Stefano Marseglia

AG Seminar at Utrecht University

I have been organizer of the AGSeminar between February and November 2023 when Woonam Lim has taken over.

The webpage has moved HERE !!!

Past Talks (until November 07, 2023)


  • November 07 (11:15-12:15, BBG 017) Gerard van der Geer (UvA-Beijing-Luxembourg) The cycle class of the supersingular locus
    Abstract: Deuring gave a formula for the number of supersingular elliptic curves in characteristic p. We generalize this to a formula for the cycle class of the supersingular locus in the moduli space of principally polarized abelian varieties of given dimension g. The formula determines the class up to a multiple and shows that it lies in the tautological ring. We also give the multiple for g up to 4. This is joint work with S. Harashita.
  • October 31 (11:15-12:15, BBG 161) Caleb Springer (University College London) Doubly isogenous genus-2 curves over finite fields
    Our main question is the following: Can you tell the difference between two curves defined over a finite field if you only know their zeta functions and the zeta functions of certain covers? This distinguishing problem is partially motivated by a possible strategy for developing a deterministic polynomial-time algorithm for factoring polynomials over finite fields due to Kayal and Poonen. Ultimately, this talk is concerned with cases where the distinguishing problem is not solvable due to the existence of "very similar" curves. Specifically, we study a family of genus-2 curves with a dihedral action and show that so-called doubly isogenous pairs of curves are surprisingly common in this family. We also provide an explanation of this phenomena which corrects the naive heuristics. This is joint work with Arul, Booher, Groen, Howe, Li, Matei and Pries.
  • October 24 (11:15-12:15, HFG 409) Denis Nesterov (Vienna) Unramified Gromov-Witten and Gopakumar-Vafa invariants
    Abstract: Kim, Kresch and Oh defined moduli spaces of unramified stable maps, which are natural generalisations of (compactified) Hurwitz spaces for a target of an arbitrary dimension. Just like Hurwitz spaces, which are smooth irreducible varieties after normalisation, moduli spaces of unramified stable maps are 'better' compactifications than moduli spaces of stable maps. Pandharipande conjectured that unramified Gromov-Witten invariants of a projective threefold are equal to Gopakumar-Vafa (BPS) invariants in the case of Fano classes (classes that intersect negatively with the canonical class) and primitive Calabi-Yau classes (trivial intersection). After a gentle introduction to unramified Gromov-Witten theory, we will discuss a work in progress which aims to prove the conjecture. This provides a geometric construction of Gopakumar-Vafa invariants in these cases. The proof is based on a certain wall-crossing technique.
  • October 10 (11:15-12:15, HFG 409) Shivang Jindal (Edinburgh)
    2d Cohomological Hall Algebras for Cyclic Quivers and and Integral form of Affine Yangian.
    Abstract: In 2012, Schiffmann and Vasserot considered a Hall algebra type construction on the cohomology of moduli of sheaves supported on points on a plane and used it to prove AGT conjecture. However due to the mysterious nature of the moduli of representations of pre-projective algebra, these algebras are very hard to study and are often highly non trivial. They are conjectured to be the same as Maulik-Okounkov Yangians which has further applications in Quantum Cohomology. In this talk, my goal is to give an introduction to these algebras and explain how one can use tools from Cohomological DT theory to study these algebras. In particular, I will explain how for the case of cyclic quiver, this algebra turn out to be a half of the universal enveloping algebra of the Lie algebra of matrix differential operators on torus, while its deformation turn out be an explicit integral form of Affine Yangian of gl(n).
  • September 28 (9:15-10:15, BBG - 223) Annette Huber-Klawitter (Freiburg) Periods and o-minimality
    Abstract: Periods are numbers obtained by integrating algebraic differential forms over semi-algebraic domains. The set contains interesting numbers like logarithms of algebraic numbers or the values of the Riemann zeta function at integral points. The linear or algebraic relations between them is a classical topic of transcendence theory. Grothendieck gave a more conceptual interpretation in terms of the pairing between singular and algebraic de Rham cohomology of algebraic varieties over the rationals. This leads to the (wide open) Period Conjecture predicting all relations between periods. Kontsevich and Zagier suggested to extend the theory to the so called exponential periods appearing in the theory of irregular connections. The conceptual part of the story has been worked out by Hien and Fresan-Jossen. In joint work with Johan Commelin an Philipp Habegger we show that all exponential periods can be written as volumes in a certain o-minimal structure. This hints at a deeper connection between periods and o-minimiality.
