Oberseminar Algebraische Geometrie

We meet regularly on Tuesdays in the building E2 4, room SR6 or online in Zoom, starting at noon or at 2pm (see below).

08.04.2025	Yuta Takada (Tokyo): Dynamical degrees of automorphisms of K3 surfaces with Picard number 2 It is known that the dynamical degree of an automorphism of a K3 surface is either 1 or a Salem number, and the question of which Salem numbers are realizable has been considered. We determine the set of the dynamical degrees of automorphisms of projective K3 surfaces with Picard number 2. This extends the result by Hashimoto, Keum, and Lee.
15.04.2025 (at 13:30!)	Stavros Papadakis (Ioannina): The g-conjecture for simplicial spheres By definition, the i-th face number of a convex polytope is the number of its faces of dimension i. An interesting open problem is the complete characterisation of all possible face numbers of convex polytopes of dimension d, where d>3 denotes an integer. Around 1980, Billera, Lee and Stanley solved the corresponding problem for the special classes of simple and simplicial convex polytopes, proving a conjecture of McMullen. In 2018, Adiprasito achieved the complete characterization of the face numbers of simplicial spheres. The aim of the talk is to give an introduction to these results and discuss a second proof of Adiprasito's result using characteristic 2 generic anisotropy.
22.04.2025	Matteo Penegini (Genoa): Arithmetic Zariski multiplets of irreducible plane curves Multicanonically embedded surfaces in projective space give rise to irreducible branch curves via projection from generic axes. Building on our previous work, we transfer results from the moduli space of surfaces to equisingular strata of plane curves. For instance, the faithful action of the Galois group on the connected components of the moduli spaces of surfaces isogenous to a product, as established by Bauer, Catanese, and Grunewald, gives rise to many arithmetic Zariski multiplets. This is a joint work with M. Loenne.
29.04.2025	Pietro Beri (Nancy): On the projective duality of Kummer fourfolds Despite being one of the families of hyper-Kähler manifolds introduced by Beauville in 1983, not many projective models of Kummer manifolds are known. In this talk, we describe the projective model of a Kummer fourfold of a principally polarized abelian surface and how it relates via projective duality to the projective model studied by Benedetti, Manivel, and Tanturri in 2019. Surprisingly, the description involves Coble cubic, introduced more than a century ago (1917) by Coble himself, and a duality result conjectured by Dolgachev and proven by Ortega and Nguyen (2003) concerning moduli spaces of sheaves on curves. This is a joint work with Agostini, F. Giovenzana and Ríos-Ortiz.
06.05.2025	TBA
14.05.2025 (Wednesday!)	Roberto Pignatelli (Trento): TBA TBA
20.05.2025	TBA
27.05.2025	No seminar
05.06.2025 (Thursday!)	Upper-Rhine and Tributaries Algebraic Geometry Seminar, Strasbourg (3 talks)
(10.06.2025)	(TBA)
17.06.2025	Davide Frapporti (Milano): TBA TBA
24.06.2025	Dario Weissmann (IMPAN): Distinguishing algebraic spaces from schemes We introduce local invariants of algebraic spaces which measure how far they are from being a scheme. In the setting of a stack admitting a separated (good) moduli space this also yields a criterion for when the moduli space is a scheme. As an application we identify all separated good moduli spaces of vector bundles over a smooth projective curve which are schemes. This is joint work with Andres Fernandez Herrero and Xucheng Zhang.
01.07.2025	Thamarai Valli Venkatachalam (UCL): TBA TBA
07.07.2025 (Monday at 2pm!)	Jihao Liu (Peking University): TBA TBA
15.07.2025	Matthias Schütt (Hannover): TBA TBA

10.10.2024	Upper-Rhine and Tributaries Algebraic Geometry Seminar, Stuttgart (3 talks)
29.10.2024	Matthias Zach (RPTU Kaiserslautern): Homological indices for mappings on stratified spaces Let us define the Milnor fiber M of a holomorphic map germ f: (Cⁿ, 0)→(C^k, 0) as the preimage of a regular value ‘v’ of f restricted to some sufficiently small ball B around the origin in the domain Cⁿ. We may ask: “What do we know about the topology of M?” It is well known that if f defines a complete intersection with an isolated singularity, then M is homotopy equivalent to a bouquet of spheres of real dimension n-k and the number of these spheres is called the Milnor number μ(f) of f. Lê and Greuel independently proved a formula to compute μ(f) as the colength of a certain analytic module. In the case where the domain is not smooth, say f: (X, 0)→(C^k, 0) with (X, 0) a subset of (Cⁿ, 0) some reduced complex analytic space, the situation is more subtle. We will discuss how to generalize the above results to this setup and establish various formulas to compute “Milnor numbers” associated to such map germs as Euler characteristic of complexes of coherent analytic sheaves. Time permitting, we will elaborate some applications to determinantal singularities.
12.11.2024	Aryaman Patel (UdS): Higgs bundles, symmetric differentials and topology I will talk about the classical correspondence between topologically trivial semistable Higgs bundles and representations of the fundamental group. This provides a link between the geometry and topology of smooth projective varieties. This link can easily be generalized to normal projective varieties, which I will explain. If time permits, I will talk about the Hitchin morphism for some special kinds of varieties.
26.11.2024	Domenico Valloni (EPFL): Brauer classes in positive characteristic In this talk we use differential forms to define supersingular Brauer classes in positive characteristic. We show that these classes can be used to describe the Brauer group of supersingular K3 surfaces together with their Artin invariant. Finally, we explain their role in the Brauer-Manin obstruction, and we give purely geometrically conditions so that the Brauer-Manin obstruction is the only obstruction for the Hasse principle. This is a joint work with Alexei Skorobogatov.
10.12.2024	Erik Paemurru (UdS): Birational rigidity of Fano 3-folds with compound A_k singularities A Fano variety is birationally rigid if it is the only Mori fibre space in its birational class, implying its nonrationality. I will discuss a recent joint work with Igor Krylov, Takuzo Okada and Jihun Park on birational rigidity of families of Fano 3-folds with compound A₁ singularities, as well as its generalisation to compound A_k singularities.
17.12.2024	Niklas Müller (Duisburg-Essen): Minimal Projective Varieties satisfying 3c₂=c₁² It is a classical fact that the Chern classes of any minimal smooth projective surface X satisfy the so-called Bogomolov-Miyaoka-Yau inequality 3c₂(X)-c₁²(X)≥0 and it is known explicitly for which surfaces equality is attained. More generally, if X is a minimal projective variety of dimension n, Miyaoka proved that (3c₂(X)-c₁²(X))Hⁿ⁻²≥0 for any ample divisor H on X. In this talk I want to discuss the structure of those varieties X attaining equality. In particular, we will see that abundance holds for such varieties. This is joint work with M. Iwai and S.-I. Matsumura.
07.01.2025	Nikolaos Tsakanikas (EPFL): Singular Enriques varieties In this talk I will introduce the class of primitive Enriques varieties. I will discuss the basic properties of these objects, showing in particular that the smooth ones are Enriques manifolds, and I will also present some examples of (singular) primitive Enriques varieties. Finally, I will sketch the proof of the following termination statement: if X is an Enriques manifold and B is an R-divisor on X such that the pair (X,B) is log canonical, then any (K_X+B)-MMP terminates.
14.01.2025	Thomas Peternell (Bayreuth): Compactifications of homology cells and the complex projective space I will discuss the following problem: given a projective manifold X and a submanifold Y such that X\Y is a homology cell, is X a projective space?
21.01.2025	Federico Fallucca (Milano-Bicocca): Classification of (k,p)-covers of the plane with geometric genus 3 A (k,p)-cover of the plane is a compact complex smooth surface X with an automorphism subgroup isomorphic to Z_p^k, where p is a prime number, such that the quotient is P². There are several motivations for investigating these surfaces. For instance, (k,p)-covers of the plane are always regular, and those with geometric genus 3 are good candidates for surfaces with a high degree of the canonical map, of which there are few examples in the literature. Furthermore, they are of interest as potential "Triple K3 Burgers", a class of surfaces due to Laterveer where Voisin's conjecture (on the pullback of 0-cycles on a surface) can be examined. Additionally, bidouble and iterated bidouble covers of rational surfaces have been studied by Garbagnati and Penegini to produce examples for investigating the Infinitesimally Torelli Property, the Mumford-Tate conjecture, and the Tate conjecture. In this seminar, I will briefly discuss the description of (k,p)-covers of P² in the language of abelian coverings by Pardini. I will then present the classification theorem of (k,2)-covers of P² with geometric genus 3 (a joint work with Pignatelli), and the more recent classification of (k,p)-covers of P² with p>2. Furthermore, I will show the degree of the canonical map for each case and discuss which ones are Triple K3 Burgers. Finally, time permitting, I will outline the approach used to achieve this classification.
28.01.2025	Emma Brakkee (Leiden): Bounding Brauer groups of K3 surfaces using moduli spaces The Brauer group of an algebraic variety is a group with many applications, in particular to the study of rational points. For a K3 surface over a number field, the transcendental part of its Brauer group is finite. It was shown by Cadoret-Charles that the size of its p-primary torsion is uniformly bounded for K3 surfaces in one-dimensional families. We give a different proof of this result for one-dimensional families of K3 surfaces with a polarization by a fixed lattice. To be precise, we construct moduli spaces of K3 surfaces with a lattice polarization and a Brauer class, and use the geometry of their complex points to prove boundedness of Brauer groups for the K3 surfaces they parametrize. I will explain the construction and give a sketch of the proof of our boundedness result. This is joint work in progress with D. Bragg and T. Várilly-Alvarado.
04.02.2025	Christian Gleissner (Bayreuth): Higher dimensional Beauville manifolds and groups In this talk we consider rigid compact complex manifolds of general type that are realized as quotients of a product of curves by a free action of a finite group. They serve as higher-dimensional analogues of Beauville surfaces. Using uniformization, we outline the theory to characterize these manifolds through specific combinatorial data associated with the group. This leads to the notion of an n-fold Beauville structure. As an application, we provide some classification results of these manifolds in dimension three. This is a work in progress with Federico Fallucca and Noah Ruhland.

