Thu, 04 Feb 2016

Basic aspects of n-homological algebra

Peter Jorgensen

Abstract: n-homological algebra was initiated by Iyama
via his notion of n-cluster tilting subcategories.
It was turned into an abstract theory by the definition
of n-abelian categories (Jasso) and (n+2)-angulated categories
The talk explains some elementary aspects of these notions.
We also consider the special case of an n-representation finite algebra.
Such an algebra gives rise to an n-abelian
category which can be "derived" to an (n+2)-angulated category.
This case is particularly nice because it is
analogous to the classic relationship between
the module category and the derived category of a
hereditary algebra of finite representation type.

Tue, 26 Jan 2016

Operator algebras associated with subshifts

Evgenios Kakariadis

A subshift is characterized by a set of allowable words on $d$ symbols. In a sense it encodes the allowable operations an automaton performs. In the late 1990's Matsumoto constructed a C*-algebra associated to a subshift, deriving initially his motivation from the work of Cuntz-Krieger. These C*-algebras were then studied in depth in a series of papers. In 2009 Shalit-Solel discovered a relation of the subshift algebras with their variants of operator algebras related to homogeneous ideals. In particular a subshift corresponds to a monomial ideal under this prism.

In a recent work with Shalit we take a closer look at these cases and study them in terms of classification programmes on nonselfadjoint operator algebras and Arveson's Programme on the C*-envelope. We investigate two nonselfadjoint operator algebras from one SFT and show that they completely classify the SFT: (a) up to the same allowable words, and (b) up to local conjugacy of the quantized dynamics. In addition we discover that the C*-algebra fitting Arveson's Programme is the quotient by the generalized compacts, rather than taking unconditionally all compacts as Matsumoto does. Actually there is a nice dichotomy that depends on the structure of the monomial ideal.

Nevertheless in the process we accomplish more in different directions. This happens as our case study is carried in the intersection of C*-correspondences, subproduct systems, dynamical systems and subshifts. In this talk we will give the basic steps of our results with some comments on their proofs.

Tue, 05 Jun 2012

Artin groups of large type: from geodesics to Baum-Connes

Professor S. Rees
I’ll report on my recent work (with co-authors Holt and Ciobanu) on Artin

groups of large type, that is groups with presentations of the form

G = hx1, . . . , xn | xixjxi · · · = xjxixj · · · , 8i 3. (In fact, our results still hold when some, but not all

possible, relations with mij = 2 are allowed.)

Recently, Holt and I characterised the geodesic words in these groups, and

described an effective method to reduce any word to geodesic form. That

proves the groups shortlex automatic and gives an effective (at worst quadratic)

solution to the word problem. Using this characterisation of geodesics, Holt,

Ciobanu and I can derive the rapid decay property for most large type

groups, and hence deduce for most of these that the Baum-Connes conjec-

ture holds; this has various consequence, in particular that the Kadison-

Kaplansky conjecture holds for these groups, i.e. that the group ring CG

contains no non-trivial idempotents.


Tue, 15 Nov 2011

15:45 - 16:45

Noncommutative mirror symmetry for punctured surfaces

Raf Bocklandt

A dimer model on a surface with punctures is an embedded quiver such that its vertices correspond to the punctures and the arrows circle round the faces they cut out. To any dimer model Q we can associate two categories: A wrapped Fukaya category F(Q), and a category of matrix factorizations M(Q). In both categories the objects are arrows, which are interpreted as Lagrangian subvarieties in F(Q) and will give us certain matrix factorizations of a potential on the Jacobi algebra of the dimer in M(Q).

We show that there is a duality D on the set of all dimers such that for consistent dimers the category of matrix factorizations M(Q) is isomorphic to the Fukaya category of its dual,  F((DQ)). We also discuss the connection with classical mirror symmetry.

Tue, 01 Jun 2010

17:00 - 18:00

The cluster category of Dynkin type $A_\infty$

Peter Jorgensen
\ \ The cluster category of Dynkin type $A_\infty$ is a ubiquitous object with interesting properties, some of which will be explained in this talk.


\ \ Let us denote the category by $\mathcal{D}$. Then $\mathcal{D}$ is a 2-Calabi-Yau triangulated category which can be defined in a standard way as an orbit category, but it is also the compact derived category $D^c(C^{∗}(S^2;k))$ of the singular cochain algebra $C^*(S^2;k)$ of the 2-sphere $S^{2}$. There is also a “universal” definition: $\mathcal{D}$ is the algebraic triangulated category generated by a 2-spherical object. It was proved by Keller, Yang, and Zhou that there is a unique such category.


\ \ Just like cluster categories of finite quivers, $\mathcal{D}$ has many cluster tilting subcategories, with the crucial difference that in $\mathcal{D}$, the cluster tilting subcategories have infinitely many indecomposable objects, so do not correspond to cluster tilting objects.


\ \ The talk will show how the cluster tilting subcategories have a rich combinatorial

structure: They can be parametrised by “triangulations of the $\infty$-gon”. These are certain maximal collections of non-crossing arcs between non-neighbouring integers.


\ \ This will be used to show how to obtain a subcategory of $\mathcal{D}$ which has all the properties of a cluster tilting subcategory, except that it is not functorially finite. There will also be remarks on how $\mathcal{D}$ generalises the situation from Dynkin type $A_n$ , and how triangulations of the $\infty$-gon are new and interesting combinatorial objects.

Mon, 18 May 2009
Oxford-Man Institute

Recent problems involving moments determinacy of distributions.

Dr Jordan Stoyanov

If a distribution, say F, has all moments finite, then either F is unique (M-determinate) in the sense that F is the only distribution with these moments, or F is non-unique (M-indeterminate).  In the latter case we suggest a method for constructing a Stieltjes class consisting of infinitely many distributions different from F and all having the same moments as F.  We present some shocking examples involving distributions such as N, LogN, Exp and explain what and why.  We analyse conditions which are sufficient for F to be M-determinate or M-indeterminate.  Then we deal with recent problems from the following areas:


(A)  Non-linear (Box-Cox) transformations of random data.

(B) Distributional properties of functionals of stochastic processes.

(C) Random sums of random variables.


If time permits, some open questions will be outlined.  The talk will be addressed to colleagues, including doctoral and master students, working or having interests in the area of probability/stochastic processes/statistics and their applications. 

Tue, 22 Apr 2008

17:00 - 18:00

Totally Disconnected, Locally Compact Groups & Geometric Group Theory

Udo Baumgartner
As a small step towards an understanding of the relationship of the two fields in the title, I will present a uniformness result for embeddings of finitely generated, virtually free groups as cocompact, discrete subgroups in totally disconnected, locally compact groups.
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