Amenability of groups and GG-sets

This text surveys classical and recent results in the field of amenability of groups, from a combinatorial standpoint. It has served as the support of courses at the University of G\"ottingen and the \'Ecole Normale Sup\'erieure. The goals of the text are (1) to be as self-contained as possible, so as to serve as a good introduction for newcomers to the field; (2) to stress the use of combinatorial tools, in collaboration with functional analysis, probability etc., with discrete groups in focus; (3) to consider from the beginning the more general notion of amenable actions; (4) to describe recent classes of examples, and in particular groups acting on Cantor sets and topological full groups

Applications of infinite-dimensional geometry and Lie theory

Habilitation thesisHabilitationsschriftInfinite-dimensional manifolds and Lie groups arise from problems related to differential geometry, fluid dynamics, and the symmetry of evolution equations. Among the most prominent examples of infinite-dimensional manifolds are manifolds of (differentiable) mappings and the diffeomorphism groups Diff(K), where K is a smooth and compact manifold. The group Diff(K) is an infinite-dimensional Lie group which arises naturally in fluid dynamics if K is a three-dimensional torus. The motion of a particle in the fluid corresponds, under periodic boundary conditions, to a curve in Diff(K). As a working definition, an infinite-dimensional Lie group will be a group which at the same time is an infinite-dimensional manifold that turns the group operations into smooth mappings. An infinite-dimensional manifold will be a topological space which is locally (in charts) homeomorphic to an open subset of an infinite-dimensional space. Moreover, we require the change of charts to be smooth. Beyond the realm of Banach spaces, the usual concept of smoothness is no longer available and we replace it with the requirement that all directional derivatives exist and induce continuous mappings, the so called Bastiani calculus. Infinite-dimensional Lie groups and their homogeneous spaces will be the objects of our main interest. In conjunction with Lie theory, we exploit tools from (infinite-dimensional) Riemannian geometry. Recall that a Riemannian metric on a manifold is a choice of inner product for every tangent space which ”depends smoothly” on the basepoint. Generalising Riemannian geometry to infinite-dimensional manifolds, one faces in general the problem that there are no (smooth) partitions of unity. Further, the inner products will in general not be compatible with the topology of the tangent spaces as they are not Hilbert spaces. Thus the finite-dimensional definition of a Riemannian metric (what we will call a ’strong Riemannian metric’) has to be relaxed to admit relevant examples beyond the Hilbert manifold setting. This leads to the notion of a ’weak Riemannian metric’, i.e. a smooth choice of inner products on each tangent space which do not necessarily induce the topology of the tangent space. Constructing weak Riemannian metrics on manifolds of mappings from the L2-inner product, the resulting metrics are studied for example in shape analysis, fluid dynamics and optimal transport. The present thesis explores structures from infinite-dimensional Lie theory and Riemannian geometry, their interplay and applications in three main topics: - Connections between infinite-dimensional Lie groups and higher geometry, - Hopf algebra character groups as Lie groups, and - Applications of the interplay between Lie theory and Riemannian geometry.publishedVersio

Polynomial growth and property RDpRD_p for \'etale groupoids with applications to KK-theory

We investigate property $RD_p$ for \'etale groupoids and apply it to $K$-theory of reduced groupoid $L^p$-operator algebras. In particular, under the assumption of polynomial growth, we show that the $K$-theory groups for a reduced groupoid $L^p$-operator algebra is independent of $p\in (1, \infty)$. We apply the results to coarse groupoids and graph groupoids.Comment: Comments welcom

Groups, operator algebras and approximation

Two main objects of the research in this thesis are countable discrete groups and their operator algebras (C*-algebras and von Neumann algebras). Discrete groups are often succesfully studied using geometric and ergodic-theoretic methods, the corresponding areas of mathematics being called geometric resp. measured group theory. This thesis has a cumulative form: each chapter is a research article, and therefore has its own abstract and bibliography. The majority of these publications have been peer-reviewed and published in various journals

Recent Advances in Graph Partitioning

We survey recent trends in practical algorithms for balanced graph partitioning together with applications and future research directions