23.04.2024 (at noon!)	Mastervortrag: Robin Lahni
30.04.2024 (at 2pm)	Eric Pichon-Pharabod (Paris-Saclay): Periods of elliptic surfaces via effective homology The period matrix of a smooth complex projective variety X encodes the isomorphism between its singular homology and its algebraic De Rham cohomology. Numerical approximations with sufficient precision of the entries of this matrix, called periods, allow to recover some algebraic invariants of the variety, such as the Picard rank in the case of surfaces. I will present a method relying on the computation of an effective description of the homology for obtaining such numerical approximations of the periods of hypersurfaces and elliptic surfaces.
07.05.2024 (at 2pm)	Tariq Syed (Mainz): Motivic cohomology of cyclic coverings Many examples of topologically contractible smooth affine complex varieties are given by cyclic coverings. In this talk, we present new results on the motivic cohomology of such cyclic coverings.
21.05.2024 (at 2pm)	Francesco Denisi (Paris Cité): On some cones of divisors on irreducible holomorphic symplectic manifolds In this talk, I will discuss some cones of divisors on projective irreducible holomorphic symplectic (IHS) manifolds. Firstly, I will provide a structure theorem for the pseudo-effective cone of a projective IHS manifold, which generalises a well known result by Kovács on K3 surfaces. Secondly, I will describe the duals of certain cones of divisors, called "cones of k-ample divisors", naturally defined using the augmented base loci of big divisors. This duality result, which is contained in a joint work with A. D. Ríos Ortiz, generalises (in the case of IHS manifolds) a well known theorem by Boucksom-Demailly-Păun-Peternell, stating that the dual of the pseudo-effective cone of a smooth complex projective variety is equal to the closed cone of mobile curves.
28.05.2024 (at 2pm)	Simon Brandhorst (UdS): Ramification of maps between moduli spaces of Enriques surfaces Barth and Peters showed that a generic complex Enriques surface has exactly 527 isomorphism classes Elliptic fibrations. We show that every Enriques surface has precisely 527 isomorphism classes of elliptic fibrations when counted with multiplicity. Their reducible singular fibers and the multiplicities can be calculated explicitly. The statements holds over any algebraically closed field of characteristic not two. To explain these results, we construct a moduli space of complex elliptic Enriques surfaces and study the ramification behavior of the forgetful map to the moduli space of unpolarized Enriques surfaces. Curiously, the ramification indices of a similar map compute the hyperbolic volume of the rational polyhedral fundamental domain appearing in the Morrison-Kawamata cone conjecture. This is work in progress with Víctor González-Alonso.
18.06.2024 (at 2pm)	Simone Billi (Genova): Automorphisms and degenerations of double EPW-cubes Double EPW-sextics and double EPW-cubes are among the few locally complete families of projective IHS manifolds. O'Grady constructed a symplectic resolution of mildly singular double EPW-sextics, which turn out to be birational to Hilbert squares of a K3 surfaces. The group of automorphisms of a double EPW-sextic was described by Kuznetsov. As an application, examples of double EPW-sextics with a maximal group of symplectic automorphisms were constructed firstly by Debarre and Mongardi, and later by Billi and Wawak. Adopting similar techniques, we construct a symplectic resolution of a mildly singular doube EPW-cube and prove that in general this is not birational to a moduli space of (twisted) sheaves on a K3 surface. We also give a description of the automorphism group of a double EPW-cube, arguing similarly to the case of double EPW-sextics. We give a lattice-theoretical classification of groups of symplectic automorphisms of manifolds of K3^[3] type and, as an application of the previous results, we construct examples of double EPW-cubes with some of the maximal groups of automorphisms. Based on a joint work in progress with S. Muller and T. Wawak.
25.06.2024	Upper-Rhine and Tributaries Algebraic Geometry Seminar, Strasbourg (3 talks)
16.07.2024 (at noon!)	Massimiliano Alessandro (UdS): On the Classification of Surfaces of General Type with p_g=q=2 The classification of smooth minimal complex projective surfaces of general type is a classical and long-standing research topic. It turns out that even constructing those with small invariants is very challenging. In this talk we describe a new construction method for surfaces of general type with p_g=q=2, providing explicit and global equations for some families of surfaces. This is a joint work with Fabrizio Catanese.

24.10.2023	Bachelorvortrag: Laura Stokkermans Frank-Olaf Schreyer (UdS): Hyperelliptic Curves and Ulrich sheaves on the complete intersection of two quadrics Using the connection between hyperelliptic curves, Clifford algebras, and complete intersections X of two quadrics, we describe Ulrich bundles on X and construct some of minimal possible rank. This is joint work with David Eisenbud based in part on unpublished work with our dear friend the late Ragnar Buchweitz.
07.11.2023 (online, at noon!)	Zhan Li (SUSTech): On the relative Morrison-Kawamata cone conjecture for Calabi-Yau fiber spaces A fibration with a relatively trivial canonical divisor is called a Calabi-Yau fiber space. The Morrison-Kawamata cone conjecture relates the birational geometry of a Calabi-Yau fiber space to the convex geometry of a movable cone. Assuming the minimal model program conjectures, we show that the cone conjecture of the generic fiber implies the cone conjecture of the Calabi-Yau fiber space. As an application, the finiteness of minimal models of a Calabi-Yau fiber space in relative dimensions less or equal to 2 is established. This work is partially joint with Hang Zhao. Omprokash Das (Tata Institute): Recent developments in the minimal model program for Kähler varieties Some of the fundamental theorems of MMP, such as Mori’s Bend-Break, Base-point free theorem, etc. fail on compact Kahler manifolds. Despite these failures, in a series of three pairs in 2015-16, Campana, Peternell and Höring established that the MMP holds for compact Kahler threefolds with terminal singularities. In this talk I will explains some of the main tools and techniques developed by these authors. Recently we have extended these results to DLT pairs in dim 3 and some partial cases in dim 4, and also introduced an inductive approach to the contraction problem. In this talk I will survey these results and explain some of our techniques. These are part of joint work with Christopher Hacon.
21.11.2023 (online!)	Erik Paemurru (UdS): Explicit Sarkisov links I will review techniques how to construct Sarkisov links as restrictions of toric 2-ray links. As an application, in a joint work with T. Guerreiro and L. Campo, we consider hypersurfaces Γ in a linear subspace of projective n-space Pⁿ and hypersurfaces X of Pⁿ containing Γ. We show that the blowup of X along Γ is a Mori dream space, we compute its Mori chamber decomposition and its associated birational models.
12.12.2023	Annalisa Grossi (Paris-Saclay): Birational equivalence and deformation equivalence for Hyperkähler manifolds Huybrechts proved that if X and Y are birational HK manifolds they have to be deformation equivalent, but this implication is far from being an equivalence. It is then natural to ask which additional assumptions are needed to get that two HK manifolds of the same deformation type are birational. In this talk I will give a gentle introduction on lattice theory for HK manifolds, then I will provide a lattice-theoretic criterion to determine when two HK manifolds of OG10 type are birational (joint work with C. Felisetti and F. Giovenzana), and I will show an application to the Li-Pertusi-Zhao variety.
19.12.2023 (online, at noon!) (cancelled!)	Zheng Xu (AMSS Beijing): On the abundance conjecture for threefolds in positive characteristic Over the past decade, the Minimal Model Program (MMP) has been largely established for threefolds over perfect fields of characteristic >3. A central conjecture remaining is the abundance conjecture, which predicts that if (X,B) is a projective log canonical threefold pair over an algebraically closed field of characteristic >3 and K_X+B is nef, then K_X+B is semiample. In this talk, I will discuss some recent results towards this conjecture. For example, I will explain my proof of this conjecture in the case when the numerical dimension of K_X+B equals to 2.
09.01.2024 (at 12:15 in U.39!)	Jonathan Spreer (Sydney): Triangulations of manifolds and their local modifications Consider the following two popular problems from combinatorial topology: 1. Given a piecewise linear manifold M, can we construct a triangulation of M? 2. Given two triangulations, do they represent the same piecewise linear manifold? In dimension two, both of these problems are trivial. In dimension three, both have practical solutions (in many cases). In dimension four, however, they can become interesting and difficult. I will discuss both problems by means of the K3 surface, a simply connected piecewise linear four-manifold: Two triangulations exist of 4-manifolds homeomorphic to the K3 surface. One of them is piecewise linear homeomorphic (diffeomorphic) to the K3 surface. The other one is highly symmetric with the smallest number of simplices – but its piecewise linear type is still unknown. I will present the methods used to construct both triangulations, and a list of attempts to prove their piecewise linear equivalence via local modifications, such as bi-stellar moves.
23.01.2024 (at 12:15 in U.39!)	Stevell Muller (UdS): On symplectic birational transformations of OG10-type hyperkähler manifolds via cubic fourfolds We know thanks to the work of L. Giovenzana, A. Grossi, C. Onorati and D. Veniani that OG10-type hyperkähler manifolds do not admit any non-trivial finite symplectic automorphisms. What about non-regular symplectic birational transformations? Given a cubic fourfold V, one can construct a hyperkähler manifold XV of OG10-type following a construction of R. Laza, G. Saccà, C. Voisin. Such manifolds are known as LSV manifolds. It can be shown that any symplectic automorphism on V induces a symplectic birational transformation on XV. In a couple of works with L. Marquand, we study and classify all possible cohomological actions on the OG10-lattice which can be realised as symplectic birational transformations. By investigating further the induced action on cohomology, we exhibit a criterium to decide which of these actions can be realised as induced from a cubic fourfold on an associated LSV manifold.
30.01.2024	Víctor González Alonso (Hannover): Infinitesimal rigidity of certain modular morphisms The Torelli morphism maps (the isomorphism class of) a smooth complex projective curve to its polarized jacobian variety. It has been recently proved by Farb that this is the only non constant holomorphic map from the moduli space of curves to that of principally polarized abelian varieties, and Serván has recently proved a similar result for the Prym morphism. One can equivalently state these results on "global rigidity" by saying that a certain moduli space of morphisms consists of just one point. But it could still happen that this point is not reduced, or in other words, the morphism admits non-trivial infinitesimal deformations. In this talk I will present a joint work with Giulio Codogni and Sara Torelli showing that this is not the case if one considers the moduli stacks. The proof uses the Fujita decomposition of the Hodge bundle of a family of curves, and can be applied to other morphisms involving moduli of smooth curves.