  • September 19 (11:15-12:15, HFG 610) Dusan Dragutinovic (Utrecht) Ekedahl-Oort types of stable curves
    Abstract: In this talk, I will present some invariants of curves in positive characteristic p, such as the p-rank, the a-number, or the Ekedahl-Oort type, and discuss intrinsic ways to define them. The main focus will be on Moonen's definition of Ekedahl-Oort types of smooth curves in terms of Hasse-Witt triples. I will show that we can extend this definition to all stable curves. The description we obtain in this manner enables us to compute the dimensions of certain loci of curves. Finally, I will mention some new examples in characteristics p = 2 and p = 3.
    [This week also: Intercity Number Theory Seminar in Utrecht]
  • September 12 (11:15-12:15, HFG 610) Carolina Tamborini (Utrecht) Hodge theory and projective structures on compact Riemann surfaces
    Abstract: A projective structure on a compact Riemann surface is an equivalence class of projective atlases, i.e. an equivalence class of coverings by holomorphic coordinate charts such that the transition functions are all Moebius transformations. Any compact Riemann surface admits two canonical projective structures: one coming from uniformization's theorem, and one from Hodge theory. These yield two (different) families of projective structures over the moduli space Mg of compact Riemann surfaces. We wish to compare them and give a characterization of the Hodge theoretic family.
  • September 5 (11:15-12:15, BBG 109) Nicoló Piazzalunga (Rutgers) 4G Networks
    Abstract: I'll introduce the equivariant K-theoretic Donaldson-Thomas theory for toric Calabi-Yau fourfolds, and construct its four-valent vertex with generic plane partition asymptotics. String-theoretically, this is the count of BPS states of a system of D0-D2-D4-D6-D8-branes in the presence of a large Neveu-Schwarz B-field. The talk is based on 2306.12995, 2306.12405 and ongoing work.
  • June 28 (11:30-12:30, HFG 409) Sarah Arpin (Leiden) Adding level structure to supersingular elliptic curve isogeny graphs
    Abstract: The classical Deuring correspondence provides a roadmap between supersingular elliptic curves and the maximal orders which are isomorphic to their endomorphism rings. Building on this idea, we add the information of a cyclic subgroup of prime order N to supersingular elliptic curves, and prove a generalisation of the Deuring correspondence for these objects. We also study the resulting ell-isogeny graphs supersingular elliptic curve with level-N structure, and the corresponding graphs in the realm of quaternion algebras. The structure of the supersingular elliptic curve ell-isogeny graph underlies the security of a new cryptographic signature protocol, SQISign, which is proposed to be resistant against both classical and quantum attack.
  • June 23 (14:00-15:00, HFG 409) Aline Zanardini (Leiden University) Pencils of plane cubics revisited
    Abstract: In recent joint work with M. Hattori we have considered the problem of classifying linear systems of hypersurfaces (of a fixed degree) in some projective space up to projective equivalence via geometric invariant theory (GIT). And we have obtained a complete and explicit stability criterion. In this talk I will explain how this criterion can be used to recover Miranda's description of the GIT stability of pencils of plane cubics.
  • June 21 (11:30-12:30, BBG 165) Lou van den Dries (University of Illinois at Urbana-Champaign) Transseries and Hardy Fields
    Abstract: Transseries are formal series involving exp and log. The differential field of transseries is a universal domain for algebraic differential equations with asymptotic side conditions (in analogy with the field of complex numbers being a universal domain for algebraic geometry). I will then apply this to Hardy fields. Until recently there were only very limited ways of extending Hardy fields, but recently we have achieved a more or less complete overview. I will start from scratch and define/explain transseries and Hardy fields, including examples. (Joint work with Matthias Aschenbrenner and Joris van der Hoeven.)
