Seminar
Friday, 18 January 2019

14:15, Room U2205
Catharina Stroppel (Bonn): Semiinfinite highest weight categories
Abstract: We briefly recall the classical highest weight categories theory (following ClineParshallScott, Donkin and Ringel) for finite dimensional algebras in a language which allows generalizations to stratified algebras and infinite situations. In particular we formulate aspects of tilting theory and Ringel duality in a semiinfinite setting. If time allows we will mention some explicit examples for this construction related to diagram algebras and categorifications.

BGTS Colloquium
16:15, Lecture Hall H5
Sabine Jansen (München): Condensation, big jump and heavy tails: from phase transitions to probability
Abstract: Ice melts, water evaporates  these are everyday experiences of phase transitions. The explanation of this macroscopic phenomenon from microscopic laws belongs to the realm of statistical physics, which treats matter as a composite system made up of many individual "agents" with random behavior. From a mathematician's point of view, a fully rigorous understanding still eludes us. The search for it leads to questions in probability that open up surprising connections: toy models for surface tension of liquid droplets build on heavytailed variables used in insurance mathematics; a big jump made by a random walker is a condensation phenomenon in disguise. The talk explains some of these connections and presents open problems and partial answers.
For a regular email announcement please contact birep.
Future Talks
Friday, 25 January 2019

14:15, Room U2205
Sebastian Opper (Paderborn): On autoequivalences and derived invariants of gentle algebras
Abstract: This talk will be about derived equivalences of gentle algebras and the group of autoequivalences of their derived categories. In joint work with Plamondon and Schroll, we attached a surface to every gentle algebra and showed that its geometry encodes of the triangulated structure of the derived category. I will explain how a derived equivalence of gentle algebras gives rise to a homeomorphism between their associated surfaces and how this leads to a complete derived invariant of gentle algebras which generalizes the combinatorial invariant of AvellaAlaminos and Geiss. Finally, I will talk about applications to groups of autoequivalences of gentle algebras and their connection to mapping class groups.

15:30, Room U2205
Frederik Marks (Stuttgart): Flat ring epimorphisms and localisations of commutative noetherian rings
Abstract: We study different types of localisations of a commutative noetherian ring. In particular, we are interested in the following questions: When is a flat ring epimorphism a universal localisation in the sense of Schofield? And when is such a universal localisation a classical ring of fractions? We approach these questions using the theory of support and local cohomology, and by analysing the specialisation closed subset of the spectrum associated with a flat ring epimorphism. As for the first question, we show that all flat ring epimorphisms are universal localisations when the underlying ring is either locally factorial or of Krull dimension one. If time permits, we will also comment on the situation for more general rings, which turns out to be significantly more complicated and diverse. Finally, we show that an answer to the question of when universal localisations are classical depends on the structure of the Picard group of the underlying ring. This talk is based on joint work with Lidia Angeleri Hügel, Jan Stovicek, Ryo Takahashi and Jorge Vitória.
Friday, 01 February 2019

Room U2205
Wassilij Gnedin (Bochum): tba

Room U2205
Severin Barmeier (Münster): Diagrams of algebras, categories of coherent sheaves and deformations
Abstract: Given a complex algebraic variety X, the restriction of its structure sheaf to a finite cover of affine open sets can be viewed as a diagram of (commutative) algebras. Deformations of a diagram obtained in this way correspond precisely to deformations of the category of
(quasi)coherent sheaves as an Abelian category (after W. Lowen and M. Van den Bergh).
We describe the higher deformation theory explicitly via Linfinity algebras for X covered by two affine opens and explain the connection to
"classical" deformations of the complex structure and deformation quantizations by means of examples. This is joint work with Y. Frégier.
Seminar Archive
Friday, 11 January 2019

14:15, Room U2205
William CrawleyBoevey (Bielefeld): Decomposition of persistence modules
Abstract: I shall discuss the decomposition of pointwise finitedimensional persistence modules. A persistence module indexed by the real plane is said to be middle exact if for each rectangle in the plane, the associated 3term exact sequence of vector spaces is exact in the middle. I shall outline a new proof of a theorem of Cochoy and Oudot classifying such middle exact modules. They arise in the study of interlevel set persistence homology, answering a question of Botnan and Lesnick. This is joint work with Magnus Botnan.