12.04.2023 (Wednesday!)	Grégoire Menet (Lille): G₂-geometry via algebraic geometry The G₂-manifolds are one of the sporadic cases of irreducible Riemannian manifolds in the Berger classification. In this talk, we will see how G₂-manifolds can be constructed using objects from algebraic geometry.
25.04.2023	Isabel Stenger (UdS): Cones of divisors on P³ blown up at eight very general points Let X be P³ blown up at eight very general points. Then X is an example of a smooth projective threefold whose anticanonical divisor is nef but not semiample. In this talk we present an explicit description of the cone of nef divisors and the cone of effective divisors on X. Moreover, we show that a certain Weyl group acts with a rational polyhedral fundamental domain on the effective movable cone of X. This is a joint work with Z. Xie.
02.05.2023	Eleni Agathocleous (CISPA): Elliptic Curves of j-invariant zero and the Ideal Class Group We will present the method of descent on elliptic curves of j-invariant zero, admitting a rational 3-isogeny, λ. We will focus on a family E_D of j-invariant zero elliptic curves that we have constructed, and their λ-isogenous curves E_D', where D' = -3D are both fundamental discriminants. We will then present some of our results related to the Selmer and Tate-Shafarevich group of these elliptic curves, and show an explicit relation between these groups, and the 3-part of the ideal class group of the underlying quadratic number fields Q(√D) and Q(√D').
09.05.2023	Zhixin Xie (UdS): Nakayama-Zariski decomposition and the termination of flips In this talk we will discuss the termination of flips conjecture, which is one of the main open problems of the Minimal Model Program. We will focus on the termination of flips for pseudoeffective projective pairs and relate the problem to the behaviour of the Nakayama-Zariski decomposition under the operations of a Minimal Model Program. This is joint work with V. Lazić.
16.05.2023	Martin Bies (Kaiserslautern): F-Theory and Singular Elliptic Fibrations I will present a powerful geometric approach to string theory called F-theory, and its mathematical foundations. The primary objects of interest are singular elliptically fibered Calabi–Yau manifolds and their resolutions. I will briefly introduce and discuss elliptic fibrations, as well as various associated quantities and techniques we are interested in, and will briefly allude to their physical significance. This talk is associated with an ongoing effort with Andrew Turner (UPenn) and Matthias Zach (RPTU Kaiserslautern-Landau) and many other contributers to the OSCAR computer algebra system to create computer algebra tools for doing F-theory computations. If time permits, I hope to discuss cutting edge technology to extract information about the vector-like spectrum in F-theory. The vector-like spectrum encodes among others the famous Higgs boson. Therefore, this information is crucial to tell if an F-theory construction resembles the physics of our everyday experience. On the mathematics side, these efforts take us into the realm of nodal curve, root bundles and their Brill-Noether theory.
23.05.2023	Bachelorvorträge: Laura Stokkermans, Sabrina Hinrichs
06.06.2023	Cécile Gachet (Berlin): The nef cone conjecture for certain Calabi-Yau varieties obtained as fiber products In the late eighties and the early nineties, a sequence of papers by Schoen, Namikawa, and Grassi and Morrison introduced and studied Calabi-Yau threefolds obtained as fiber products of two general rational elliptic surfaces over the projective line. These Calabi-Yau threefolds inherit many properties directly from their two factors, which creates a rich geometry (with infinitely many extremal rays spanning the nef cone), but which is yet accounted for (by the large automorphism group). The (nef) cone conjecture predicts that such a balance is achieved in a much larger degree of generality, by arbitrary Calabi-Yau varieties. In this talk reporting on my joint work with Hsueh-Yung Lin and Long Wang, I will explain how we construct some Calabi-Yau varieties of arbitrary dimension, again as fiber products of two factors over the projective line. Our examples satisfy the nef cone conjecture, and have nef cones spanned by infinitely many extremal rays. Interestingly, they still inherit the properties of their nef cones straight from their two factors, but that often does not apply to their automorphism groups. While fleshing out our construction and sketching the ideas of our proof, I will discuss in what degree of generality nef cones of fiber products can actually be described in terms of the nef cones of their two factors.
20.06.2023	Lie Fu (Strasbourg): Constructing maximal real varieties via moduli spaces The topology of real algebraic varieties is a classical and fascinating subject. One central result is the Smith-Thom inequality bounding the total F₂-Betti numbers of the real locus from above by that of the complex variety. Maximal real varieties are the ones that attain the equality. We explore a new source of construction of maximal real varieties given by taking moduli spaces of objects (vector bundles, sheaves, cycles etc.) on maximal varieties. The talk is based on arXiv:2303.03368.
27.06.2023	David Eisenbud (Berkeley): Finite and infinite free resolutions I will survey some of the history and motivations of the study of free resolutions, from Cayley and Hilbert through Auslander, Buchsbaum and Serre on the finite case and Tate, Gulliksen, Roos, Avramov and others on the infinite case. I'll also describe some phenomena recently discovered in the infinite case by Hai Long Dao, Michael Brown, Prashanth Sridhar and myself.
04.07.2023	Enrica Floris (Poitiers): Normal split subvarieties of a homogeneous manifold Van de Ven proved in 1959 that the subvarieties of the projective space whose normal short exact sequence splits are linear subspaces. In this talk I will explain a generalisation of this result to rational homogeneous varieties: a subvariety of a rational homogeneous variety whose normal exact sequence splits is a rational homogeneous variety. This is a joint work with Andreas Höring.
11.07.2023	Stavros Papadakis (Ioannina): The Hibi-Ohsugi conjecture for IDP Gorenstein Lattice Polytopes A lattice polytope P is a convex polytope whose vertices have integer coordinates. Given a field K there is an associated commutative graded K-algebra K[P]. The polytope is called Gorenstein if K[P] is Gorenstein and IDP if for all positive integers k every point with integer coordinates of the dilation polytope kP is a sum of k points of P with integer coordinates. Counting the number of points with integer coordinates of kP for all positive integers k leads to the notion of the h-vector of P and Ehrhart theory. In 2006 Hibi and Ohsugi conjectured that the h-vector of an IDP Gorenstein lattice polytope is unimodal. The aim of the presentation is to discuss a recent proof of the conjecture in joint work with Karim Adiprasito, Vasiliki Petrotou and Johanna Steinmeyer.
18.07.2023	Nikolaos Tsakanikas (EPFL): Comparison and uniruledness of asymptotic base loci Given a normal projective variety X and a Q-Cartier Q-divisor D on X, one attaches to D various so-called asymptotic base loci, which measure, in some sense, the failure of positivity of D. In this talk I will focus on the canonical class D=K_X+B+M of a log canonical generalized pair (X,B+M). Specifically, I will present various comparison results among the asymptotic base loci associated with D=K_X+B+M and I will also discuss the uniruledness of the components of those loci. If time permits, I will give some examples that demonstrate the optimality of those results as well as some applications of those results, especially to varieties with nef anti-canonical class. This talk is based on joint work in progress with Zhixin Xie.