  • June 14 (11:30-12:30, HFG 409) Gergely Berczi (Aarhus) Tautological intersection theory of Hilbert scheme of points
    Abstract: While the Hilbert scheme of points on surfaces is pretty well-understood, the Hilbert scheme over manifolds presents a mixture of pathological and unknown behaviour: our knowledge of their components, singularities and deformation theory is very limited. After a brief survey we report on a new approach to calculate tautological intersection numbers of geometric subsets which play crucial role in enumerative geometry applications. We present a Chern-Segre-type positivity conjecture for tautological integrals coming from global singularity theory.
  • June 7 : (11:30-12:30, HFG 409) Jeongseok Oh (Imperial College, KIAS) Koszul factorisations of moduli spaces and invariants
    Abstract: Invariants on moduli spaces are defined to be integrations over the virtual fundamental classes or Euler characteristics of the virtual structure sheaves. Roughly a virtual structure sheaf is a Koszul resolution of the structure sheaf and the virtual fundamental class is its Chern character (multiplied by Todd class). In fact their first algebraic constructions by Li-Tian and Behrend-Fantechi look different. Here we explain these two are the same in a reasonable circumstance. Furthermore, we explain the use of Koszul factorisations rather than Koszul resolutions gives rise to Kiem-Li localisations of Li-Tian and Behrend-Factechi constructions. These are joint works with Bumsig Kim and Bhamidi Sreedhar. We can use these to produce invariants of moduli spaces of stable sheaves on Calabi-Yau 4-folds as explained in the joint work with Richard Thomas.
  • May 24 (11:30-12:30, HFG 409) Nancy Abdallah (University of Borås) Lefschetz properties of Artin Gorenstein algebras in low codimensions
    Abstract: The weak and strong Lefschetz properties (WLP and SLP) has been much studied for Artinian algebras. Codimension two Artinian algebras over a field of characteristic zero have the strong Lefschetz property. It is open whether Artin Gorenstein (AG) algebras of codimension c=3 satisfy the weak or strong Lefschetz properties. Harima, Migliore, Nagel, and Watanabe proved that complete intersection algebras of codimension three satisfy the weak Lefschetz property. For c=4, Gondim showed that WLP always holds for regularity r <= 4 and gives a family where WLP fails for any r >= 7, building on an earlier example of Ikeda of failure for r=5. In this talk we go through an overview of Lefschetz properties over AG algebras and we give some recent results in codimensions 3 and 4. In codimension 4 we explore relations between Lefschetz properties of an AG algebra A and the free resolutions of A. Lefschetz properties of the Milnor algebras of projective hypersurfaces will be discussed.
  • May 17 (11:30-12:30, HFG 409) Samir Canning (ETH) Semi-tautological systems and the cohomology of the moduli space of curves
    Abstract: I will introduce the notion of semi-tautological systems, which are systems of subalgebras with minimal set of functoriality properties of the cohomology rings of the moduli spaces of stable curves. They are designed to study the structure of the cohomology of the moduli spaces of stable curves beyond the tautological ring. I will give a criterion for a given semi-tautological system to span all of cohomology in a given degree. Using this criterion and other results about the moduli space of curves, both topological and algebro-geometric, I will give several applications. These applications include a complete description of the thirteenth cohomology of the moduli space of stable n pointed curves of genus g for all g,n and that all cohomology classes of sufficiently high degree are tautological. This is joint work in progress with Hannah Larson and Sam Payne.
  • May 10, (11:30-12:30 MIN 2.01) Thorsten Schimannek (CNRS LPTHE) Counting curves on non-Kaehler Calabi-Yau 3-folds with Topological Strings
    Abstract: In general, a projective Calabi-Yau threefold with nodal singularities does not admit a Kaehler small resolution. This happens in particular if the exceptional curves are torsion in homology. In this talk we will discuss how the classical relationship between topological string theory, enumerative geometry and mirror symmetry generalizes to this setting. After recalling some of the ideas from the smooth case, we will both explain the physical intuition behind the generalization and translate it into a concrete mathematical proposal. At the end of the talk, if time permits, we will highlight some open questions that follow from this proposal, related to Donaldson-Thomas theory, non-commutative geometry, and FJRW-theory.
