ICTS

For a punctured surface S and a split reductive algebraic group G such as SL_n or PGL_n, Fock and Goncharov (and Shen) consider two types of moduli spaces parametrizing G-local systems on S together with certain data at punctures. These moduli spaces yield versions of higher Teichmüller spaces, and are equipped with special coordinate charts, making them birational to cluster varieties. Fock and Goncharov’s duality conjectures predict the existence of a canonical basis of the algebra of regular functions on one of these spaces, enumerated by the tropical integer points of the other space. I will give an introductory overview of this topic, briefly explain recent developments involving quantum topology and mirror symmetry of log Calabi-Yau varieties, and present some open problems if time allows.

Let $S$ an oriented surface of type $(g, n)$. We are interested in geodesics in the curve complex $\mathcal C(S)$ of $S$. In general, two $0$-simplexes in $\mathcal C(S)$ have infinitely many geodesics connecting the two simplexes. On the other hand, we may find two $0$-simplexes in $\mathcal C(S)$ so that they have only finitely many geodesics between them.

In this talk, we consider the spectrum of the number of geodesics with length $d (\geq 2)$ in $\mathcal C(S)$, which is denoted by $\mathfrak{Sp}_d(S)$. That is, it is the set of $k\in \mathbb N$ of the number of geodesics of length $d$ connecting two $0$-simplexes in $\mathcal C(S)$. Our main theorem asserts that $\mathfrak{Sp}_2(S)$ is completely determined in terms of $(g, n)$. This is a joint work with Ryo Matsuda (Kyoto) and Kayoko Oie (Nara).

Will be a basic introduction to hyperbolic geometry including the construction of surfaces with such a geometry, their geometric invariants, and the dynamics of the geodesic flow.

Our starting points consist of the simultaneous uniformization theorem for surface groups and the mating construction for polynomials. In part I of the talk, we describe a hybrid construction that simultaneously uniformizes a polynomial and a surface. We provide two constructions for some genus zero orbifolds and polynomials lying in the principal hyperbolic component:
1) For punctured spheres with possibly order 2 orbifold points using orbit equivalence
2) Generalizing (1) to orbifolds that have, in addition, an orbifold point of order > 2. This uses a factor dynamical system.
We conclude by describing the analog of the Bers slice in this context.
In the second part, we will characterize the combinations of polynomials and Fuchsian genus zero orbifold groups as explicit algebraic functions. This allows us to embed the 'product' of Teichm{\"u} spaces of genus zero orbifolds and parameter spaces of polynomials in a larger ambient space of algebraic correspondences.
We will discuss compactifications of such copies of Teichm{\"u}ller spaces in the space of correspondences, and end with a host of open questions.

Will focus on finding topological, geometric, and analytic conditions for which the geodesic flow exhibits ergodic behavior.

The discrete isoperimetric inequality states that the regular $n$-gon has the largest area among all $n$-gons with a fixed perimeter $p$. In this talk, we extend the discrete isoperimetric inequality to disconnected regions in the hyperbolic plane, i.e., we permit the area to be divided between regions. We provide the necessary and sufficient conditions to ensure that the result holds for multiple polygons with areas that add up.
This is a joint work with Arya Vadnere.

In the first part, I will give an introduction to Thurston's metric on Teichmuller space. In the second part, I will talk about convex structures on tangent spaces of Teichmuller space with respect to the norm induced by Thurston's metric. The latter part includes my joint work with Assaf Bar-Natan and Athanase Papadopoulos.

A hyperbolic quasifuchsian (or more generally convex co-compact) manifold $M$ contains a smallest non-empty geodesically convex subset, its convex core. The boundary of this convex core has a hyperbolic induced metric, and is pleated along a measured geodesic lamination. Thurston asked whether the induced metric, or the the measured pleating lamination, uniquely determine $M$. In the first part, we will explain why the answer is positive for the measured pleating lamination (joint w/ Bruno Dular). In the second part, we will put this problem in a more general frramework concerning the boundary data of convex subsets in hyperbolic manifolds or in hyperbolic space.
 