26.10.2022	Gebhard Martin (Bonn): On the non-degeneracy of Enriques surfaces I will report on joint work with G. Mezzedimi and D. Veniani on the classification of Enriques surfaces S with non-degeneracy nd(S)≤3, where nd(S) is the maximal length of sequences of the form (F_1,...,F_n), where \|2F_i\| is a genus one pencil on S and F_i∙F_j=1-δ_ij. In particular, we prove that nd(S)≥4 for all S if char(k)≠2. As a consequence, we show that, in these characteristics, all Enriques surfaces arise via Enriques' original construction as a minimal resolution of a sextic in P³ which is non-normal along the edges of a tetrahedron and all Enriques surfaces are birational to normal quintics in P³.
02.11.2022 (online)	Xavier Roulleau (Marseille): On order 3 generalized Kummer surfaces and Fourier-Mukai partners Let A be an abelian surface which possesses a symplectic order 3 automorphism s. The minimal resolution of the quotient of A by s is a K3 surface X called a generalised Kummer surface. A natural question (analogous to the one raised by Shioda in the case of the classical Kummer surfaces) is to understand in the case when two abelian surfaces A, B with symplectic automorphisms of order 3, give the same generalised Kummer surface, is it true that A and B isomorphic? Or else, how are they related? A first step is to prove that these surfaces A, B are Fourier-Mukai partners. This will be the topic of the talk; the tool we use are of lattice theoretic nature. Joint work with A. Sarti.
09.11.2022	Anna Bot (Basel): A smooth complex rational affine surface with uncountably many nonisomorphic real forms A real form of a complex algebraic variety X is a real algebraic variety whose complexification is isomorphic to X. Many families of complex varieties have a finite number of nonisomorphic real forms, but up until recently no example with infinitely many had been found. In 2019, Lesieutre constructed a projective variety of dimension six with infinitely many nonisomorphic real forms, and this year, Dinh, Oguiso and Yu described projective rational surfaces with infinitely many as well. In this talk, I’ll present the first example of a rational affine surface having uncountably many nonisomorphic real forms.
23.11.2022	Daniele Agostini (Tübingen): Effective Torelli theorem Torelli's theorem is a foundational result of classical algebraic geometry, asserting that a smooth curve can be recovered from its Jacobian. There are many effective proofs of this result, that can even be implemented on a computer. In this talk, I will present this circle of ideas. In particular, I will focus on a method based on the KP equation in mathematical physics, that I have recently implemented together with Türkü Çelik and Demir Eken.
07.12.2022	at 2pm (in presence): Davide Veniani (Stuttgart): Symplectic rigidity of O'Grady's manifolds Mukai classified all symplectic groups of automorphisms of K3 surfaces as possible subgroups of one of the Mathieu groups. Since then, the proof of Mukai's theorem has been simplified using lattice theoretical techniques, and extended to higher dimensional hyperkähler manifolds. In two joint works with L. Giovenzana (Loughborough), A. Grossi (Chemnitz) and C. Onorati (Roma Tor Vergata), we studied possible cohomological actions of symplectic automorphisms of finite order on the two sporadic deformation types found by O'Grady in dimension 6 and 10. In particular, we showed that, in dimension 10, all symplectic automorphisms are trivial. In my talk, I will explain the connection between our proof and the sphere packing problem. at 4pm (online): Joaquín Moraga (UCLA): Coregularity of Fano varieties In Algebraic Geometry, there are three building blocks of varieties: Canonically polarized, Calabi–Yau, and Fano varieties. In this talk, we will introduce an invariant, the coregularity, that allows us to understand the geometry of Fano varieties. The coregularity is an integer between zero and the dimension of the variety. All toric varieties have coregularity 0. In this talk, we explore how Fano varieties of coregularity 0 resemble toric varieties. This is joint work with Stefano Filipazzi, Mirko Mauri, Fernando Figueroa and Junyao Peng.
11.01.2023	Stevell Muller (UdS): Finding complete intersections in projective space with prescribed symmetry In this talk I present an algorithm used to compute defining ideals of certain (smooth) complete intersections fixed under a linear action of a group on their ambient projective space. In particular, this method allows us to get a parametrization of all candidate ideals for each class of linear actions of the given group on the corresponding projective space. I illustrate the use of this algorithm with the case of polarized K3 surfaces of genus 5. If time permits, I want to list a couple of questions arising after this first work of my PhD.
25.01.2023	Susanna Zimmermann (Orsay): Involutions of the real plane The birational involutions of the complex plane have been classified by Bayle-Beauville in 2000. In particular, they show that if an involution is not linearisable, then its conjugacy class is determined by the fixed curve of the involution. This fails over the real numbers and I'll present a family of non-conjugate involutions all fixing the same curve. I'll also present a rough classification of birational involutions of the real plane and motivate how to attack such a classification.
01.02.2023	Sönke Rollenske (Marburg): Standard stable Horikawa surfaces In 1976 Horikawa classified minimal surfaces of general type with K_X²=2p_g(X)-4, the equality case of Noether's inequality. If K_X² is divisible by 16, then there are two irreducible families and it is a long-standing open problem if these parametrise diffemorphic surfaces. Considering these surfaces inside the moduli space of stable surfaces, I will construct an anticipated intersection of the two components and some unexpected extra components. The talk is based on joint work with Julie Rana.

13.04.2022	Vladimir Lazić (UdS): The Nonvanishing problem for varieties with nef first Chern class Let X be a threefold with mild singularities such that c₁(X) is nef. In this talk I will present a very recent proof that then the numerical class of c₁(X) is effective. This result (joint work with Thomas Peternell, Nikolaos Tsakanikas and Zhixin Xie) is the positive curvature counterpart of the famous result of Miyaoka from the 1980s, which showed that –c₁(X) is effective as soon as it is nef.
27.04.2022	Sokratis Zikas (Basel): Rigid birational involution of projective space and cubic 3-folds The Cremona group Cr_n(k) is the group of birational transformations of the projective n-space over a field k. The study of these groups dates back to the 19th century with some of the central questions still being open. In this talk, using modern techniques based on the Minimal Model Program, I will explain how to construct families of birational involutions on the projective 3-space which do not fit in an elementary relation of Sarkisov links. Using these involutions, we can endow Cr₃(C) with a free product structure. As corollaries, we effectively reprove its non-simplicity as well as answer some question regarding its group of automorphisms. Counterpart results hold for the group of birational transformations of a smooth cubic 3-fold.
04.05.2022	Quentin Posva (EPFL): Sources and springs in positive characteristic, and applications The singular varieties appearing in the KSBA moduli theory of canonically polarised manifolds are called stable varieties: these are the varieties with an ample canonical sheaf, and semi-log canonical singularities. In particular, stable varieties need not be normal. Various technical challenges arise: among them is the failure of the MMP for non-normal varieties. In characteristic zero, a solution to this problem was developed by Kollár, who established a dictionary between stable varieties and their normalisations. This dictionary relies on the theory of sources and springs of log canonical centers. In this talk I will recall this theory, and explain how it can be generalised to positive characteristic in low dimensions. Then I will discuss some applications and open questions.
11.05.2022	Frank-Olaf Schreyer (UdS): Extensions of paracanonical curves of genus 6 Two-regular subschemes of Pⁿ have been studied and classified by Bertini, Eisenbud, Harris et. al. for a long time. The study of 3-regular subschemes whose ideal is generated by cubics was initiated by Sijong Kwak and co-authors. The first open case for their classification are 3-regular varieties of codimension 3 and degree 10. In case of curves these are paracanonical embedded curves. A non-degenerate variety Y⊆Pⁿ⁺¹ of dimension dim X+1, which is not a cone, is an extension of a variety X⊆Pⁿ if there exists an hyperplane H⊆Pⁿ⁺¹ such that X≅Y∩H. In this talk I will report on joint work with Hoang Truong where we classify extensions of a general paracanonical curve of genus 6 and degree 10 in P⁴. Our main result says that there are 26 different families of extensions for a given general paracanonical curve C⊆P⁴.
18.05.2022 at 4pm!	Jingjun Han (Fudan): Birational boundedness of rationally connected log Calabi-Yau pairs with fixed index In this talk, I will introduce M^cKernan-Prokhorov's conjecture on the boundedness of rationally connected Calabi-Yau varieties. This conjecture is a natural generalization of the Borisov– Alexeev–Borisov (BAB) Conjecture which was solved by Birkar. I will show that the set of rationally connected projective varieties X of a fixed dimension such that (X,B) is klt, and -m(K_X+B) is Cartier and nef for some fixed positive integer m, is bounded modulo flops. This is a joint work with Chen Jiang.
08.06.2022 at 2:30pm!	Fulvio Gesmundo (UdS): Lower bounds for algebraic branching programs via intersection theory Algebraic branching programs (ABP) define one of the algebraic versions of the class of problems which can be solved in polynomial time. In this seminar, I will explain how to characterize ABPs via restrictions of a special class of polynomials and how geometric methods allow us to determine lower bounds in this class. In particular, I will focus on recent work with Ghosal, Ikenmeyer and Lysikov, where we use intersection theoretic properties of an associated hypersurface to give lower bounds on the ABP complexity.
29.06.2022	Robin Lahni (UdS, Bachelorseminar): Numerical criteria for ampleness and nefness Lukas Braun (Freiburg): Reductive quotients of klt varieties In this talk, I will explain the proof of the recent result, obtained together with Daniel Greb, Kevin Langlois, and Joaquin Moraga, that reductive quotients of klt type varieties are of klt type. If time permits, I will also discuss several applications of our result, e.g. on quotients of Fano type varieties, good moduli spaces, and collapsing of homogeneous bundles.
06.07.2022 (Thursday!) at 11am!	Shin-ichi Matsumura (Tohoku/Bayreuth): On projective manifolds with pseudo-effective tangent bundle In this talk, I will introduce a structure theorem of a projective manifold with pseudo-effective tangent bundle, comparing to the structure theorem for nef cotangent bundle by Demailly-Peternell-Schneider. Specifically, I show that such a manifold admits a smooth fibration onto a flat projective manifold whose general fiber is rationally connected. The proof is based on the theory of foliations, but an analog of the structure theorem is proved from the viewpoint of MMP. I would like to explain a comparison of the viewpoints of foliations and MMP. This talk is partially based on joint work with Genki Hosono and Masataka Iwai.
08.07.2022 (Friday!) at 10am!	Jie Liu (Beijing): Bounding algebraic ranks of Fano foliations One of the central problems in the theory of holomorphic foliations on projective varieties is to find conditions that guarantee the existence of algebraic leaves and the notion of algebraic rank was introduced to measure the algebraicity of leaves. In this talk, I will talk about how we can get a lower bound for algebraic ranks of Fano foliations using global and local invariants related to the positivity of anti-canonical divisors.
13.07.2022	Joachim Jelisiejew (Warsaw): Geometry of minimal border rank tensors Concice tensors in C^m⊗C^m⊗C^m which have border rank m are classically important in complexity theory. In the talk I will present a new approach to these tensors, centered around the notion of 111-algebra. It yields both new results, like set-theoretic equations for m=5, as well as a new perspective and abundance of open questions, ranging from computational, to birational, to deformation theoretic. This is a joint work with J.M. Landsberg and Arpan Pal, building on the work of Buczyńska-Buczyński.
02.09.2022 (Friday!) at 2pm!	Robin Lahni (UdS, Bachelorseminar): Numerical criteria for ampleness and nefness, II