  • May 3, (13.30-14.30, HFG 409) Chia-Fu Yu (Academia Sinica, Taiwan) Arithmetic invariants on supersingular Ekedahl-Oort (EO) strata for g=4
    Abstract: The supersingular locus is one of main interests in algebraic geometry in characteristic p, and can be described in terms of polarised flag type quotients (PFTQs) in the sense of Li and Oort. The description for g=3 is rather explicit and is exploited by Karemaker and Yobuko and myself for investigating the arithmetic invariants of supersingular abelian threefolds, namely, the endomorphism rings and automorphism groups of them, confirming Oort's conjecture for g=3. In this talk we shall explain a general method for investigating the arithmetic invariants on supersingular EO strata, and report the progress of the joint work with Karemaker for g=4.
  • April 26, (13:30-15:30, HFG 409) Minicourse by Carolina Tamborini (UU) Families of Galois covers of the line: examples and construction
  • April 25, (13:30-15:30, HFG 610) Minicourse by Carolina Tamborini (UU) Some results on totally geodesic subvarieties in the Torelli locus
  • April 20, (13:30-15:30, HFG 610) Minicourse by Carolina Tamborini (UU) Torelli map, its differential, its second fundamental form, and the Lie bracket map
  • April 19, (13:30-14:30, HFG-409) research talk by Oliver Leigh (Uppsala) The Blowup Formula for the Instanton Part of Vafa-Witten Invariants on Projective Surfaces
    Abstract: In this talk I will present a blow-up formula for the generating series of virtual χ_y-genera for moduli spaces of sheaves on projective surfaces. The formula is related to a conjectured formula for topological χ_y-genera of Göttsche, and is a refinement of a formula of Vafa-Witten relating to S-duality. I will also discuss the proof of the formula, which is based on the blow-up algorithm of Nakajima-Yoshioka for framed sheaves on ℙ^2. This talk is based on joint work with Nikolas Kuhn and Yuuji Tanaka.
  • April 18, (13:30-15:30, HFG 610) Minicourse by Carolina Tamborini (UU) Siegel space as symmetric space
  • April 5, (13:30, HFG-409) research talk by Fabian Reede (Hannover) Line bundles on noncommutative algebraic surfaces
    Abstract: Let X be a complex algebraic surface. Let A be a (noncommutative) coherent O_X-algebra whose generic stalk is a central division algebra. Then the pair (X,A) can be thought of as a noncommutative algebraic surface. This talk deals with the moduli space of line bundles on (X,A), i.e., of locally free A-modules of rank one, in the cases where X has Kodaira dimension zero or is Fano. It turns out that the moduli space can be compactified by adding torsion-free A-modules of rank one. We will study e.g. the smoothness of this compactification and the deformation theory of the sheaves involved. This is partly joint work with U. Stuhler and with N.Hoffmann.
  • March 22, (13:30, HFG-409) research talk by Sara Mehidi (Bordeaux) Extending torsors via log schemes
    Abstract: We give here an approach of the problem of extending torsors defined on the generic fiber of a family of curves. The question is to extend each of the structural group and the total space of the torsor above the family. This problem has been studied by many researchers, starting by the first ideas of Grothendieck who solved the case of a constant group of order prime to the residual characteristic. When we are interested in algebraic varieties from an arithmetic point of view, it is natural to consider torsors under a finite flat group that is not necessarily constant: we talk about fppf torsors. In fact, we know from the literature that there are cases where the problem does not have a solution in this setting.
    So the idea is to look for a solution in a larger category, namely the category of logarithmic torsors. We will show in particular that the existence of such an extension amounts to extending group functors and morphisms between them. Then, we will compute the obstruction for the extended log torsor to lift into an fppf one. Finally, we give an example of a computation of an extension of a torsor over a given hyperelliptic curve as an application of our results.