15:45, Room U2205
Jörg Schürmann (Münster): Degenerate affine Hecke algebras and Chern classes of Schubert cells
Abstract: We explain in the context of complete flag varieties X=G/B the relation between Chern classes of Schubert cells and convolution actions of degenerate affine Heckealgebras as in the work of Ginzburg. This is based on the Lagrangian approach via characteristic cycles. As an application we show that the two cohomological Weyl group actions constructed by Ginzburg and AluffiMihalcea coincide. These Weyl group actions permute the (equivariant) Chern classes of the corresponding Schubert cells. This is joint work with P. Aluffi, L. Mihalcea and C. Su.
Friday, 23 November 2018

BiBo Seminar in Bochum
14:00, Room IA 171 (60 minutes)
Arif Dönmez (Bochum): Moduli of representations of onepoint extensions
Abstract: We study the moduli spaces of (semi)stable representations of onepoint extensions of quivers by rigid representations and derive results on their geometric properties with homological methods.

BiBo Seminar in Bochum
15:30, Room IA 171 (30 minutes)
Julia Sauter (Bielefeld): An invitation to relative geometry of representations
Abstract: Following Auslander and Solberg, relative homological algebra replaces Ext^1 by a subfunctor Ext^1_F. In this setup it is natural to replace the representation space of quiver representations by locally closed subsets where (certain) Homdimensions are fixed (their closures are usually referred to as rank varieties). I would like to use relative homological algebra to study these spaces and explain as a first step the relative Voigt's lemma. I would end with many conjectures and would hope to find some interested people who would like to work on this with me.

BiBo Seminar in Bochum
16:45, Room IA 171 (60 minutes)
Christof Geiss (Mexico City): Real Schur roots and rigid representations
Abstract: This is a report on joint work with B. Leclerc and J. Schröer.
Let F be a field. In previous work we constructed, associated to a symmetrizable generalized Cartan matrix C with symmetrizer D and and orientation Ω, an 1IwangaGorenstein Falgebra H:=H(C,D,Ω), defined in terms of a quiver with relations.
We show that the rigid indecomposable locally free Hmodules are parametrized, via their rank vector, by the real Schur roots associated to (C,Ω). Moreover, if M is such a module then it is free as if module over its endomorphism ring, and this ring is of the form F[x]/(x^n) for some n. This allows us to classify the left finite bricks of H in terms of the real Schur roots associated to (C^t,Ω). The main tool to prove our results is a F[[x]]order, which permits us to relate locally free Hmodules with modules over the canonical F((x))species associated to the combinatorial data (C,D,Ω).
Friday, 16 November 2018

13:15, Room U2205
Olaf Schnürer (Paderborn): Smoothness of derived categories of algebras
Abstract: We report on joint work with Alexey Elagin and Valery Lunts where we prove smoothness in the dg sense of the bounded derived category of finitely generated modules over any finitedimensional algebra over a perfect field. More generally, we prove this statement for any algebra over a perfect field that is finite over its center and whose center is finitely generated as an algebra. These results are deduced from a general sufficient criterion for smoothness.

14:30, Room U2205
Lidia Angeleri Hügel (Verona): Silting complexes over hereditary rings
Abstract: I will report on joint work with Michal Hrbek. Given a hereditary ring, we use the lattice of homological ring epimorphisms to construct compactly generated tstructures in its derived category. This leads to a classification of all (not necessarily compact) silting complexes over the Kronecker algebra.
Friday, 09 November 2018

14:15, Room U2205
Kevin Coulembier (Sydney): Tensor categories in positive characteristic
Abstract: Tensor categories are abelian klinear monoidal categories satisfying some natural additional properties. Archetypical examples are the representation categories over affine (super)group schemes. P. Deligne gave very succinct intrinsic criteria for a tensor category to be equivalent to such a representation category, over fields k of characteristic zero. These descriptions are known to fail badly in prime characteristics. In this talk, I will present analogues in prime characteristic of these criteria, admittedly less succinct, but still intrinsic. Time permitting, I will comment on the link with a recent conjecture of V. Ostrik extending Deligne’s work in a different direction.
Friday, 19 October 2018

14:15, Room U2205
Sarah Scherotzke (Münster): The Chern character and categorification
Abstract: The Chern character is a central construction which appears in topology, representation theory and algebraic geometry. In algebraic topology it is for instance used to probe algebraic Ktheory which is notoriously hard to compute, in representation theory it takes the form of classical character theory. Recently, Toen and Vezzosi suggested a construction, using derived algebraic geometry, which allows to unify the various Chern characters. We will categorify this Chern character. In the categorified picture algebraic Ktheory is replaced by the category of noncommutative motives.

