Trimester Seminar

Venue: HIM, Poppelsdorfer Allee 45, Lecture Hall

Tuesday, October, 24th, 10:00

Noncommutative crepant resolutions of quotient singularities for reductive groups 

Michel Van den Bergh (Universiteit Hasselt)

Blackboard photos

Monday, October, 30th, 14:45

A universal source of braid group actions

David Jordan (University of Edinburgh)


One of the reasons we love quantum groups is that they are the best-understood examples of braided tensor categories, and these are a systematic way to build representations of braid groups, which can then be studied using Lie theory (and its deformations).  We can then ask, what about braid groups of other surfaces, is there a similar Lie theoretic construction of representations of these?  In this talk I'll explain a positive answer to this question, and we'll consider lots of examples.  It will turn out that the categories we find in this way are connected in a surprising way to important examples in (classical) geometric representation theory, like Harish-Chandra D-modules, adjoint orbits, character sheaves, etc.  I'll discuss a bit of this as well.

Blackboard Photos

Thursday, November, 2nd, 14:45

Representation Theory of Tensor Categories

Tim Weelinck (University of Edinburgh)


Tensor categories are categories with a multiplication structure and in this sense categorify algebras. There is a natural notion of module over a tensor category and so the fundamental question looms: 'can we classify the modules of our favourite tensor category?'. As in the case of algebras, this is very difficult in general. However, if the algebra happens to be a Frobenius algebra we have powerful tools available to study its representation theory. We will recall those tools and then explain that rigid tensor categories (where objects have duals) are categorified Frobenius algebras. This analogy proves very fruitful and provides us with powerful tools to study the representation theory of rigid tensor categories. We conclude by giving a detailed example of a module over the tensor category of representations of a quantum group, and explain how the reflection equation algebra arises naturally there.

Blackboard photos

Monday, November, 6th, 14:45

From A-infinity algebras to bocses via Koszul duality

Julian Külshammer (Universität Stuttgart)

Related Talk 


Exceptional collections frequently appear in algebraic and symplectic geometry as well as in representation theory. In representation theory, algebras having a full exceptional collection in their module category are called quasi-hereditary. In this talk, I will explain how to use a variant of Koszul duality for A-infinity algebras to construct a bocs (= a coring in a category of bimodules over an algebra) given an exceptional collection. On this level, also Ringel duality can be seen as a special case of Koszul duality. If time permits, we will show in an example, how the language of bocses can help to understand the category of modules filtered by the exceptional collection. This reports on joint work with Steffen Koenig and Sergiy Ovsienko as well as Agnieszka Bodzenta.

Blackboard photos

Tuesday, November, 14th, 10:00

On symmetric spaces for Kac-Moody groups

Guido Pezzini (La Sapienza University in Rome)


In the talk I will report on a research project aimed at studying symmetric spaces for Kac-Moody groups, joint with Bart Van Steirteghem. Our work is motivated by the relevant connections that symmetric spaces, and more in general spherical varieties, have with other areas of mathematics such as representation theory and symplectic geometry.

We expect that similar interactions will be possible in the infinite-dimensional setting of Kac-Moody groups too, in particular with the representation theory of such groups and also with multiplicity free symplectic (Fréchet) manifolds under the action of loop groups.

Our goals include defining a structure of infinite-dimensional algebraic variety (ind-variety) on such symmetric spaces, studying functions defined by matrix coefficients, and define compactifications. We will also highlight some peculiarities that arise with Kac-Moody groups of affine type, and that show quite a different behavior from the classical finite-dimensional case.

Blackboard photos

Wednesday, November, 15th, 10:00

On equations defining the affine Grassmannian of SLn

Oded Yacobi (University of Sydney)


The affine Grassmannian Gr of a semisimple Lie group G is an important infinite dimensional variety that appears in geometric representation theory.  This talk concerns the projective geometry of Gr when G=SLn.  More precisely, in this case Gr naturally embeds into the Sato Grassmannian, which is a limit of finite dimensional Grassmannians Gr(n,2n) as n → ∞. We are interested in the equations defining the embedding Gr ⊂ SGr. 

Kreiman, Lakshmibai, Magyar and Weyman constructed linear equations on SGr which vanish on Gr and conjectured that these equations suffice to cut out the affine Grassmannian.  We recently proved this conjecture by reducing it to a question about finite dimensional Grassmannians.  I'll describe our method of proof and mention some conjectures that arise from our work.  I'll motivate this discussion by relating our work to the problem of describing the equations of an interesting class of singular varieties: the nilpotent orbit closures in positive characteristic.

This is joint work with Dinakar Muthiah and Alex Weekes.

Blackboard photos

Friday, November, 17th, 16:15

Perverse sheaves and knot contact homology

Wai-kit Yeung (Indiana University)


In this talk, I will present a universal construction, called homotopy braid closure, that produces invariants of links in R3 starting with a braid group action on objects of a (model) category. Applying this construction to the natural action of the braid group Bn on the category of perverse sheaves on the two-dimensional disk with singularities at n marked points, we obtain a differential graded (DG) category that extends knot contact homology in the sense of L. Ng. As an application, we show that the category of finite-dimensional modules over the 0-th homology of this DG category is equivalent to the category of perverse sheaves on R3 with singularities at most along the link.

This is joint work with Yu. Berest and A. Eshmatov.


Tuesday, December, 12th, 14:45

Mukai flops and P-twists via non-commutative crepant resolutionsy

Wahei Hara (Waseda University)


In this talk I would like to discuss three derived equivalences, mutation equivalence for NCCRs of minimal nilpotent orbit closure of type A, Kawamata-Namikawa equivalence for Mukai flop and P-twists on the cotangent bundle of a projective space. We will observe correspondences among these equivalences, which can be regarded as a higher-dimensional analog of results of Donovan and Wemyss.