  • March 15, (13:30, HFG-409) research talk by Henry Liu (Oxford): Multiplicative vertex algebras and wall-crossing in equivariant K-theory
    Abstract: I will give an overview of recent progress in wall-crossing for equivariant K-theoretic invariants of moduli of sheaves, based on a new framework of Joyce. A multiplicative version of vertex algebras plays a central role in this and related stories. I will give some applications to refined Vafa-Witten theory, the 3-fold DT/PT correspondence, and, if time permits, also some speculation about modularity and S-duality.
  • March 1, (13:30, HFG-409) research talk by Stefano Marseglia (UU): Cohen-Macaulay type of endomorphism rings of abelian varieties over finite fields
    Abstract: In this talk, we will speak about the (Cohen-Macaulay) type of the endomorphism ring of abelian varieties over a finite field with commutative endomorphism algebra. We will exhibit a condition on the type of End(A) implying that A cannot be isomorphic to its dual. In particular, such an A cannot be principally polarised or a Jacobian. This is partly joint work with Caleb Springer.
  • February 15, (13:30, BBG-069) research talk by Andreas Braun (Durham): Hodge classes on Calabi-Yau fourfolds
    Abstract: A crucial question in string theory concerns its set of solutions, particularly those giving rise to an effective four-dimensional description. The most general such solutions are found by specifying a Calabi-Yau fourfold equipped with an elliptic fibration, together with a Hodge class. Without assuming any background I will briefly explain how these objects arise in physics, and which constraints they need to obey. After formulating some of the central questions and conjectures that arise in this context, I will discuss some recent progress.
  • February 8, (13:30, HFG-409) research talk by Hyeonjun Park (KIAS): Cosection localization via derived algebraic geometry
    Abstract: Cosection localization is one of the most powerful tools in virtual enumerative geometry. In this talk, we revisit cosection localization from the perspective of derived algebraic geometry. I will explain derived reduction by (-1)-shifted closed 1-forms and localization through homotopical intersection theory. I will also provide an intrinsic description of cosection-localized virtual cycles using (-2)-shifted symplectic structures. This is based on joint works with Younghan Bae and Martijn Kool, with Dhyan Aranha, Adeel Khan, Alexei Latyntsev, and Charanya Ravi, and with Young-Hoon Kiem.
  • February 1, (11:00, HFG-409) research talk by Olivier de Gaay Fortman (Hannover): Real moduli spaces, unitary Shimura varieties and non-arithmetic lattices
    Abstract: Hodge theory can sometimes be used to identify a moduli space of complex varieties with a complex ball quotient, or an open subset of such a space. I will explain that similar things happen for moduli of real varieties. Real moduli spaces of smooth varieties are often not connected, however - to get a connected moduli space one is led to allow some singularities. It turns out that, very similar to the way in which the connected components of the space of smooth varieties embed into the larger moduli space, real arithmetic ball quotients can be glued together to form a large real ball quotient. Unitary Shimura varieties provide the right framework for this glueing procedure. I will explain how this works, constructing non-arithmetic lattices in PO(n,1) for every n.
  • January 18, (10:00, HFG-409) research talk by Mick van Vliet (UU): Tame geometry and Hodge theory
    Abstract: Tame geometry, made precise by the concept of o-minimal structures, has recently led to some interesting developments in algebraic geometry. In the first half of this talk I will motivate and explain the definition of o-minimal structures, and review some remarkable theorems that hold in the resulting framework of tame geometry. In the second half of the talk, based on work of Bakker, Klingler, and Tsimerman (1810.04801), I will give an overview of a recent application of tame geometry to Hodge theory.
  • January 11, (11:00, HFG-409) research talk by Lars Halvard Halle (University of Bologna): Degenerations of Hilbert schemes and relative VGIT
    Abstract: This talk will be a report on joint work with K. Hulek and Z. Zhang. First I will explain how some central results in VGIT can be extended to a relative setting. After this, I will discuss an application of relative VGIT to the study of certain degenerations of Hilbert schemes of points.


