Séminaires

Extension of Differential Forms, Uniformization, Miyaoka-Yau inequalities and the topological characterization of certain klt varieties (with Daniel Greb and Thomas Peternell)

The first part of this overview talk begins with a non-technical overview of minimal model theory, explaining why any classification theory of complex-projective manifolds always needs to consider singular varieties. The talk describes the relevant singularities in brief, mentions methods that have been developed to study them and will ideally convey an idea what classification results one might hope to expect.

The second part describes some of the theory that has been developed over the last years and mentions some of the more concrete applications.

To an algebraic variety X we associate its group of birational transformations Bir(X). In this talk, we will see the following theorem: If X is an algebraic variety such that Bir(X) is isomorphic to Bir(P^n), where P^n is the n-dimensional projective space, then X is birational to P^n. In other words, the group structure of Bir(X) determines whether X is rational or not. In another direction, I will explain that Borel subgroups of Bir(X), i.e. maximal connected solvable subgroups, are of derived length <= 2 dim(X) with equality if and only if X is rational and the Borel subgroup is standard. This is joint work with Regeta and Van Santen.

In this talk I will present a joint work with C. Casagrande, in which we study smooth complex Fano 4-folds with a rational fibration onto a 3-fold. After an introduction on the setting and motivation, I will discuss our main result: if X is Fano 4-fold with a rational fibration onto a 3-fold and it is not a product of surfaces, then the Picard number of X is at most 9, and the bound is sharp. Moreover, I will present a classification result in a special case within the setting above, and show new examples of Fano 4-folds with large Picard number.

The Shafarevich conjecture asks whether the universal covering of a compact Kähler manifold is holomorphically convex. In this work, we explore a similar question regarding the holomorphic convexity of intermediate coverings. We prove that if the intermediate covering of a compact Kähler surface admits a faithful reductive representation for its covering group and does not have two ends, then it is holomorphically convex. The main techniques employed include the analysis of the degeneracy loci of the Levi form and the properties of subanalytic functions.

A fundamental result in Complex Geometry is the Kobayashi-Hitchin correspondence, stating that a holomorphic vector bundle on a Kähler manifold is poly-stable (as defined by Mumford, Takemoto) if and only if it admits a Hermitian metric solving the Hermite-Einstein equation. It has now become clear that there exist many possible different stability notions for vector bundles, that are of great interest in Algebraic Geometry and String Theory. It is natural to wonder if these stabilities are also tied to the existence of Hermitian metrics with special curvature properties. In this talk, based on joint work with Julien Keller (arXiv:2405.03312[math.DG]), we will consider a class of « polynomial » equations for the curvature of rank 2 holomorphic vector bundles on compact projective surfaces, and a corresponding class of polynomial stability conditions for these bundles. We will then explain how each of these stability conditions is related to the existence of a Hermitian metric satisfying the corresponding equation. This refines and partially confirms a conjectural correspondence between Bridgeland stability conditions and PDEs on holomorphic vector bundles, formulated by Dervan, McCarthy, and Sektnan.

I will introduce a method to compute efficiently and with arbitrary precision a basis of harmonic functions with singularities in a Riemann surface of any genus. This basis is a powerful tool to approximate harmonic functions with spectral speed, using the method of particular solutions: we give a full characterization of the speed of convergence in any genus, depending on the singularities of the harmonic extension.

We introduce the canonical model singularities (CMS) criterion for bigness of the cotangent bundle for surfaces. The CMS-criterion for bigness involves invariants for canonical singularities that we describe and we give formulas for A_n singularities. The CMS-criterion leads to conjectures and some answers about the geography and the possible ratios $c_1^2/c_2$ of surfaces with big cotangent bundle. Two cases are naturally separated: regular and irregular surfaces. For regular surfaces we apply the CMS-criterion to show the existence of deformations of hypersurfaces in $\mathbb{P}^3$ with big cotangent bundle for degree $d\ge 8$ and give an example of the regular surface with big  cotangent bundle with ratio close to 1/5. For irregular surfaces we show that there are examples with ratio as close to 1/5 as possible. If time permits, we talk about ratios  below 1/5.