15:30, Room U2205
Jörg Feldvoss (Mobile, Alabama): Cohomological Vanishing Theorems for Leibniz Algebras
Abstract: Leibniz cohomology was introduced by Bloh and Loday as a noncommutative analogue of ChevalleyEilenberg cohomology of Lie algebras. It turned out that Leibniz cohomology works more generally for Leibniz algebras which are a non(anti)commutative version of Lie algebras. Many results for Lie algebras have been proven to hold in this more general context.
In the talk I will start from scratch and define Leibniz algebras, Leibniz (bi)modules, and their cohomology. Then I will explain the Leibniz analogues of vanishing theorems for the ChevalleyEilenberg cohomology of semisimple and solvable Lie algebras due to Whitehead, Dixmier, and Barnes. In particular, we obtain the second Whitehead lemma for Leibniz algebras and the rigidity of semisimple Leibniz algebras in characteristic zero. The latter results were conjectured to hold for quite some time. Our main tools are the cohomological analogues of two spectral sequencesof Pirashvili for Leibniz homology and a spectral sequence due to Beaudouin.
All this is joint work with Friedrich Wagemann.
Friday, 24 August 2018

14:15, V2200
Hiroyuki Minamoto (Sakai): Finite dimensional graded IwanagaGorenstein algebras and Happel's functor
Abstract: Let A be a finite dimensional algebra and T(A)= A + D(A) be the trivial extension by the dual bimodule D(A). Happel constructed a functor H from the derived category of A to the stable category of graded T(A)modules and proved that it is fully faithful and gives an equivalence precisely when A is of finite global dimension. Happel's functor is generalized to a finite dimensional graded IwanagaGorenstein algebra S and gives a functor from the derived category of the Beilinson algebra, a finite dimensional algebra constructed from S, to the stable category of graded CohenMacaulay modules. In this talk, first we give two characterizations of S such that Happel's functor is fully faithful or equivalence. Next we show that Happel's functor admits a fully faithful left adjoint which has left adjoint provided that the degree 0 subalgebra of S is of finite global dimension. If time permits, we give several applications. This is a joint work with Kota Yamaura.
Friday, 20 July 2018

14:15, Room T2205
Baptiste Rognerud (Bielefeld): The derived category of the Tamari lattice is fractionally CalabiYau
Abstract: In this talk, I will introduce an interesting family of indecomposable objects in the bounded derived category of the Tamari lattice. Then, I will give a combinatorial description of the action of the Serre functors on these objects and explain how we can deduce that the bounded derived category is fractionally CalabiYau.

15:30, Room T2205
Fan Xu (Beijing): RingelHall algebras and categorification
Abstract: The aim of this talk is to generalize Lusztig's construction of quantum groups to RingelHall algebras. We construct the geometric analog of Green's theorem on the comultiplication of a RingelHall algebra. It is an extension version of the comultiplication of a quantum group defined by Lusztig. As an application, we show that the Hopf structure of a RingelHall algebra can be categorified under Lusztig's framework. This is based on joint work with Jie Xiao and Minghui Zhao.
Friday, 13 July 2018

15:00, Room T2205
Louis Rowen (Ramat Gan): Algebraic systems and exterior semialgebra
Abstract: In this talk, we describe negation maps and `systems', and their application to linear algebra in a rather general framework that includes tropical algebra, hyperfields and fuzzy rings.
The usual definition of Grassmann (exterior) algebras generalizes directly to semialgebras, and has a builtin negation map for elements of degree > 1, so the theory of systems can be applied directly to Gatto's theory, unifying results of linear algebra from different perspectives including the classical perspective.
This will include joint work with Akian, Gaubert, Gatto, Jun, Knebusch, and Mincheva, and does not require prerequisites.
Friday, 06 July 2018

