorial arguments.

This is joint work with João Araújo (Universade Aberta/CEMAT), James Mitchell (University of St Andrews), and Csaba Schneider (Universidade Federal de Minas Gerais).

Title:

Abstract: A measurable transformation $T$ of a probability space $(\Omega,\mathcal{B},m)$ is quasi invariant if it preserves the $\sigma$-ideal of measure $0$ sets. An old question of Halmos, which was answered in the affirmative by Ornstein and L. Arnold, is whether there exists such transformations which have recurrent dynamics but there exists no $\sigma$-finite, $m$-absolutely continuous $T$ invariant measures. Such systems are called type $III$. The type $III$ transformations can be further classified according to their Krieger types $III_\lambda, 0 \leq \lambda \leq 1$ where being type $III_1$ is equivalent to the Maharam extension being ergodic.

In this talk we will discuss these notions and more in the context of the dynamics of the shift with respect to products measures (not necessarily i.i.d.). If time permits we will discuss an application of these results to symmetric $\alpha$-stable processes, some extensions to the case of the shift of inhomogeneous Markov chain and the construction of a new class of Anosov diffeormorphisms of the torus.

Title:

Abstract: In the talk we will survey a novel domain of computational group theory: computing with infinite linear groups. We will provide an introduction to the area, and will discuss available methods and algorithms. Special consideration will be given to the most recent developments in computing with arithmetic groups and its applications. This talk is aimed at a general mathematical audience.

13th Oct, 2016: DOUBLE BILL:

Title:

Abstract: Bernoulli convolutions are arguably the simplest fractal measures on the unit interval, parametrized by a factor t between 0 and 1. They have been studied for almost 80 years, without much success in the overlapping case. Only for countably many parameters it is exactly known whether the measure admits a density function. We introduce these measures from the viewpoints of probability, fractals, number systems, and dynamical systems. Then we present a new approach which represents the whole parametric family by a function of two parameters. The structure of that function is studied with computer assistance.

Title:

Abstract: I will talk about some similarities between Fractal Geometry and Probability Theory, in particular the Markov Chains, Random Walks and Iterated Function System. I will then talk about some recent progress on projections and slices of random and deterministic fractal measures on the plane.

Title:

Abstract: A (bar-joint) framework (G,p) is a graph G, along with a placement p of its vertices into R^d. A framework is said to be universally rigid if any other (G,q) in *any dimension* $D\geq d$ that has the same edge lengths as (G,p) is related to (G,p) by a rigid body motion. I'll describe an algebraic characterisation of which graphs G have generic universally rigid frameworks (G,p) and a close connection to a widely used semidefinite programming algorithm for the graph realisation or "distance geometry" problem.

Joint work with Bob Connelly and Shlomo Gortler.

Title:

Abstract: TBA

Title:

Abstract: Graphons are uncountable limits of sequences of finite graphs. Their invention in 2006 by Lovasz and Szegedy revolutionized both the finite and the infinite graph theory by bringing an unforeseen connection. Graphons, also known as combinatorial limits can be seen as certain ultraproducts, which makes them amenable to study using the methods of logic. We shall give a very general talk about this concept and at the end present some joint results with Tomasic.

Title:

Abstract: TBA

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