Past Topology Seminar

The Eilenberg-Ganea conjecture is the statement that every group of cohomological dimension two admits a two-dimensional classifying space.  This problem is unsolved after 50 years.  I shall discuss the background to this question and negative answers to some other related questions.  This includes recent joint work with Martin Fluch.

I will discuss quasi-isometries of the free group that preserve an

equivariant pattern of lines.

There is a type of boundary at infinity whose topology determines how

flexible such a line pattern is.

For sufficiently complicated patterns I use this boundary to define a new

metric on the free group with the property that the only pattern preserving

quasi-isometries are actually isometries.

Chataur and Menichi showed that the homology of the free loop space of the classifying space of a compact Lie group admits a rich algebraic structure: It is part of a homological field theory, and so admits operations parametrised by the homology of mapping class groups.  I will present a new construction of this field theory that improves on the original in several ways: It enlarges the family of admissible Lie groups.  It extends the field theory to an open-closed one.  And most importantly, it allows for the construction of co-units in the theory.  This is joint work with Anssi Lahtinen.

I will discuss new rigidity and rationality phenomena

(related to the phenomenon of Arnold tongues) in the theory of

nonabelian group actions on the circle. I will introduce tools that

can translate questions about the existence of actions with prescribed

dynamics, into finite combinatorial questions that can be answered

effectively. There are connections with the theory of Diophantine

approximation, and with the bounded cohomology of free groups. A

special case of this theory gives a very short new proof of Naimi’s

theorem (i.e. the conjecture of Jankins-Neumann) which was the last

step in the classification of taut foliations of Seifert fibered

spaces. This is joint work with Alden Walker.

The concordance group of classical knots C was introduced

over 50 years ago by Fox and Milnor. It is a much-studied and elusive

object which among other things has been a valuable testing ground for

various new topological (and smooth 4-dimensional) invariants. In

this talk I will address the problem of embedding C in a larger group

corresponding to the inclusion of knots in links.

A longstanding question in geometric group theory is the following. Suppose G is a hyperbolic group where all finitely generated subgroups of infinite index are free. Is G the fundamental group of a surface? This question is still open for some otherwise well understood classes of groups. In this talk, I will explain why the answer is affirmative for graphs of free groups with cyclic edge groups. I will also discuss the extent to which these techniques help with the harder problem of finding surface subgroups.

Automorphisms of right-angled Artin groups interpolate between $Out(F_n)$ and $GL_n(\mathbb{Z})$. An active area of current research is to extend properties that hold for both the above groups to $Out(A_\Gamma)$ for a general RAAG. After a short survey on the state of the art, we will describe our recent contribution to this program: a study of how higher-rank lattices can act on RAAGs that builds on the work of Margulis in the free abelian case, and of Bridson and the author in the free group case.

We introduce the notion of a real cubing. Roughly speaking, real cubings are to CAT(0) cube complexes what real trees are to simplicial trees. We develop an analogue of the Rips’ machine and establish the structure of groups acting nicely on real cubings.

In this talk I'll explain how to build CAT(0) cube complexes and construct Lipschitz maps between them. The existence of suitable Lipschitz maps is used to prove that the asymptotic dimension of a

CAT(0) cube complex is no more than its dimension.

The notion of an E-infinity ring spectrum arose about thirty years ago,

and was studied in depth by Peter May et al, then later reinterpreted

in the framework of EKMM as equivalent to that of a commutative S-algebra.

A great deal of work on the existence of E-infinity structures using

various obstruction theories has led to a considerable enlargement of

the body of known examples. Despite this, there are some gaps in our

knowledge. The question that is a major motivation for this talk is

`Does the Brown-Peterson spectrum BP for a prime p admit an E-infinity

ring structure?'. This has been an important outstanding problem for

almost four decades, despite various attempts to answer it.

I will explain what BP is and give a brief history of the above problem.

Then I will discuss a construction that gives a new E-infinity ring spectrum

which agrees with BP if the latter has an E-infinity structure. However,

I do not know how to prove this without assuming such a structure!

The Farrell-Jones Conjecture predicts a homological formula for K-and L-theory of group rings. Through surgery theory it is important for the classification of manifolds and in particular the Borel conjecture. In this talk I will give an introduction to this conjecture and give an overview about positive results and open questions.

Distortion is an asymptotic invariant of the embeddings

of finitely generated algebras. For group embeddings,

it has been introduced by M.Gromov. The main part of

the talk will be based on a recent work with Yu.Bahturin,

where we consider the behavior of distortion functions

for subalgebras of associative and Lie algebras.

In the 1990's Haagerup discovered a new subfactor, and hence a new topological quantum field theory, that has so far proved inaccessible by the methods of quantum groups and conformal field theory. It was the subfactor of smallest index beyond 4. This led to a classification project-classify all subfactors to as large an index as possible. So far we have gone as far as index 5. It is known that at index 6 wildness phenomena occur which preclude a simple listing of all subfactors of that index. It is possible that wildness occurs at a smaller index value, the main candidate being approximately 5.236.

The goal of this talk is to construct new examples of hyperbolic

aspherical complexes. More precisely, given an aspherical simplicial

complex P and a subcomplex Q of P, we are looking for conditions under

which the complex obtained by attaching a cone of base Q on P remains

aspherical. If Q is a set of loops of a 2-dimensional complex, J.H.C.

Whitehead proved that this new complex is aspherical if and only if the

elements of the fundamental group of P represented by Q do not satisfy

any identity. To deal with higher dimensional subcomplexes we use small

cancellation theory and extend the geometric point of view developed by

T. Delzant and M. Gromov to rotation families of groups. In particular

we obtain hyperbolic aspherical complexes obtained by attaching a cone

over the "real part" of a hyperbolic complex manifold.

We give an introduction to the Kakimizu complex of a link,

covering a number of recent results. In particular we will see that the

Kakimizu complex of a knot may be locally infinite, that the Alexander

polynomial of an alternating link carries information about its Seifert

surfaces, and that the Kakimizu complex of a special alternating link is

understood.

I'll describe an approach to perturbative quantum field theory
which is philosophically similar to the deformation quantization approach
to quantum mechanics. The algebraic objects which appear in our approach --
factorization algebras -- also play an important role in some recent work
in topology (by Francis, Lurie and others).  This is joint work with Owen
Gwilliam.

Coarse geometry provides a very useful organising point of view on the study
of geometry and analysis of discrete metric spaces, and has been very
successful in the context of geometric group theory and its applications. On
the other hand, the work of Carlsson, Ghrist and others on persistent
homology has paved the way for applications of topological methods to the
study of broadly understood data sets. This talk will provide an
introduction to this fascinating topic and will give an overview of possible
interactions between the two.

Topological spaces and manifolds are commonly used to model configuration
spaces of systems of various nature. However, many practical tasks, such as
those dealing with the modelling, control and design of large systems, lead
to topological problems having mixed topological-probabilistic character
when spaces and manifolds depend on many random parameters.
In my talk I will describe several models of stochastic algebraic topology.
I will also mention some open problems and results known so far.