Homotopy type and volume of locally symmetric manifolds

We consider locally symmetric manifolds with a fixed universal covering, and construct for each such manifold M a simplicial complex R whose size is proportional to the volume of M. When M is non-compact, R is homotopically equivalent to M, while when M is compact, R is homotopically equivalent to M\N, where N is a finite union of submanifolds of fairly smaller dimensions. This expresses how the volume controls the topological structure of M, and yields concrete bounds for various finiteness statements which previously had no quantitative proofs. For example, it gives an explicit upper bound for the possible number of locally symmetric manifolds of volume bounded by v>0, and it yields an estimate for the size of a minimal presentation for the fundamental group of a manifold in terms of its volume. It also yields a number of new finiteness results.Comment: 54 pages, amsc

Limits of finite homogeneous metric spaces

We classify the metric spaces that can be approximated by finite homogeneous ones.Comment: 9 page

Convergence groups are not invariably generated

It was conjectured in [KLS14] that non-elementary word hyperbolic groups are never invariably generated. We show that this is indeed the case even for the much larger class of convergence groups.Comment: 7 page

On fixed points and uniformly convex spaces

The purpose of this note is to present two elementary, but useful, facts concerning actions on uniformly convex spaces. We demonstrate how each of them can be used in an alternative proof of the triviality of the first $L_p$-cohomology of higher rank simple Lie groups, proved in [BFGM].Comment: 2 page

A lecture on Invariant Random Subgroups

Invariant random subgroups (IRS) are conjugacy invariant probability measures on the space of subgroups in a given group G. They can be regarded both as a generalization of normal subgroups as well as a generalization of lattices. As such, it is intriguing to extend results from the theories of normal subgroups and of lattices to the context of IRS. Another approach is to analyse and then use the space IRS(G) as a compact G-space in order to establish new results about lattices. The second approach has been taken in the work [7s12], that came to be known as the seven samurai paper. In these lecture notes we shall try to give a taste of both approaches.Comment: 19 pages, 5 figures. Based on a mini course given in Ghys' birthday --- conference Geometries in Action (2015), Oberwolfach workshop on Locally Compact Groups (2014) and Ventotene's conference on Manifolds and Groups (2015

Kazhdan-Margulis theorem for Invariant Random Subgroups

Given a simple Lie group $G$, we show that the lattices in $G$ are weakly uniformly discrete. This is a strengthening of the Kazhdan-Margulis theorem. Our proof however is straightforward --- considering general IRS rather than lattices allows us to apply a compactness argument. In terms of p.m.p. actions, we show that for every $\epsilon$ there is an identity neighbourhood $U$ which intersects trivially the stabilizers of $1-\epsilon$ of the points in every non-atomic $G$-space.Comment: 4 page

An Aschbacher--O'Nan--Scott theorem for countable linear groups

The purpose of this note is to extend the classical Aschbacher--O'Nan--Scott theorem for finite groups to the class of countable linear groups. This relies on the analysis of primitive actions carried out in a previous paper. Unlike the situation for finite groups, we show here that the number of primitive actions depends on the type: linear groups of almost simple type admit infinitely (and in fact unaccountably) many primitive actions, while affine and diagonal groups admit only one. The abundance of primitive permutation representations is particularly interesting for rigid groups such as simple and arithmetic ones.Comment: 8 page

The dynamics of Aut(F_n) on redundant representations

We study some dynamical properties of the canonical Aut(F_n)-action on the space R_n(G) of redundant representations of the free group F_n in G, where G is the group of rational points of a simple algebraic group over a local field. We show that this action is always minimal and ergodic, confirming a conjecture of A. Lubotzky. On the other hand for the classical cases where G=SL(2,R) or SL(2,C) we show that the action is not weak mixing, in the sense that the diagonal action on R_n(G)^2 is not ergodic.Comment: Some of the statements and arguments rely on the assumption that the algebraic group G is simply connected. This assumption, which was missing in the previous version, is not necessary in the archimedean cases, but it is needed in the non-archimedean case

A topological Tits alternative

Let k be a local field, and G a linear group over k. We prove that either G contains a relatively open solvable subgroup, or it contains a relatively dense free subgroup. This result has applications in dynamics, Riemannian foliations and profinite groups.Comment: 43 page

Equicontinuous actions of semisimple groups

We study equicontinuous actions of semisimple groups and some generalizations. We prove that any such action is universally closed, and in particular proper. We derive various applications, both old and new, including closedness of continuous homomorphisms, nonexistence of weaker topologies, metric ergodicity of transitive actions and vanishing of matrix coefficients for reflexive (more generally: WAP) representations.Comment: 28 pages. The introduction has been improved. (We also extended the discussion about week topologies.