Location

The talks happen on Mondays from 11:00 to 12:00 in the classroom D2.314 and we stream them via zoom:https://uni-paderborn-de.zoom-x.de/j/62698976462?pwd=YkZSN2NnTDl3WGYvUEdmVG1yVXEwQT09

Password: 952895

Upcoming events:

15.07 - Lukas KlawuhnTitle: Why everyone should know association schemes: An introduction to Delsarte Theory

Abstract:

Interesting combinatorial structures can often be characterised as special subsets of association schemes. In his PhD thesis, Philippe Delsarte developed powerful linear programming techniques to prove non-existence and uniqueness results for such structures. Ideas of this type were fundamental in the work for which Marina Viazovska was awarded the Fields medal in 2022. This talk will begin with an overview of Delsarte theory.

The association scheme of the symmetric group is well known. We will introduce it, and then study generalised permutations. They act on the set {1,2,...,n}, whose elements are coloured with one of r possible colours. We consider different notions of transitivity and interpret these algebraically in the appropriate association scheme . We will give existence results showing that there exist transitive sets of generalised permutations that are small compared to the size of the group. Many of these results extend results previously known for the symmetric group.

No particular knowledge of association schemes will be required to appreciate this talk.

Past events:

22.04 Stepan MaximovTitle: Regular decompositions and integrable systems

Abstract:

An important way of obtaining integrable systems is the r-matrix method. Generalized r-matrices can be constructed using Lie algebra decompositions: they can be viewed as generating series of decompositions.

In this talk, we look at certain decompositions, called regular, of the Lie algebra of formal Laurent series g((x)) and present partial classification results. The r-matrices corresponding to such decompositions give rise to particularly nice Gaudin models.

29.04 - Milan Niestijl

Title: Representation theory of infinite-dimensional Lie groups

Abstract:

After introducing infinite-dimensional Lie groups, we shall discuss the study of their unitary representations. In particular, I will indicate the main challenges that appear in the infinite-dimensional context, and what type of results one might hope for. To that end, a so-called 'positive energy' condition is introduced, which isolates a mathematically interesting class of representations and is motivated from physics. We will explore the consequences of this condition, and present some classification results.

06.05 - Carsten Peterson

Title: The Poincare series of a Coxeter group and its applications

Abstract:

The Chevalley-Solomon theorem provides a formula for the Poincare series of a Coxeter group G, namely the generating function for the number of elements of each length in the group (the length of an element is the length of its shortest representative as a product of Coxeter generators). In this talk we shall present a proof of this theorem which involves studying the action of G on its "Coxeter complex", which is a simplicial complex encoding the combinatorics of the group, and the associated action of G on the chain complex of the Coxeter complex. We shall also discuss why this formula is useful in computing the orders of finite groups of Lie type (i.e. semisimple algebraic groups over finite fields) and the volume of certain naturally defined subsets of buildings.

13.05 - Alexandre Maksoud

Title: Modular forms and congruences

Abstract:

This is a friendly introduction to the topic of modular form, with an emphasis on congruences mod p between their coefficients. We'll start with Ramanujan's famous congruence mod 691 and we'll explain how congruences are related to special values of L-functions.

27.05 - Charly Schwabe

Title: Elliptic Weyl groups

Abstract:

After recalling some basic theory about finite and affine root systems, we will introduce their generalization, the extended affine root systems. We will then classify the elliptic Weyl groups associated to them and investigate a special element called Coxeter transformation. We shall finally use this Coxeter element to motivate a connection to representation theory (and the logo of the CRC).

3.06 - Daniel Perniok

Title: Representation theory of quivers

Abstract:

Originating in fundamental linear algebra problems, the representation theory of quivers has proved useful in many other areas such as representation theory of Lie algebras and Lie groups, geometric group theory or categorification of quantum groups. In this talk we focus on basic definitions and comprehensible examples. Finally we consider the Kronecker quiver and how it gives a different perspective to the study of coherent sheaves on the projective line.

10.06 - Praful Rahangdale

Title: Poisson and related structures

Abstract:

Poisson structure is a Foliation on the manifold, which partitions it into leaves. These leaves have a Symplectic structure, and transverse to the leaves, there is a Lie algebra structure, so Poisson geometry amalgamates Foliation theory, Symplectic geometry, and Lie theory.

The original motivation for studying Poisson structure comes from classical mechanics. Phase spaces of classical mechanical systems are modelled on Poisson manifolds, and the dynamics of the system are governed by a distinguished function called the Hamiltonian of the system. The time evolution of the system is given by the flow of the vector field corresponding to the Hamiltonian. The system is constrained to submanifolds, which are the level sets of functions which Poisson commute with the Hamiltonian, and we hope to constrain the dynamics enough to completely solve the problem.

In this talk, we shall first review the basics of Poisson geometry. Then, we will discuss the local description of a Poisson manifold according to Weinstein splitting theorem, as a product of a symplectic manifold with another manifold transverse to it whose Poisson structure vanishes at the point. I will introduce symplectic realizations of Poisson manifolds, and we will see that any Poisson manifold can be viewed as the quotient of a symplectic manifold. Finally, we will discuss Lie bialgebras, the algebraic counterpart of Poisson Lie groups, and the one-to-one correspondence between connected, simply connected Poisson Lie groups and finite-dimensional Lie bialgebras.

17.06 - Kyungmin Rho

Title: An introduction to homological mirror symmetry

Abstract:

Recent developments in homological mirror symmetry have significantly connected symplectic geometry, algebraic geometry, and representation theory. Fukaya categories of Riemann surfaces have emerged as powerful tools for dealing with representation theory of tame categories. These categories include categories of matrix factorizations, derived categories of coherent sheaves of singular complex curves and derived categories of some finite-dimensional algebras. In this talk, we will explore these extensive fields by examining concrete examples and their correspondences.

24.06 - Abhijit Aryampilly Jayanthan

Title: Étale and profinite-étale vector bundles on elliptic curves

Abstract:

Over an algebraically closed field, vector bundles on elliptic curves were classified by Atiyah in 1957. He showed that for a chosen rank and degree, the indecomposable vector bundles are in one to one correspondence with the closed points of the elliptic curve. Further, every vector bundle can be written as a direct sum of its indecomposable subbundles, completing this classification.

The universal profinite-étale cover of an elliptic curve (over some p-adically complete algebraically closed field) is represented by an perfectoid space. With the help of Atiyah's classification, along with some results by Ben Heuer, one is able to classify the collection of all étale (equivalently Zariski) vector bundles which become trivial on some profinite-étale cover of the elliptic curve.

01.07 - Mingyu Ni

Title: O-minimality and Hodge Theory

Abstract:

It was noticed in 1980s that many properties of semi-algebraic sets could be derived from several simple axioms, which is now known as o-minimal structures. These structures provide a candidate of the framework to develop the notion of tame topology, as envisaged by Grothendieck in 1984, and the techniques have profound applications to number theory and arithmetic geometry. More recently, after the work of B. Bakker, Y. Brunebarbe, B. Klingler, and J. Tsimerman, o-minimal structures have become an important tool in the study of Hodge theory. In this expository talk I would start with basic definitions and examples of o-minimal structures, with a view from and towards algebraic geometry, then discuss the particular applications to Hodge theory.

08.07 - Cancelled

Organisers:

Abhijit Aryampilly Jayanthan, Stepan Maximov