Next Seminar: December 1 14:15

Liviu Ornea (Bucarest)

Algebraic cones on LCK manifolds with potential

Title and Abstract Link Teams
Abstract: I will present an equidistribution result for families of (non-degenerate) subvarieties in a family of abelian varieties. Using this result, one can deduce a uniform version of the classical Bogomolov conjecture for curves embedded in their Jacobians, namely that the number of torsion points lying on them is uniformly bounded in the genus of the curve. This has been previously only known in a few select cases by work of David-Philippon and DeMarco-Krieger-Ye. Furthermore, one can deduce a rather uniform version of the Mordell conjecture by complementing a result of Dimitrov-Gao-Habegger: The number of rational points on a smooth algebraic curve defined over a number field can be bounded solely in terms of its genus and the Mordell-Weil rank of its Jacobian. Again, this was previously known only under additional assumptions (Stoll, Katz-Rabinoff-Zureick-Brown). All these results have been recently generalized beyond curves in joint work with Ziyang Gao and Tangli Ge, but I will focus on the simpler case of curves.
Abstract: Smooth minimal surfaces of general type with \(K^2=1,\ p_g=2\), and \(q=0\) constitute a fundamental example in the geography of algebraic surfaces, and the 28-dimensional moduli space \(\mathbf{M}\) of their canonical models admits a modular compactification \(\overline{\mathbf{M}}\) via the minimal model program. We describe eight new irreducible boundary divisors in such compactification parametrizing reducible stable surfaces. Additionally, we study the relation with the GIT compactification of \(\mathbf{M}\) and the Hodge theory of the degenerate surfaces that the eight divisors parametrize. This is joint work with Patricio Gallardo, Gregory Pearlstein, and Zheng Zhang.
Abstract: Inspired from ideas in topology, Koszul modules turned out to have important algebro-geometric applications for instance to (i) Green's Conjecture on syzygies of canonical curves, (ii) stabilization of cohomology of projective varieties in arbitrary characteristics and (iii) a resolution of an effective form of an important conjecture of Suciu's on Chen invariants of hyperplane arrangements. I will discuss new developments related to this circle of ideas obtained in joint work with Aprodu, Raicu and Suciu.
Abstract: One says a scheme, or an algebraic stack, has the resolution property if every coherent sheaf is the quotient of a locally free sheaf. Although this is a fundamental and widely used property in algebraic geometry, it is still poorly understood. After giving the appropriate definitions, we will explain the two most important sources of non-examples: (1) affine group schemes G/S which cannot be embedded into GL_n but which are forms of embeddable group schemes, and (2) cohomological Brauer classes which are not represented by Azumaya algebras. After describing a new way to construct non-trivial vector bundles on schemes and stacks, we introduce the notion of an R-unipotent morphism and characterize it geometrically. We will then present a surprising local to global principle: a locally R-unipotent morphism over a base with enough line bundles is globally R-unipotent. To conclude, we will explain why the unipotent analogues of (1) and (2) above cannot occur.
Abstract: I will report on some recent results on the geometry of irreducible symplectic varieties that are deformation of moduli spaces of sheaves on K3 surfaces. In the first part of the talk I will introduce the main characters of this story and I will talk about their deformations: this will also include a joint work in progress with A. Perego and A. Rapagnetta on (undesingularisable) singular moduli spaces of sheaves. In the second part I will mostly focus on moduli spaces of O'Grady type and their symplectic desingularisation, presenting an older result on their ample cone (joint with G. Mongardi), and illustrating a recent application to symplectic automorphisms (joint with L. Giovenzana, A. Grossi and D.C. Veniani).
Abstract: I will discuss joint work with Chris Peters which extends rigidity results of Arakalov, Faltings and Peters to period maps arising from families of complex algebraic varieties which are non-necessarily proper or smooth. Inspired by recent work with P. Gallardo, L. Schaffler, Z. Zhang, I will discuss two classes of elliptic surfaces which can be presented as hypersurfaces in weighted projective spaces which have a unique canonical curve. In each case, we will show that infinitesimal Torelli fails for \(H^2\) of the compact surface, but is restored when one considers the period map for the complement of the canonical curve. We will also show that these period maps are rigid, in the sense that they do not admit any horizontal deformations which keep the source and target fixed.
Abstract: On abelian varieties it is possible to extend the notion of base point freeness (as well as other holomorphic properties) to rational multiples of polarizations. It is therefore natural to consider the base point freeness threshold of a given polarization \(L\), namely the infimum of all rational numbers \(x\) such that \(xL\) is base point free. I will report on recent and less recent methods and results, due to various authors, on this invariant.
Abstract: We know by Falting's theorem that a curve \(C\) of genus \(g>1\) defined over the rationals has a finite number of rational points, but there is no general procedure to provably compute the set \(C(\mathbb{Q})\). When the rank of the Mordell-Weil group \(J(\mathbb{Q})\) (with \(J\) the Jacobian of \(C\)) is smaller than \(g\) we can use Chabauty method, i.e. we can embed \(C\) in \(J\) and, after choosing a prime p, we can view \(C(\mathbb{Q})\) as a subset of the intersection of \(C(\mathbb{Q}_p)\) and the closure of \(J(\mathbb{Q})\) inside the p-adic manifold \(J(\mathbb{Q}_p)\); this intersection is always finite and computable up to finite precision. Minhyong Kim has generalized this method by inspecting (possibly non-abelian) quotients of the fundamental group of \(C\). His ideas have been made effective in some new cases by Balakrishnan, Dogra, Muller, Tuitman and Vonk: their "quadratic Chabauty method" works when the rank of the Mordell-Weil group is strictly less than \(g + s -1\) (with s the rank of the Neron-Severi group of \(J\)). In the seminar we will give a reinterpretation of the quadratic Chabauty method, only using the Poincaré torsor of \(J\) and a little of formal geometry, and we will show how to make it effective. This is joint work with Bas Edixhoven.
The conference "WINTER MEETING IN ALGEBRA AND GEOMETRY 2022" is a Winter meeting in Algebra and Geometry in Rome that takes place in the rione Monti. For more info here is the Link to the Official Webpage

The seminar is usually held every Thursday at 14:15-15:45 in Aula M1. This year seminars will be held in a blended format: in person (for more info you can send an email to an organizer) and on the platform Microsoft Teams (online).

The seminar is organized by Luca Schaffler and Amos Turchet and maintained by the Geometry Group of the Department of Mathematics and Physics at the Roma Tre University.

We acknowledge the support of the grants PRIN2020: Curves, Ricci flat varieties and their interactions, PRIN2017: Advances in Moduli Theory and Birational Classification, and the support of the Department of Mathematics and Physics at Roma Tre University.

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