20.10.2021	Nikolaos Tsakanikas (UdS): Termination of flips for pseudo-effective 4-folds The Minimal Model Program (MMP) plays a fundamental role in the classification theory of projective varieties. One of the main open problems of the MMP in higher dimensions is the termination of flips conjecture, which predicts that any sequence of flips terminates. In this talk I will sketch the proof of the termination of flips conjecture for pseudo-effective log canonical generalized pairs of dimension 4. This is joint work with Guodu Chen.
27.10.2021	Zhixin Xie (UdS): Rationally connected threefolds with nef anticanonical divisor Complex projective varieties with nef anticanonical divisor appear as natural generalisations of Fano varieties. Fano manifolds are classified up to dimension three and many of their properties are well studied. However, the classification of manifolds with nef anticanonical divisor is more complicated as new phenomena arise and many results for Fano manifolds no longer hold for this class of varieties. In this talk, we will first look at some examples in dimension two. Then we will discuss some properties of rationally connected threefolds with nef anticanonical divisor. We will consider the case where the anticanonical divisor is not semi-ample, and will give a classification result in this case.
10.11.2021	Giacomo Mezzedimi (Hannover): The Potential Hilbert Property on Enriques and K3 surfaces The problem of determining "how many" rational points an algebraic variety has lies at the heart of Number Theory and Arithmetic Geometry. One way to formalize this idea is provided by the (weak) Hilbert Property, a modern generalization of Hilbert's classical irreducibility theorem, concerning the distribution of the rational points on the variety and on its finite covers. An important conjecture by Campana and Corvaja-Zannier predicts a strong interplay between the Hilbert Property and the density of rational points on the variety. In this talk I will report on a joint work with D. Gvirtz-Chen in which we solve this conjecture for Enriques surfaces and for a wide class of K3 surfaces.
17.11.2021	Stefano Filipazzi (EPFL): On the boundedness of elliptic Calabi-Yau threefolds In this talk, we will discuss the boundedness of Calabi-Yau threefold admitting an elliptic fibration. First, we will survey some ideas to address weak forms of boundedness of varieties admitting an elliptic fibration. Then, we will switch focus to Calabi-Yau varieties and discuss how the Kawamata-Morrison cone conjecture comes in the picture when studying boundedness properties for this class of varieties. To conclude, we will see how this circle of ideas applies to the case of elliptic Calabi-Yau threefolds. This talk is based on work joint with C.D. Hacon and R. Svaldi.
24.11.2021	Giovanni Mongardi (Bologna): Irrational Gushel Mukai threefolds We construct an explicit complex smooth Fano threefold with Picard number 1, index 1 and degree 10 (a Gushel-Mukai threefold) and prove that it is not rational by showing that its intermediate jacobian has a faithful PSL(2,F₁₁) action. The construction is based on a very special double EPW sextic. This is joint work with O. Debarre.
15.12.2021	Christian Lehn (TU Chemnitz): Singular varieties with trivial canonical class We will present recent advances in the field of singular varieties with trivial canonical class obtained in joint work with Bakker and Guenancia building on work by many others. This includes the decomposition theorem which says that such a variety is up to a finite cover isomorphic to a product of a torus, irreducible Calabi-Yau (ICY) and irreducible symplectic varieties (ISV). The proof uses a reduction argument to the projective case which in turn is possible due to advances in deformation theory and a certain result about limits of Kähler Einstein metrics in locally trivial families.
14.01.2022 (Friday!) at 9am!	Shigeyuki Kondo (Nagoya): Coble surfaces with finite automorphism group A Coble surface is a rational surface which appears as a degeneration of Enriques surfaces. Thus Coble surfaces inherit many properties of Enriques surfaces and complement the theory of Enriques surfaces. A general Coble surface has an infinite group of automorphisms and hence it is natural to classify Coble surfaces with finite automorphism group as in the case of Enriques surfaces. We will give the classification of such surfaces: there are exactly 9 isomorphism classes of such surfaces in characteristic p≠2 and 9 families in p=2. In case of p=2, this is a joint work with Toshiyuki Katsura.
02.02.2022	Isabel Stenger (UdS): On the numerical dimension of Calabi-Yau threefolds of Picard number 2 Lesieutre showed that various definitions of the numerical dimension do not coincide for a pseudoeffective R-divisor on a specific complete intersection Calabi-Yau threefold. We show that Lesieutre's example is not a sporadic one and generalize his method of proof. This is joint work with Michael Hoff.

19.05.2021	Yvonne Neuy (UdS, Bachelorseminar): Factorization of polynomials Sven Manthe (UdS, Bachelorseminar): Generation of local integral unitary groups
07.07.2021	Hans-Christian Graf von Bothmer (Hamburg): Rigid, not infinitesimally rigid surfaces with ample canonical bundle It was a long standing problem of Morrow and Kodaira whether there are compact complex manifolds X with Def(X) a non reduced point. The first examples answering this question in the affirmative were given by Bauer and Pignatelli in 2018. These are certain surfaces of general type that have nodal canonical models. These canonical models are rigid and infinitesimally rigid, while their desingularizations are still rigid, but not infinitesimally rigid anymore. One can therefore ask, if this situation is typical for rigid, not infinitesimally rigid surfaces of general type, or if it is possible to have examples with smooth canonical models. We answer this question also in the affirmative by constructing such a surface X via line arrangements and abelian covers.
14.07.2021	Firoozeh Aga (UdS/Fraunhofer ITWM): Algorithmic generation of automorphism groups of K3-surfaces A complex K3 surface S is a compact connected Kähler complex surface, with trivial canonical bundle and H¹(S,O_S)=0. The Fermat quartic, given by the equation V(x₀⁴+ x₁⁴+x₂⁴+x₃⁴)∊P³, is an example of such a surface. In this talk, we begin by introducing some important properties of K3 surfaces. Next, we will look at the notions from Lattice theory, and other invariants required to compute the automorphism groups of these surfaces. We will briefly discuss the algorithm by R. E. Borcherds to compute these automorphism groups. Following which we will look at a generalization of his algorithm by Ichiro Shimada. I will also give a list of examples for which the automorphism groups have already been computed using these methods. Next, I will present the computational tools (GPI-Space, Julia, and their embedding) required to parallelize algorithms and explore different components of GPI-Space. Using a small example, we will design a Petri net and see how we can benefit (in terms of reduced computation time) from a ’well designed’ Petri net. Finally, I will present a list of tasks that I am currently working on. These could be potentially used to improve the algorithms mentioned above by using different parallel programming models.
23.07.2021 (Friday!) at 10am!	Keiji Oguiso (Tokyo): Real forms of a smooth complex projective variety This talk is based on a series of joint works with Professors Tien-Cuong Dinh and Xun Yu. After Lesieutre, there were several works related to a long standing open question since Kharlamov: Is there a smooth complex projective rational surface with infinitely many real forms up to isomorphisms, i.e., with infinitely many ways to describe it by a system of equations with real coefficients up to isomorphism over the real number field? After a brief introduction to the real form problem, and in particular, a way to reduce the problem to a problem on the conjugacy classes of involutions in the automorphism group, I would like explain our affirmative answer to the question, with a proof using a special Kummer surface of product type and its rich geometry. We then explain the results in higher dimensions and/or non-negative Kodaira dimensions, together with related open problems.
28.07.2021	Yvonne Neuy (UdS, Bachelorseminar): Factorization of polynomials, II