14:15, Room T2205
Ulrich Thiel (Sydney): Finitedimensional graded algebras with triangular decomposition
Abstract: I will discuss a new approach to the representation theory of selfinjective finitedimensional graded algebras with triangular decomposition (such as restricted enveloping algebras, Lusztig’s small quantum groups, hyperalgebras, finite quantum groups, restricted rational Cherednik algebras, etc). We show that the graded module category of such an algebra is a highest weight category and has a tilting theory in the sense of Ringel. We can then show that the degree zero part of the algebra (the "core") is cellular and can construct a canonical highest weight cover à la Rouquier of it. The core captures essential information of the representation theory of the original algebra, hence we can approach the latter with these additional structures. This is joint work with Gwyn Bellamy (Glasgow).

15:30, Room T2205
Yann Palu (Amiens): Nonkissing complex and tautilting over gentle algebras
Abstract: This is a report on a joint paper with Vincent Pilaud and PierreGuy Plamondon. The nonkissing complex is a simplicial complex introduced by T. McConville who studied some of its lattice theoretic aspects. After explaining the properties of the nonkissing complex that seem the most relevant to representation theory, I will relate it to tautilting theory, as defined by AdachiIyamaReiten. This allows to generalise nonkissing to a more general set up by making use of gentle algebras.
Wednesday, 27 June 2018

14:15, Room U2205 (90 minutes)
Giovanni Cerulli Irelli (Rome): Cell decompositions and algebraicity of cohomology for quiver Grassmannians
Abstract: I will report on a joint project with F. Esposito (Padova), H. Franzen (Bochum) and M. Reineke (Bochum), arXiv: 1804.07736. We show that the cohomology ring of a quiver Grassmannian associated with a rigid quiver representation has property (S): there is no odd cohomology and the cycle map is an isomorphism; moreover, its Chow ring admits explicit generators defined over any field. From this, we deduce the polynomial point count property. By restricting the quiver to finite or affine type, we are able to show a much stronger assertion: namely, that a quiver Grassmannian associated to an indecomposable (not necessarily rigid) representation admits a cellular decomposition. As a corollary, we establish a cellular decomposition for quiver Grassmannians associated with representations with a rigid regular part. Finally, we study the geometry behind the cluster multiplication formula of Caldero and Keller, providing a new proof of a slightly more general result.
Friday, 15 June 2018

14:15, Room U2113
Jan Geuenich (Bielefeld): Tilting modules for the Auslander algebra of the truncated polynomial ring
Abstract: I present the classification of the tilting modules for the Auslander algebra of the truncated polynomial ring. More precisely, I construct an isomorphism between the tilting poset and a finite interval in the braid group. This extends an isomorphism described by Iyama and Zhang between the classical tilting poset and the symmetric group with the opposite left weak order.

15:30, Room U2113
Estanislao Herscovich (Grenoble): The curved A_infinitycoalgebra of the Koszul codual of a filtered dg algebra
Abstract: In this talk I will present a result allowing to compute the coaugmented curved A_infinitycoalgebra structure of the Koszul codual of a filtered dg algebra over a field k. This provides a generalisation of a result by B. Keller, which described the A_infinitycoalgebra structure of the Koszul codual of a nonnegatively graded connected algebra. As an application, I will show how to compute the coaugmented curved A_infinitycoalgebra structure of the Koszul codual of a PBW deformation of an NKoszul algebra, extending a previous result by G. Fløystad and J. Vatne.
Friday, 08 June 2018

14:15, Room T2205
Mikhail Gorsky (Cologne): Extended Hall algebras
Abstract: Hall algebras play an important role in representation theory and algebraic geometry. The Hall algebra of an exact or a triangulated category captures information about the extensions between objects. It turns out that in some cases twisted and extended Hall algebras of triangulated categories are welldefined even when their nonextended counterparts are not. I will explain how to associate a twisted extended Hall algebra to a triangulated category, when the latter arises as the homotopy category of a hereditary exact model category or as an orbit category of certain kind. I will discuss applications of this constructions to graded quiver varieties and to categorification of modified quantum group.

