Given an automorphism of the free group, we consider the mapping torus defined with respect to the automorphism. If the automorphism is atoroidal, then the resulting free-by-cyclic group is hyperbolic by work of Brinkmann. In addition, if the automorphism is fully irreducible, then work of Kapovich-Kleiner proves the boundary of the group is homeomorphic to the Menger curve. However, their proof is very general and gives no tools to further study the boundary and large-scale geometry of these groups. In this talk, I will explain how to construct explicit embeddings of non-planar graphs into the boundary of these groups whenever the group is hyperbolic. Along the way, I will illustrate how our methods distinguish free-by-cyclic groups which are the fundamental group of a 3-manifold. This is joint work with Yael Algom-Kfir and Arnaud Hilion.[-]

In these lectures, we will review what it means for a 3-manifold to have a hyperbolic structure, and give tools to show that a manifold is hyperbolic. We will also discuss how to decompose examples of 3-manifolds, such as knot complements, into simpler pieces. We give conditions that allow us to use these simpler pieces to determine information about the hyperbolic geometry of the original manifold. Most of the tools we present were developed in the 1970s, 80s, and 90s, but continue to have modern applications.[-]

In these lectures, we will review what it means for a 3-manifold to have a hyperbolic structure, and give tools to show that a manifold is hyperbolic. We will also discuss how to decompose examples of 3-manifolds, such as knot complements, into simpler pieces. We give conditions that allow us to use these simpler pieces to determine information about the hyperbolic geometry of the original manifold. Most of the tools we present were developed in the 1970s, 80s, and 90s, but continue to have modern applications.[-]

In these lectures, we will review what it means for a 3-manifold to have a hyperbolic structure, and give tools to show that a manifold is hyperbolic. We will also discuss how to decompose examples of 3-manifolds, such as knot complements, into simpler pieces. We give conditions that allow us to use these simpler pieces to determine information about the hyperbolic geometry of the original manifold. Most of the tools we present were developed in the 1970s, 80s, and 90s, but continue to have modern applications.[-]

Bestvina-Mess showed that the duality properties of a group $G$ are encoded in any boundary that gives a Z-compactification of $G$. For example, a hyperbolic group with Gromov boundary an $n$-sphere is a PD$(n+1)$ group. For relatively hyperbolic pairs $(G,P)$, the natural boundary - the Bowditch boundary - does not give a Z-compactification of G. Nevertheless we show that if the Bowditch boundary of $(G,P)$ is a 2-sphere, then $(G,P)$ is a PD(3) pair.
This is joint work with Genevieve Walsh.[-]

We are interested in the structure of the set of homomorphisms from a fixed (but arbitrary) finitely generated group G to the groups in some fixed family (such as the family of 3-manifold groups). I will explain what one might hope to say in different situations, and explain some applications to relatively hyperbolic groups and acylindrically hyperbolic groups, and some hoped-for applications to 3-manifold groups.
This is joint work with Michael Hull and joint work in preparation with Michael Hull and Hao Liang.[-]

A group is said to be coherent if every finitely generated subgroup is finitely presented. This property is enjoyed by free groups, and the fundamental groups of surfaces and 3-manifolds. A group that is not coherent is incoherent, and it is very interesting to try and understand which groups are coherent. We will discuss some of the geometric and topological aspects of this question, particularly quasi-convexité and algebraic fibers. We show that free-by-free and surface-by-free groups are incoherent, when the rank and genus are at least 2. The proof uses an understanding of fibers and also the RFRS property. this is joint work with Robert Kropholler and Stefano Vidussi.[-]

'I am a geometric topologist, and I'm interested in problems in both geometric topology and geometric group theory. I study groups acting on spaces in a variety of contexts: groups acting on hyperbolic space with quotient the complement of a knot in S3, groups acting on trees, how to make a "good" space for a group to act on, and the many ways a particular group can act on a particular space. I also like to understand the geometry of these spaces.

I was trained (if a mathematician can be trained) as a 3-manifold topologist. Work that came out of my thesis showed that hyperbolic 2-bridge knot complements are virtually fibered. The relevant point is that every 2-bridge knot complement has a finite cover which is very nice geometrically: it is the complement of a link of great circles in S3. I've studied when a 3-manifold has a cover which contains an embedded incompressible surface, by using eigenspaces of covering group action. That every closed hyperbolic 3-manifold has such a cover is known as the virtually Haken conjecture. My current research on knot complements studies the question of commensurability: When do two manifolds or orbifolds have a common finite-sheeted cover? Commensurability is an equivalence relation on manifolds and orbifolds which is very rich even when restricted to knot complements. It tells us a lot about the geometry of the knot complement. For example, the shape of the cusp of a knot complement restricts its commensurability class.

Recently, I've been working on some questions about groups generated by involutions and the type of spaces they can act on. When does a right-angled Coxeter group act by reflections in hyperbolic space? When does the automorphism group of a reflection group act on a CAT(0) space? My approach to these group theoretical questions is deeply influenced by 3-dimensional hyperbolic manifolds and orbifolds. In turn, geometric group theory informs my research on manifolds and orbifolds.'

CIRM - Chaire Jean-Morlet 2018 - Aix-Marseille Université[-]