04.11.2020	Yeongrak Kim (UdS): Ulrich bundles on cubic fourfolds
11.11.2020	Andreas Braun (Durham): String Compactifications and Arithmetic String theory gives a strong motivation for studying elliptic fibrations and Hodge cycles on K3 surfaces, as well as higher-dimensional Calabi-Yau varieties. After explaining the relevant data of the setup and giving a rudimentary physics dictionary, I will review some recent results and highlight open problems.
02.12.2020	Ann Sophie Cenkel (UdS, Bachelorseminar): Borsuk-Ulam Theorem and its applications
09.12.2020	Edgar Costa (MIT): Effective obstruction to lifting Tate classes from positive characteristic We present an algorithm that takes a smooth hypersurface over a number field and computes a p-adic approximation of the obstruction map on the Tate classes of a finite reduction. This gives an upper bound on the "middle Picard number" of the hypersurface. The improvement over existing methods is that our method relies only on a single prime reduction and gives the possibility of cutting down on the dimension of Tate classes by two or more. The obstruction map comes from p-adic variational Hodge conjecture and we rely on the recent advancement by Bloch-Esnault-Kerz to interpret our bounds. This is joing work with Emre Sertoz.
20.01.2021 at 4pm!	Klaus Altmann (FU Berlin): Displaying the universal extension of toric line bundles Line bundles L on projective toric varieties can be understood as formal differences (P−Q) of convex polyhedra in the character lattice. We show how it is possible to use this language for understanding the cohomology of L by studying the set-theoretic difference (Q\P). Moreover, when interpreting these cohomology groups as certain Ext-groups, we demonstrate how the approach via (Q\P) leads to a direct description of the associated extensions. The first part is joint work with Jarek Buczinski, Lars Kastner, David Ploog, and Anna-Lena Winz; the second is joint work with Amelie Flatt and Lutz Hille.
27.01.2021	Sönke Rollenske (Marburg): Stratifications in the moduli space of stable surfaces The Gieseker moduli space of surfaces of general type admits a modular compactification, the moduli space of stable surfaces. Our knowledge about the "new" surfaces in the boundary is still limited and I will discuss different possibilities to organise them, in particular a Hodge-theoretic approach proposed by Green, Griffiths, Laza, and Robles. Everything will be illustrated with examples and many pictures. This is based on joint work with B. Anthes, M. Franciosi, R. Pardini.
03.02.2021	Stefan Kebekus (Freiburg): Brauer-Manin obstruction on a simply connected fourfold and a Mordell theorem in the orbifold setting Almost one decade ago, Poonen constructed the first examples of algebraic varieties over global fields for which Skorobogatov's étale Brauer-Manin obstruction does not explain the failure of the Hasse principle. By now, several constructions are known, but they all share common geometric features such as large fundamental groups. In joint work with Jorge Pereira (IMPA) and Arne Smeets (Nijmegen), we construct simply connected fourfolds over global fields of positive characteristic for which the Brauer-Manin machinery fails. Contrary to earlier work in this direction, our construction does not rely on major conjectures. Instead, we establish a new diophantine result of independent interest: a Mordell-type theorem for Campana's "geometric orbifolds" over function fields of positive characteristic. Along the way, we also construct the first example of simply connected surface of general type over a global field with a non-empty, but non-Zariski dense set of rational points.
17.02.2021	Ann Sophie Cenkel (UdS, Bachelorseminar): Borsuk-Ulam Theorem and its applications, II

29.04.2020	Frank-Olaf Schreyer (UdS): Hyperelliptic curves, complete intersection of two quadrics and their Ulrich complexity
06.05.2020	Cédric Bonnafé (Montpellier): Singular K3 surfaces and complex reflection groups Joint work with A. Sarti. Singular K3 surfaces are the K3 surfaces with maximal Picard number, namely 20. I will explain how to construct families of K3 surfaces with big Picard number using invariants of finite complex reflection groups of rank 4, each family containing some singular ones. This extends earlier work of Barth-Sarti for two reasons: firstly, we obtain many more examples by considering all reflection groups of rank 4 and secondly, our proofs involve more theory of complex reflection groups and avoid as much as possible (but not completely) a case-by-case analysis.
13.05.2020	Calum Spicer (King's College London): Mori theory and foliations We will explain some of the ideas behind the study of the birational geometry of foliations, as well as indicating some recent progress in the case of codimension one foliations on threefolds. We will suggest some relations between the study of the birational geometry of foliations and the algebraicity of leaves of foliations. Features joint work with P. Cascini and R. Svaldi.
27.05.2020	Niklas Müller (UdS, Bachelorseminar): Vanishing theorems in algebraic geometry Sascha Blug (UdS, Masterseminar): Gonality and relative canonical syzygies of curves on toric surfaces
10.06.2020	Fanjun Meng (Northwestern): Pushforwards of klt pairs under morphisms to abelian varieties In this talk, I will discuss some positivity properties of pushforwards of klt pairs under morphisms to abelian varieties, which include global generation (after pullback by an isogeny), Chen-Jiang decomposition and some other related ones. These extend previous results about pushforwards of pluricanonical bundles of smooth projective varieties to the singular setting. I will also apply them to some effective results.
01.07.2020	Andreas Höring (Nice): A nonvanishing conjecture for cotangent bundle Let X be a smooth complex projective manifold, and let K_X be its canonical bundle. If X is not covered by rational curves (or equivalently K_X is pseudoeffective), the nonvanishing conjecture claims that some positive multiple of the canonical bundle has a non-zero global section. In this talk I will discuss an analogous conjecture for the cotangent bundle Ω_X: assume that Ω_X pseudoeffective (I will give the definition in the talk). Then some symmetric power S^mΩ_X has a non-zero global section. We will see that this conjecture is quite non-trivial, even for surfaces. This is joint work with Thomas Peternell.
26.08.2020	Niklas Müller (UdS, Bachelorseminar): Vanishing theorems in algebraic geometry, II Sascha Blug (UdS, Masterseminar): Gonality and relative canonical syzygies of curves on toric surfaces, II

23.10.2019	Vladimir Lazić (UdS): Abundance for uniruled pairs which are not rationally connected
06.11.2019	Patrick Graf (Bayreuth): Reflexive differential forms in positive characteristic Given a differential form on the smooth locus of a normal variety defined over a field of positive characteristic, we discuss under what conditions it extends to a resolution of singularities (possibly with logarithmic poles). Our main result works for log canonical surface pairs over a perfect field of characteristic at least seven. We also give a number of examples showing that our results are sharp in the surface case, and that they fail in higher dimensions. Giovanni Staglianò (Catania): Kuznetsov's Conjecture and rationality of cubic fourfolds We recall the main conjectures about the important classical problem (still unsolved) on the rationality of smooth cubic hypersurfaces in P⁵, cubic fourfolds for short, and we present recent contributions in favour of these conjectures. We will also briefly illustrate similar conjectures and results for Gushel-Mukai fourfolds, that is for smooth quadric hypersurfaces in del Pezzo fivefolds. The talk is based on some joint works with Francesco Russo and the recent collaboration with Michael Hoff.
13.11.2019	Eileen Oberringer (UdS, Bachelorseminar): Riemann-Roch theorem and applications Lisa Karst (UdS, Bachelorseminar): Bézout's theorem and applications Igor Schlegel (UdS, Bachelorseminar): Dimension theory and applications
08.01.2020	Andreas Knutsen (Bergen): Moduli of polarized Enriques surfaces Moduli spaces of polarized Enriques surfaces have several components, even if one fixes the degree of the polarization. I will present some results concerning how to determine the various irreducible components and in some cases their unirationality and uniruledness.
15.01.2020	Frank Gounelas (TU München): Curves on K3 surfaces Bogomolov and Mumford proved that every projective K3 surface contains a rational curve. Since then, a lot of progress has been made by Bogomolov, Chen, Hassett, Li, Liedtke, Tschinkel and others, towards the stronger statement that any such surface in fact contains infinitely many rational curves. In this talk I will present joint work with Xi Chen and Christian Liedtke completing the remaining cases of this conjecture in characteristic zero, reproving some of the main previously known cases more conceptually and extending the result to arbitrary genus.
29.01.2020	Enrica Floris (Poitiers): On Mori fibre spaces of dimension 4 and their automorphism group In this talk we will explain how the study of Mori fibre spaces with an action of an algebraic connected group is related to the study of maximal connected subgroups of the Cremona group. We will present some examples of Mori fibre spaces of dimension 4 fibred onto the projective line. This is a work in progress joint with Jérémy Blanc.
05.02.2020	Yongqiang Zhao (Westlake): Scrollar syzygies and Galois representations The theory of scrollar syzygy resolutions was introduced by Schreyer in his work on Green's conjecture for canonical curves. Apart from its applications to syzygy theory, it is also closely related to the parametrization theory of curves having small gonality, which has important applications to the theory of Hurwitz spaces. A priori, it has nothing to do with representation theory. In this talk, however, we will discuss some recent observations on its connections with Galois representation theory. We will give a (conjectural) complete description of all syzygy bundles through representation theory. Wouter Castryck (Leuven): Recovering a secret ideal class modulo squares The "discrete logarithm problem for class group actions" is about finding a secret ideal class [J] in the class group of an imaginary quadratic order O, upon input of two elliptic curves E, E' (over a finite field), both having endomorphism ring O, that are obtained from one another through the isogeny-wise action of [J]. This problem was proposed by Couveignes as a substitute for the classical discrete logarithm problem in elliptic curve cryptography, and has recently attracted a lot of attention, thanks to the quantum computing threat. In this talk we will introduce this problem in more detail, and we will explain how to efficiently recover [J] modulo the subgroup of squares inside the class group of O. In many cases, this breaks the decisional version of the discrete logarithm problem for class group actions. This is joint work with Jana Sotáková and Frederik Vercauteren.
19.02.2020	Eileen Oberringer (UdS, Bachelorseminar): Riemann-Roch theorem and applications, II Lisa Karst (UdS, Bachelorseminar): Bézout's theorem and applications, II Igor Schlegel (UdS, Bachelorseminar): Dimension theory and applications, II