  • December 7, (HFG-409) research talk by Leo Herr (Leiden University): The Rhizomic Topology
    Abstract: What is a sheaf on a log scheme X? If we take the ordinary etale topology, we ignore the log structure. Taking the log étale topology, even the structure "sheaf" O_X is not a sheaf! The same goes for M_X, \overline M_X. We introduce a new "rhizomic" topology on log schemes coarser than the log etale topology. Will this be enough?
  • November 30, (HFG-409) research talk by Navid Nabijou (QMU London): Roots and logs in the enumerative forest
    Abstract: Logarithmic and orbifold structures provide two different paths to the enumeration of curves with fixed tangencies to a normal crossings divisor. Simple examples demonstrate that the resulting systems of invariants differ, but a more structural explanation of this defect has remained elusive. I will discuss joint work with Luca Battistella and Dhruv Ranganathan, in which we identify birational invariance as the key property distinguishing the two theories. The logarithmic theory is stable under strata blowups of the target, while the orbifold theory is not. By identifying a suitable system of “slope-sensitive” blowups, we define a “limit" orbifold theory and prove that it coincides with the logarithmic theory. Our proof hinges on a technique – rank reduction – for reducing questions about normal crossings divisors to questions about smooth divisors, where the situation is much-better understood.
  • November 23, (HFG-409) research talk by Mar Curco Iranzo (UU): Generalised Jacobians of modular curves and their Q-rational torsion
    Abstract: The Jacobian J0(N) of the modular curve X0(N) has received much attention within arithmetic geometry for its relation with cusp forms and elliptic curves. In particular, the group of Q-rational points on X0(N) controls the cyclic N-isogenies of elliptic curves. A conjecture of Ogg predicted that, for N prime, the torsion of this group comes all from the cusps. The statement was proved by Mazur and later generalised to arbitrary level N into what we call generalised Ogg’s conjecture. Consider now the generalised Jacobian J0(N)m with respect to a modulus m. This algebraic group also seems to be related to the arithmetic of X0(N) through the theory of modular forms. In the talk we will present new results that compute the Q-rational torsion of J0(N) for N an odd integer with respect to a cuspidal modulus m. These generalise previous results of Yamazaki, Yang and Wei. In doing so, we will also discuss how our results relate to generalised Ogg’s conjecture.
  • November 16, (HFG-409) research talk by Francesca Carocci (EPFL): BPS invariant from non Archimedean integrals
    Abstract: We consider moduli spaces of one-dimensional semistable sheaves on del Pezzo and K3 surfaces supported on ample curve classes. Working over a non-archimedean local field F, we define a natural measure on the F-points of such moduli spaces. We prove that the integral of a certain naturally defined gerbe on the moduli spaces with respect to this measure is independent of the Euler characteristic. Analogous statements hold for (meromorphic or not) Higgs bundles. Recent results of Maulik-Shen and Kinjo-Coseki imply that these integrals compute the BPS invariants for the del Pezzo case and for Higgs bundles. This is a joint work with Giulio Orecchia and Dimitri Wyss.
  • October 26, (HFG-409) research talk by Reinier Schmiermann: On Classifying Continuous Constraint Satisfaction Problems
    Abstract: The computational complexity class of the existential theory of the reals contains problems which can be reduced to checking whether a system of polynomial equations has a solution over the real numbers. The complexity of a lot of problems in computational geometry turns out to be captured by this class (they are complete for this class). These completeness proofs often use the completeness of a specific Continuous Constraint Satisfaction Problem (CCSP) as an intermediate step. We attempt to give a more systematic analysis of the computational complexity of these CCSPs, and show that a large class of CCSPs is complete for the existential theory of the reals. In this talk, I will give a introduction to computational complexity, the existential theory of the reals, and CCSPs. Then I will state our results, and give a sketch of the proof. This talk is based on joint work with Tillmann Miltzow.