15:30, Room T2205
William CrawleyBoevey (Bielefeld): A new approach to simple modules for preprojective algebras
Abstract: This is joint work with Andrew Hubery. My earlier work on the moment map for representations of quivers included a classification of the possible dimension vectors of simple modules for deformed preprojective algebras. That classification was later used to solve an additive analogue of the DeligneSimpson problem. The last step in the proof of the classification involved some general position arguments; here we give a new approach which avoids such arguments.
Friday, 25 May 2018

14:15, Room T2205
Sophiane Yahiatene (Bielefeld): Thick subcategories in hereditary abelian categories
Abstract: Let H be a connected Extfinite hereditary abelian kcategory with tilting complex. In this talk I present a grouptheoretic method to classify the thick subcategories generated by exceptional sequences. For that, we consider the Grothendieck group of H, which can be seen as a root lattice of a generalized root system and define a reflection group acting on it.
As an example we consider the category of coherent sheaves of a weighted projective line of tubular type.
(Joint work with B. Baumeister and P. Wegener)

15:30, Room T2205
Henning Krause (Bielefeld): Morphic enrichments of triangulated categories (after Keller)
Abstract: The talk presents some recent work of Bernhard Keller. The notion of a triangulated category suffers from the fact that cones are not functorial. Morphic enrichments provide a concept to overcome this problem, and basically all triangulated categories that arise in nature admit such an enrichment. A morphic enrichment of a triangulated category T is given by a recollement of triangulated categories with T at both ends, and the additive structure of this recollement determines the triangulated structure of T. This idea goes back to work of Keller (Derived categories and universal problems, Comm. Algebra 19, 1991); it turns out to be useful for defining a triangulated structure on the completion of a triangulated category.
Friday, 04 May 2018

13:15, Room T2205
Magnus Bakke Botnan (München): Representation Theory in Topological Data Analysis
Abstract: Topological data analysis (TDA) is a relatively recent approach to data analysis in which topological signatures are assigned to data. In this talk I will survey how the theoretical foundations of TDA rest on classical results from the representation theory of quivers. I will also discuss recent results in representation theory inspired by questions in TDA. This is joint work with Ulrich Bauer, Steffen Oppermann and Johan Steen.

14:30, Room T2205
Haydee Lindo (Stuttgart): Trace modules, Rigidity and Endomorphism rings
Abstract: I will speak on some recent developments in the theory of trace modules over commutative Noetherian rings. This will include applications of trace modules in understanding endomorphism rings and a discussion of ongoing work examining the relationship between trace modules and modules having no selfextensions.
Friday, 27 April 2018

14:15, Room T2205
Thomas Poguntke (Bonn): Higher Segal structures in algebraic Ktheory and Hall algebras
Abstract: One of the main results of DyckerhoffKapranov's work on higher Segal spaces concerns the fibrancy properties of Waldhausen's simplicial construction of the algebraic Ktheory of an exact category, which are in particular responsible for the associativity of various Hall algebras. We will explain their results, with an emphasis on this latter aspect. Finally, we will introduce a higher dimensional analogue of the construction, where short exact sequences are replaced by longer extensions, whose algebraic ramifications are yet to be clearly understood.
Saturday, 21 April 2018
Friday, 20 April 2018
Friday, 13 April 2018

14:15, Room T2205
Minghui Zhao (Beijing): On purity theorem of Lusztig's perverse sheaves
Abstract: Let Q be a finite quiver without loops and U the quantum group corresponding to Q. Lusztig introduced the canonical basis of the positive part of U via some semisimple perverse sheaves (Lusztig's perverse sheaf). When Q is a Dynkin quiver, Lusztig proved that any Lusztig's perverse sheaf L possesses a Weil structure such that the Frobenius eigenvalues on the stalk of the ith cohomology sheaf H^i(L) at x are equal to q^(i/2) for any krational point x, where k is the finite field of q elements. The purpose of this talk is to generalize this result to all finite quiver without loops. As an application, we shall prove the existence of a class of Hall polynomials. This is a joint work with Jie Xiao and Fan Xu.
For information on earlier talks please check the complete seminar archive.