10.04.2019	Alberto Cattaneo (MPI Bonn): Positive divisors and automorphisms of hyperkähler manifolds The movable cone of a projective hyperkähler manifold admits a locally polyhedral wall-and-chamber decomposition which encodes information on the birational models of the manifold. In the case of moduli spaces of Bridgeland-stable objects on K3 surfaces, Bayer and Macrì provided a lattice-theoretical description of the walls in this decomposition, which allows for explicit computations. We will show how to apply these results to obtain a purely arithmetic classification of the automorphism group of Hilbert schemes of points on a generic projective K3 surface.
17.04.2019	Davide Veniani (Mainz): The number of Enriques surfaces covered by a K3 surface Given the transcendental lattice of a complex K3 surface X, I will give a formula for the number of non-isomorphic Enriques surfaces covered by X. In particular, I will describe the two 'most algebraic' Enriques surfaces covered by the singular K3 surface of discriminant 7 (j.w. Ichiro Shimada). Finally, I will explain how the search for K3 surfaces with no Enriques quotients turns into a question about integral quadratic forms representing 1, and how to answer it (j.w. Simon Brandhorst).
08.05.2019	Sławomir Rams (Cracow): On lines on surfaces of general type I will present bounds on the number of lines on a smooth degree d surface in three-dimensional projective space for d=5 (joint work with M. Schuett) and d>5 (joint with T. Bauer).
15.05.2019	Ichiro Shimada (Hiroshima): The automorphism groups of Enriques surfaces covered by a Jacobian Kummer surface
22.05.2019	Nikolaos Tsakanikas (UdS): On minimal models
28.05.2019	Francesco Galuppi (MPI Leipzig): The rough Veronese variety We study signature tensors of paths from an algebraic geometric viewpoint. The signatures of a given class of paths parametrize a variety inside the space of tensors. These "signature varieties" provide both new tools to investigate paths and new challenging questions about their behavior. In this talk I will focus on the class of rough paths. They play a central role in stochastic analysis, and their signature variety has peculiar geometric properties, showing surprising analogies with the classical Veronese variety. We show that this so-called Rough Veronese is toric. This makes it much easier to study it and to perform explicit computations.
05.06.2019	Yeongrak Kim (UdS): On the Borisov-Nuer conjecture
12.06.2019	Rosemary Taylor (Warwick): On explicit constructions of codimension 4 Fano 3-folds Fano 3-folds with terminal singularities are a key family produced by the minimal model program, but their classification remains an open problem. Often research into these varieties takes place on their Gorenstein anticanonical rings. A famous classification of Gorenstein rings in codimensions 1, 2 and 3 allows us to classify Fano 3-folds in these codimensions; however, codimension 4 Fano 3-folds remain mysterious. In this talk I will introduce "unprojections". Unprojections are a graded ring method which construct "large" Gorenstein rings from "smaller" and in practice act as a good substitute for classification and structure theories. In particular, I will show how to apply this method to the case of Fano 3-folds and prove the existence of 16 new codimension 4 Fano 3-folds.
26.06.2019	Isabel Stenger (UdS): Constructing numerical Godeaux surfaces
17.07.2019	Sijong Kwak (Daejeon): The Betti tables of higher secant varieties In this talk, I'd like to consider the defining equations of higher secant varieties and the structure of the Betti tables. There are very similar structures as in the quadratic world. I'd like to call this phenomenon "Matryoshka structure" which I discussed with Fyodor Zak long time ago.

17.10.2018	Vladimir Lazić (UdS): On Generalised Abundance
24.10.2018	Vladimir Lazić (UdS): On Generalised Abundance, II
07.11.2018	Sascha Blug (UdS, Bachelorseminar): Divisors and line bundles on hyperelliptic curves
14.10.2018	Vladimir Lazić (UdS): On the B-Semiampleness Conjecture
28.11.2018	Roberto Svaldi (Cambridge): On the boundedness of Calabi-Yau varieties in low dimension I will discuss new results towards the birational boundedness of low-dimensional elliptic Calabi-Yau varieties, joint work with Gabriele Di Cerbo and work in progress with Caucher Birkar and Di Cerbo. Recent work in the minimal model program suggests that pairs with trivial log canonical class should satisfy some boundedness properties. I will show that 4-dimensional Calabi-Yau pairs which are not birational to a product are indeed log birationally bounded. This implies birational boundedness of elliptically fibered Calabi-Yau manifolds with a section, in dimension up to 5. If time allows, I will also try to discuss a first approach towards boundedness of rationally connected CY varieties in low dimension (joint with G. Di Cerbo, W. Chen, J. Han and C. Jiang).
19.12.2018	Simon Brandhorst (UdS): On the dynamical spectrum of complex surfaces
09.01.2019	Chiara Camere (Milano/MPI Bonn): Twisted sheaves on K3 surfaces, Verra fourfolds and non-symplectic involutions The object of this talk is the construction of a family of hyperkähler fourfolds of K3^[2]-type producing an example of the last missing case in the classification of non symplectic involutions. We will see that these varieties can be described geometrically in two different ways, as moduli spaces of twisted sheaves on K3 surfaces and as double covers of EPW quartics associated to Verra fourfolds. This is joint work with G. Kapustka, M. Kapustka and G. Mongardi.
16.01.2019	Fabio Bernasconi (Imperial College London): On log del Pezzo surfaces over imperfect fields and Mori fibre spaces Fibrations play a key role in the classification problems of algebraic varieties. While in characteristic zero, the general fibre of a morphism between smooth varieties is still smooth, this is no longer true in general over fields of positive characteristic (where the classical examples are quasi-elliptic fibrations). However, one can hope to bound such a bad behaviour to small primes if the generic fibre has special global properties. In this talk, I will discuss the case log del Pezzo surfaces over imperfect fields, with a particular emphasis on their cohomologies and Picard groups. This has interesting consequences to the study of threefold Mori fibre spaces onto a curve. This is a joint work with H. Tanaka.
23.01.2019	Susanna Zimmermann (Angers): Relations between Sarkisov links A result by Hacon and McKernan states that the groupoid of birational maps between Mori fibre spaces is generated by Sarkisov links. We revisit and refine a result of Kaloghiros, which presents a set of generating relations between Sarkisov links, and give a closer look at those which include Sarkisov links between conic bundles. This is joint work with Jérémy Blanc and Stéphane Lamy.

