Humboldt Universität zu Berlin
Mathem.Naturwissenschaftliche Fakultät II
Institut für Mathematik
Sommersemester 2013
Das Forschungsseminar findet mittwochs in der Zeit von 15.00  17.00 Uhr in der Rudower Chaussee 25, 12489 BerlinAdlershof, Raum 2.009 (Haus 2, Erdgeschoss), statt.
Seminar: Algebraic Geometry an der FU
10.04.2013  Timo Schürg (HU Berlin) 
Title: Derived algebraic geometry and reduced obstruction theories  
Abstract: Obstruction theories are essential ingredients in defining enumerative invariants as in GromovWitten and DonaldsonThomas theory. After reviewing this notion, I will give an overview of the added functoriality of obstruction theories available from derived algebraic geometry. As an application, the reduced obstruction theory necessary for counting curves on K3 surfaces will be constructed. 

    
17.04.2013  15:15  16:15: Ingrid Bauer (Universität Bayreuth) 
Title: Faithful actions of the absolute Galois group on moduli spaces and change of fundamental group  
Abstract: In the 60's J.P. Serre showed that there exists a field automorphism σ ∈ Gal (Q / Q), and a variety X defined over Q such that X and the Galois conjugate variety X^{σ} have no isomorphic fundamental groups, in particular they are not homeomorphic. In a joint paper with F. Catanese and F. Grunewald we give a strong sharpening of this phenomenon discovered by Serre: Theorem. If σ ∈ Gal (Q / Q) is not in the conjugacy class of the complex conjugation then there exists a surface (isogenous to a product) X such that X and the Galois conjugate variety X^{σ} have non isomorphic fundamental groups. Moreover, we give some faithful actions of the absolute Galois group Gal (Q / Q ), related among them, in particular: Theorem. The absolute Galois group Gal(Q / Q) acts faithfully on the set of connected components of the (coarse) moduli spaces of surfaces of general type We will explain the idea of the proof of the second theorem, as well as the technical difficulties. Finally we will show that the first theorem can be deduced from the second. 

                      
16:30  17:30: Fabrizio Catanese (Universität Bayreuth )  
Title: Moduli spaces of automorphism marked varieties: the case of curves and surfaces  
Abstract: For several reasons it is interesting to consider moduli spaces of triples (X,G,a) where X is a projective variety, G is a finite group, and a is an effective action G on X. If X is the canonical model of a variety of general type, then G is acting linearly on some pluricanonical model, and we have a moduli space which is a finite covering of a closed subspace M^{G} of the moduli space. In the case of curves this investigation is related to the description of the singular locus of the moduli space M_{g}, for instance of its irreducible components (due to Cornalba), and of its compactification M_{g}. In the case of surfaces there is another ocurrence of Murphy´s law, as shown in my joint work with Ingrid Bauer: the deformation equivalence for minimal models S and for canonical models differs drastically (nodal Burniat surfaces being the easiest example). In the case of curves, there are interesting relations with topology. Moduli spaces of curves with a group G of automorphisms of a fixed topological type have a description by Teichmüller theory, which naturally leads to conjecture genus stabilization for rational homology groups. I will then show two equivalent descriptions of its irreducible components, surveying known irreducibility results for some special groups. A new fine homological invariant was introduced in my joint work with Lönne and Perroni: it allows to prove genus stabilization in the ramified case, extending a theorem of Dunfield and Thurston in the easier unramified case. 

    
24.04.2013  Fabio Tonini (HU Berlin) 
Title: Stacks of ramified Galois covers  
Abstract: We introduce the notion of Galois cover for a finite group G and discuss the problems of constructing them and the geometry of the stack GCov they form. When G is abelian, we show that these problems are related to the theory of equivariant hilbert schemes and we describe certain families of Gcovers in terms of combinatorial data associated to G. In the general case, we present a correspondence between Gcovers and particular monoidal functors and study the problem of Galois covers of normal varieties whose total space is normal.  
    