  • October 19, (HFG-409) Carel Faber, preprint talk on "On the Chow and cohomology rings of moduli spaces of stable curves" by Canning and Larson, arXiv:2208.02357
  • October 12 (HFG-409), research talk by Remy van Dobben de Bruyn: A variety that cannot be dominated by one that lifts
    Abstract: The recent proofs of the Tate conjecture for K3 surfaces over finite fields start by lifting the surface to characteristic 0. Serre showed in the sixties that not every variety can be lifted, but the question whether every motive lifts to characteristic 0 is open. We give a negative answer to a geometric version of this question, by constructing a smooth projective variety that cannot be dominated by a smooth projective variety that lifts to characteristic 0.
  • October 5, research talk by Dusan Dragutinovic: Computing binary curves of genus five
    Abstract. In this talk, we will present algorithms used to determine, up to isomorphism over $\F_2$, all genus five curves defined over $\F_2$ (together with the sizes of their $\F_2$-automorphism groups). Furthermore, we will discuss the outcome considering the Newton polygons of computed curves and mention the obtained stack count $|\mathcal{M}_5(\F_2)|$.
  • September 21, Boaz Moerman, preprint talk on "Weak approximation and the Hilbert property for Campana points" by Nakahara and Streeter, arXiv:2010.12555
  • July 6 (11:00 - 12:00, HFG-611), research talk by Valentijn Karemaker
  • June 28 (11:00 - 13:00, HFG-611), research talk by Pol van Hoften
  • June 22 (11:15, HFG-610) Carolina Tamborini, "Punctual characterization of the unitary flat bundle of weight 1 PVHS and application to families of curves" by González-Alonso and Torelli, arXiv:2101.03153
  • June 8 (HFG-610), Wilberd van der Kallen, "Frobenius Splittings", arXiv:1208.3100
  • May 31 (HFG-610), Dirk van Bree, "When are two HKR isomorphisms equal?" by Huang, arXiv:2205.04439
  • May 25 (HGF-610), Marta Pieropan, "Heights on stacks and a generalized Batyrev-Manin-Malle conjecture" by Ellenberg, Satriano and Zureick-Brown, arXiv:2106.11340
  • May 9 (Duistermaat), Marta Pieropan, "Global Frobenius liftability I & II" by Achinger, Witaszek and Zdanowicz, arXiv:1708.03777 and arXiv:2102.02788
  • March 7, Sebastián Carrillo Santana, "Values of zeta-one functions at positive even integers" by Kobayashi and Sasaki, arXiv:2202.11835
  • Feb. 28, Carolina Tamborini, "The Coleman-Oort conjecture: reduction to three key cases" by Moonen, arXiv:2201.11971
  • Feb. 14, Boaz Moerman, "Tamagawa measures on universal torsors and points of bounded height on Fano varieties" by Salberger, article, and "Compter des points d'une variété torique" by de la Bretèche, article
  • Feb. 7, Sergej Monavari, "On the motive of the Quot scheme of finite quotients of a locally free sheaf" by Ricolfi, arXiv:1907.08123
  • Jan. 24, Dirk van Bree, "Using the internal language of toposes in algebraic geometry" by Blechschmidt, arXiv:2111.03685


  • Nov. 29 (HFG-611), Reinier Schmiermann, "Components and singularities of Quot schemes and varieties of commuting matrices" by Jelisiejew and Šivic, arXiv:2106.13137
  • Nov. 22 (KBG-Atlas), Dusan Dragutinovic, "The existence of supersingular curves of genus 4 in arbitrary characteristic" by Kudo, Harashita and Senda, arXiv:1903.08095
  • Nov. 8, Dirk van Bree, "Unramified division algebras do not always contain Azumaya maximal orders" by Antieau and Williams, arXiv:1209.2216
  • Nov. 1, Stefano Marseglia, "On matrices of endomorphisms of abelian varieties" by Zarhin, arXiv:2002.00290, and "Lattices in Tate modules" by Poonen and Rybakov, arXiv:2107.06363
  • Oct. 11, Carel Faber, "A non-hyperelliptic curve with torsion Ceresa class" by Beauville, arXiv:2105.07160, and "A non-hyperelliptic curve with torsion Ceresa cycle modulo algebraic equivalence" by Beauville and Schoen, arXiv:2106.08390
  • Oct. 4, Marta Pieropan, "Sums of four squareful numbers" by Shute, arXiv:2104.06966