26.04.2018	Zsolt Patakfalvi (EPF Lausanne): Positivity of the Chow-Mumford line bundle for families of K-stable Q-Fano varieties The Chow-Mumford (CM) line bundle is a functorial line bundle on the base of any family of polarized varieties, in particular on the base of families of Q-Fano varieties (that is, Fano varieties with klt singularities). It is conjectured that it yields a polarization on the conjectured moduli space of K-semi-stable Q-Fano varieties. This boils down to showing semi-positivity/positivity statements about the CM-line bundle for families with K-semi-stable/K-polystable Q-Fano fibers. I present a joint work with Giulio Codogni where we prove the necessary semi-positivity statements in the K-semi-stable situation, and the necessary positivity statements in the uniform K-stable situation, including in both cases variants assuming K-stability only for very general fibers. Our statements work in the most general singular situation (klt singularities), and the proofs are algebraic, except the computation of the limit of a sequence of real numbers via the central limit theorem of probability theory. I also present a birational geometry application to the classification of Fano varieties.
03.05.2018	Christian Lehn (TU Chemnitz): Coisotropic subvarieties in holomorphic symplectic manifolds The existence of coisotropic subvarieties in general projective irreducible symplectic varieties was conjectured by C. Voisin and is related to the structure of the Chow ring of such a variety and in particular to zero cycles. We present some methods to prove general existence results and apply them on special irreducible symplectic varieties. This is joint work with Gianluca Pacienza.
17.05.2018	Stefan Schreieder (LMU München): Stably irrational hypersurfaces of small slopes We show that a very general complex projective hypersurface of dimension N and degree at least log₂N+2 is not stably rational. The same statement holds over any uncountable field of characteristic p>>N.
24.05.2018	Christian Liedtke (TU München): A Néron-Ogg-Shafarevich criterion for K3 Surfaces Given a family of smooth and projective manifolds over a pointed disk, one may ask whether this family has good reduction, that is, whether this family can be extended to a smooth family over the whole disk. A necessary condition for this is that the monodromy actions on cohomology, that is, the actions of the fundamental group of the pointed disk on the singular cohomology groups of a general fiber, is trivial. But does the converse hold? First, we generalise the setup as follows: let R be a complete local ring with field of fractions K and residue field k (Spec K generalises the pointed disk and Spec R generalises the whole disk), and let X be a smooth and projective variety over K. In this setup, good reduction translates into finding a smooth and proper scheme over Spec R with generic fiber X. Moreover, trivial monodromy translates into the action of the absolute Galois group of K on all l-adic cohomology groups H^*(X,Q_l) being unramified (or crystalline if l=char(k)). For Abelian varieties, it is classical known by a theorem of Serre and Tate (and already established by Néron, Ogg, and Shafaravich for elliptic curves) that good reduction is equivalent to having unramified/crystalline Galois-actions on H¹. In my talk, I will first introduce the above notions and then, I will talk on joint work with Matsumoto, Chiarellotto, and Lazda, where we study K3 surfaces over K and ask whether good reduction is equivalent to having unramfied/crystalline Galois-actions on H². It turns out that this is almost true, but that the right analog of a Néron-Ogg-Shafarevich criterion for K3 surfaces is rather subtle.
21.06.2018	Hanieh Keneshlou (UdS): The unirational component of moduli stacks of 6 gonal genus 11 curves
28.06.2018	Cinzia Casagrande (Torino): Fano 4-folds with rational fibrations Smooth, complex Fano 4-folds are not classified, and we still lack a good understanding of their general properties. In the talk we will focus on Fano 4-folds with large second Betti number b₂, studied via birational geometry and the detailed study of their contractions and rational contractions. We recall that a contraction is a morphism with connected fibers onto a normal projective variety, and a rational contraction is given by a sequence of flips followed by a (regular) contraction. The main result that we want to present is the following: let X be a Fano 4-fold having a rational contraction X→Y of fiber type (with dim Y > 0). Then either X is a product of surfaces, or b₂(X) is at most 17, or Y is P¹ or P².
05.07.2018	Alessandra Sarti (Poitiers): Involutions of the Hilbert scheme of two points on a K3 surface I will show how to use K3 surfaces with special geometry to construct involutions on their Hilbert scheme of two points. Indeed by using Torelli theorem one can show the existence of automorphisms of the Hilbert scheme, but it is a difficult problem to give an explicit realization. This is a joint work with S. Boissière, A. Cattaneo and D. Markushevich.
12.07.2018	Michael Hoff (UdS): K3 surfaces of small genus with many elliptic pencils

19.10.2017	Pagona Koulakidou (UdS): Divisors as fibres of morphisms Nikolaos Tsakanikas (UdS): On varieties birational to abelian varieties
07.12.2017	George H. Hitching (OsloMet): Tangent cones to generalised theta divisors and generic injectivity of the theta map Let C be a Petri general curve of genus g. The tangent cones of the Riemann theta divisor on Pic^g-1(C) have been used in various ways by Kempf and Schreyer and by Ciliberto and Sernesi to give new proofs of Torelli's theorem. We use a related approach to study the generalised theta divisor D(V) of a semistable bundle V over C of rank r and integral slope. For large enough g, we show how a sufficiently general such V can be reconstructed from the tangent cone of D(V) at a suitable singular point. We use this to give a constructive proof and a sharpening of Brivio and Verra's theorem that the theta map from the moduli space of semistable bundles of rank r and trivial determinant to the projective space \|rΘ\| is generically injective for large values of g. (Joint work with Michael Hoff)
18.01.2018	Gianluca Pacienza (Nancy): Density of Noether-Lefschetz loci of irreducible holomorphic symplectic varieties and applications We will try to illustrate how useful such density results can be by presenting several (old and new) applications to: the existence of rational curves on projective IHS varieties, the study of relevant cones of divisors, the study of lagrangian fibrations and a refinement of Hassett's result on cubic fourfolds whose Fano variety of lines is isomorphic to to an Hilbert scheme of 2 points on a K3 surface. We also discuss Voisin's conjecture on the existence of coisotropic subvarieties on IHS varieties and relate it to a stronger statement on Noether-Lefschetz loci in their moduli spaces.
25.01.2018	Andrea Fanelli (Düsseldorf): Del Pezzo fibrations in positive characteristic In this talk, I will discuss some pathologies for the generic fibre of del Pezzo fibrations in characteristic p>0, motivated by the recent developments of the MMP in positive characteristic. The main application of the joint work with Stefan Schröer concerns 3-dimensional Mori fibre spaces.
01.02.2018	Luca Tasin (Bonn): A non-vanishing result for weighted complete intersections Let X be a smooth (or mildly singular) projective variety and let H be an ample line bundle on X. Kawamata conjectured that if H-K_X is ample, then the linear system \|H\| is not empty. I will explain that the conjecture holds true for weighted complete intersections which are Fano or Calabi-Yau, relating it with the Frobenius coin problem. This is based on a joint work with M. Pizzato and T. Sano.

18.05.2017	Christian Bopp (UdS): Moduli of lattice polarized K3 surfaces via relative canonical resolutions
01.06.2017	Sascha Blug (UdS, Bachelorseminar): Die Picard Gruppe von hyperelliptischen Kurven Jonas Baltes (UdS, Bachelorseminar): Der Riemannsche Abbildungssatz und Metriken konstanter Krümmung auf Riemannschen Flächen
08.06.2017	Janik Schug (UdS, Bachelorseminar): Puiseux-Reihen Christian Ikenmeyer (UdS): Formula size, iterated matrix multiplication, and algebraic geometry
29.06.2017	Andreas L. Knutsen (Bergen): Brill-Noether theory of curves on abelian surfaces The Brill-Noether theory of curves on K3 surfaces is well understood. Until recently, quite little has been known for curves on abelian surfaces. In the talk I will present some recent results obtained with M. Lelli-Chiesa and G. Mongardi. In particular, we show that the general curve in the linear system \|L\| on a general primitively polarized abelian surface (S,L) is Brill-Noether general, as in the K3 case. However, contrary to the K3 case, there are smooth curves in \|L\|possessing "unexpected" linear series, that is, with negative Brill-Noether number. As an application, we obtain the existence of components of special Brill-Noether loci of the expected dimension in the moduli space of curves. Thomas Peternell (Bayreuth): Descent of numerically flat vector bundles and singular ball quotients In my talk I will explain recent results with Greb, Kebekus and Taji concerning the uniformization of klt spaces whose (orbifold) Chern classes are extremal in the sense that they satisfy the Miyaoka-Yau equality.
06.07.2017	Hanieh Keneshlou (UdS): On Accola's genus bound for algebraic curves Frank-Olaf Schreyer (UdS): Horrocks splitting on products of projective spaces
03.08.2017	Michael Kemeny (Stanford): The Prym-Green conjecture for curves of odd genus We will present a proof of the Prym-Green conjecture on the resolution of a paracanonical curve of odd genus and arbitrary torsion level. The proof proceeds by using curves on ruled surfaces over an elliptic curve. These surfaces naturally arise as desingularizations of limiting K3 surfaces with elliptic singularities, and come up in Arbarello-Bruno-Sernesi's study of the Wahl map and deformations of the cone. They have the downside of being irregular, which makes the study of syzygies more complicated than for K3s, but on the upside they allow for inductive arguments on the genus of the curve, which is not possible for a K3. Joint with Gabi Farkas.
09.08.2017	Yeongrak Kim (UdS): Ulrich bundles on the intersection of two 4-dimensional quadrics A coherent sheaf F on a projective variety X is Ulrich if its pushforward by a finite degree map is trivial. Since they naturally appear in several different theories, the study of Ulrich bundles becomes important. In this talk, I will discuss two different approaches to construct Ulrich bundles on the intersection of two 4-dimensional quadrics: via Serre correspondence and via derived categories. I will also briefly explain a connection between generalized theta series. This is a joint work with Y. Cho and K.-S.Lee.
24.08.2017	Jonas Baltes (UdS, Bachelorseminar): Der Riemannsche Abbildungssatz und Metriken konstanter Krümmung auf Riemannschen Flächen (Teil II)
05.10.2017	Janik Schug (UdS, Bachelorseminar): Puiseuxreihen und ihre Anwendungen

Address

Fachrichtung Mathematik
Campus, Gebäude E2 4
Universität des Saarlandes
66123 Saarbrücken
Germany

Fachrichtung Mathematik

Fakultät für Mathematik und Informatik