We discuss some quite general properties of infinite discrete groups G acting on compact spaces. The spaces we mainly have in mind are horoboundaries of metric spaces which admit an isometric action of G. As an application, we show that the mapping class group of a surface of finite type admits a proper action on some $L^p$-space.

The Teichmüller space of a compact 2-orbifold X can be defined as the space of faithful and discrete representations of the fundamental group $pi_1$(X) of X into PGL(2,R). It is a contractible space. For closed orientable surfaces, « higher analogues » of the Teichmüller space are, by definition, (unions of) connected components of representation varieties of $pi_1$(X) that consist entirely of discrete and faithful representations. There are two known families of such spaces, namely Hitchin representations and maximal representations, and conjectures on how to find others. In joint work with Daniele Alessandrini and Gye-Seon Lee, we show that the natural generalisation of Hitchin components to the orbifold case yields new examples of higher Teichmüller spaces: Hitchin representations of orbifold fundamental groups are discrete and faithful, and share many other properties of Hitchin representations of surface groups. However, we also uncover new phenomena, which are specific to the orbifold case.

The Leibniz rule for derivations is invariant under cyclic permutations of co-multiples within the derivations' arguments. Let us explore the implications of this principle: in effect, we develop the calculus of variations on the infinite jet spaces for maps from sheaves of free associative algebras over commutative manifolds to the generators and then, quotients of free associative algebras over the linear relation of equivalence under cyclic shifts. In the frames of such variational noncommutative symplectic (super)geometry we prove the main properties of the Batalin-Vilkovisky Laplacian and variational Schouten bracket.

 

We show as by-product that the structures which arise in the variational Poisson geometry of infinite-dimensional integrable systems do actually not refer to the graded commutativity assumption.

The 8-vertex model and the XYZ spin chain have been found to emerge from gauge theories in various ways, such as 4d and 2d Nekrasov-Shatashvili correspondences, the action of surface operators on the supersymmetric indices of class-Sk theories, and correlators of line operators in 4d Chern-Simons theory. I will explain how string theory unifies these phenomena.

 

This is based on my work with Kevin Costello [arXiv:1810.01970].

 

A long-standing question of Hopf asks whether every self-map of absolute degree one of a closed oriented manifold is a homotopy equivalence. This question gave rise to several other problems, most notably whether the fundamental groups of aspherical manifolds are Hopfian, i.e. any surjective endomorphism is an isomorphism. Recall that the Borel conjecture states that any homotopy equivalence between two closed aspherical manifolds is homotopic to a homeomorphism. In this talk, we verify a strong version of Hopf’s problem for certain aspherical manifolds. Namely, we show that every self-map of non-zero degree of a circle bundle over a closed oriented aspherical manifold with hyperbolic fundamental group (e.g. negatively curved manifold) is either homotopic to a homeomorphism or homotopic to a non-trivial covering and the bundle is trivial. Our main result is that a non-trivial circle bundle over a closed oriented aspherical manifold with hyperbolic fundamental group does not admit self-maps of absolute degree greater than one. This extends in all dimensions the case of circle bundles over closed hyperbolic surfaces (which was shown by Brooks and Goldman) and provides the first examples (beyond dimension three) of non-vanishing functorial semi-norms on the fundamental classes of circle bundles over aspherical manifolds with hyperbolic fundamental groups.

I will describe a remarkable symmetric monoidal category associated to a reductive group G, which acts centrally on any G-category. This construction quantizes the universal centralizer group scheme, together with its action on Hamiltonian G-spaces used by Ngo in his proof of the Fundamental Lemma. The category and its central action appear most naturally in a Langlands dual incarnation, which is phrased in terms of convolution on the affine Grassmannian, via work of Bezrukavnikov, Finkelberg and Mirkovic. This is joint work with David Ben-Zvi.

The affine Springer fibers are geometric objects conceived for the study of orbital integrals. They have complicated geometric structures. We will explain our work on the geometry of affine Springer fibers, with emphasize on the construction of a fundamental domain, and show how the study of the affine Springer fibers can be reduced to that of its fundamental domain. As an application, we will explain how to calculate Arthur’s weighted orbital integrals via counting points on the fundamental domain.

The purpose of the talk is to explain a result in collaboration with B. Pozzetti and A. Wienhard expressing the Hausdorff dimension of certain attractors as a critical exponent. This class of attractors consists of limit sets of Anosov representations in PGLd (hence of non-conformal nature) that verify an extra open condition. If time permits, we will discuss implications of the formula to the geometry of the Hitchin component.

Séminaire Laurent Schwartz — EDP et applications