I will describe some joint work with Karen Uhlenbeck on best Lipschitz maps between surfaces. While our original motivation was to understand Thurston’s theory in Teichmueller space, it has connections with older ideas. I will remind the listeners about infinity harmonic functions, and describe our theory of infinity harmonic mappings and their dual laminations. The goal is to motivate several interesting, new and I believe hard questions in analysis and their relation to topology.

Anosov representations can be considered a generalization of convex-cocompact representations for groups of higher-rank. In this talk we are considering connected components of Anosov representations from the fundamental group of a closed hyperbolic manifold N, and which contains Fuchsian representations, and their associated domains of discontinuity. We will prove that the quotient of these domains of discontinuity are always smooth fiber bundles over N. Determining the topology of the fiber is hard in general, but we are able to describe it for representations in Sp(4,C), and for the domain of discontinuity in the space of complex Lagrangians in C^4 by using the classification of smooth 4-manifolds. This is joint work with Daniele Alessandrini, Nicolas Tholozan and Anna Wienhard.

Let O be a compact reflection n-orbifold whose underlying space is homeomorphic to a truncation n-polytope, i.e. a polytope obtained from an n-simplex by successively truncating vertices. In this talk, I will give a complete description of the deformation space of convex real projective structures on the orbifold O of dimension at least 4. Joint work with Suhyoung Choi and Ludovic Marquis.

A hyperbolic quasifuchsian (or more generally convex co-compact) manifold $M$ contains a smallest non-empty geodesically convex subset, its convex core. The boundary of this convex core has a hyperbolic induced metric, and is pleated along a measured geodesic lamination. Thurston asked whether the induced metric, or the the measured pleating lamination, uniquely determine $M$. In the first part, we will explain why the answer is positive for the measured pleating lamination (joint w/ Bruno Dular). In the second part, we will put this problem in a more general frramework concerning the boundary data of convex subsets in hyperbolic manifolds or in hyperbolic space.

Anosov representations can be considered a generalization of convex-cocompact representations for groups of higher-rank. In this talk we are considering connected components of Anosov representations from the fundamental group of a closed hyperbolic manifold N, and which contains Fuchsian representations, and their associated domains of discontinuity. We will prove that the quotient of these domains of discontinuity are always smooth fiber bundles over N. Determining the topology of the fiber is hard in general, but we are able to describe it for representations in Sp(4,C), and for the domain of discontinuity in the space of complex Lagrangians in C^4 by using the classification of smooth 4-manifolds. This is joint work with Daniele Alessandrini, Nicolas Tholozan and Anna Wienhard.

The goal of this mini-course is to give an idea of a proof of Mirzakhani's curve counting theorems, using geodesic currents. Mirzakhani proved that the number of closed curves of fixed topological type and length bounded by L on a hyperbolic surface is asymptotic to a constant times L^{6g-6}. Originally she gave very different proofs of this statement depending on whether the curves are simple or not. However, the use of geodesic currents allows one to consider the two cases as one. In the first lecture we will discuss geodesic currents and their properties, and in the second lecture give an outline of the proof.
 

Amenability of a group action is a dynamical generalisation of amenability for groups, with interesting applications in geometry and topology. Many (non-amenable) groups, like the Gromov hyperbolic groups, relatively hyperbolic groups (with suitable parabolic subgroups), mapping class groups of surfaces and outer automorphism groups of free groups admit amenable actions.

In this talk we will define amenable action of a group and outline two constructions of amenable actions for (i) acylindrically hyperbolic groups and (ii) hierarchically hyperbolic groups, which generalise some of the above classes of groups, and thereby giving a new proof of amenable action for the mapping class groups. This is based on a joint work with Partha Sarathi Ghosh.