08.05.2013  Alessandro Verra (Università Roma Tre ) 
Title: The universal Prym variety in genus < 7 via nodal conic bundles  
Abstract: In the talk various examples of families of nodal conic bundles are considered and some of them are used to dominate the universal Prym variety over the Prym moduli space in genus g < 7. Possible further applications are described. Work in progress with G. Farkas and S. Grushevsky.  
    
15.05.2013  Kein Seminar (NoGAGS in Hannover) 
    
22.05.2013  François Charles (Université de Rennes) 
Title: Arithmetic aspects of the NoetherLefschetz locus for K3 surfaces  
Abstract: Given a family of smooth projective varieties, the NoetherLefschetz locus is the set of points of the base corresponding to varieties containing strictly more divisors than the generic fiber. In the setting of complex algebraic geometry, Hodge theory has provided many tools leading to a thorough understanding of it. In this talk, we will describe some arithmetic aspects of the NoetherLefschetz locus for families of K3 surfaces, first over finite fields, leading to a proof of the Tate conjecture for K3 surfaces, and then over number fields, where we expect it to shed light on the geometry of rational curves on these surfaces. 

    
29.05.2013  Kein Seminar (Syzygies in Berlin May 2731 ) 
05.06.2013  
12.06.2013  
19.06.2013  
26.06.2013  Orsola Tommasi (Leibniz Universität Hannover) 
Title: A counterexample to the Gorenstein conjecture in genus 2  
Abstract: A main theme in the study of the cohomology of moduli spaces of curves is the study of the tautological ring, a subring generated by certain geometrically natural classes. One of the open questions is whether the tautological ring is a Gorenstein ring, as conjectured by Carel Faber in the case of smooth curves without marked points. In this talk we discuss an approach that allows to detect the existence of nontautological classes in the cohomology ring of the moduli space of stable curves of genus 2 with sufficiently many marked points, such as those constructed by Graber and Pandharipande for the compact moduli space of 20pointed curves of genus 2. We use this to prove that the Gorenstein conjecture does not hold for these spaces. This is joint work with Dan Petersen (KTH, Stockholm). 

    
03.07.2013  Sean Keel (University of Texas at Austin) 
Title: Canonical Coordinates  
Abstract: I will give more details on my proof, joint with Gross, Hacking and Kontsevich, of the (corrected) FockGoncharov dual basis conjecture, and the (positivity statement in the) FominZelevinski Laurent phenomenon conjecture, for cluster algebras. I will aim the talk at algebraic geometers, but will not assume any familiarity with cluster algebras. 

    
15.07.2013 (Montag)  Algebraic Geometry afternoon 
The event takes place in room 3.008. 

14:00  15:00: Balázs Szendrői (Oxford) 

Title: Purity and quantum cluster positivity  
Abstract: To a CalabiYau threefold or CY3 category, DonaldsonThomas theory associates numerical "sheaf counting" invariants in the first instance, which can however be refined to cohomological invariants which also carry Hodgetheoretic information. I will explain a purity result and an application to the quantum cluster positivity conjecture in the theroy of cluster algebras. This is joint work with Davison, Maulik, Schuermann. 

15:30  16:30: Marian Aprodu (Bucharest) 

Title: Chow forms of K3 surfaces and Ulrich bundles  
Abstract: An Ulrich bundle on a projective variety is a vector bundle that admits a completely linear resolution over the polynomial algebra. We prove the existence of rank2 Ulrich bundles on polarised K3 surfaces with a mild BrillNoether condition. As a consequence, we obtain explicit Pfaffian representations of the associated Chow forms. This talk is based on a joint work with Gavril Farkas and Angela Ortega. 

17:00  18:00: Maksym Fedorchuk (Boston College) 

Title: GIT stability of Hilbert and Syzygy points of canonical curves  
Abstract: I will discuss a Geometric Invariant Theory problem concerning stability of Hilbert and Syzygy points of canonical curves, explain its solution for the Hilbert points in the generic case (joint with Alper and Smyth) and explain how it is extended to obtain GIT stability of the first Syzygy point of a general canonical curve (current workinprogress, joint with Deopurkar and Swinarski). I will also give motivation and explain applications of these results coming from the minimal model program for M_{g}. 