Malgré les succès de la théorie de Hodge non abélienne de Corlette-Simpson pour exclure que de nombreux groupes de présentation finie soient groupes fondamentaux de variétés projectives lisses (ou des groupes Kähleriens), les techniques de construction manquent. La construction de Campana du groupe fondamental orbifold d'une paire orbifolde permet de considérer le groupe fondamental des compactifications orbifolds d'une variété (ou champ) quasiprojective lisse donnée $U$ qui, si quelques précautions sont prises et sous des hypothèses raisonnables - mais pas toujours faciles a vérifier, est un groupe Kählerien. En choisissant bien la variété $U$, les groupes obtenus sont potentiellement intéressants et on utilise souvent des techniques inattendues pour établir les propriétés de leurs représentations linéaires. L'exposé fera un survey de cas particulièrement intrigants ou, par exemple, $U$ est un complément d'arrangement de droites, une variété localement complexe hyperbolique non compacte ou un espace de modules de courbes pointées.[-]

Gushel-Mukai varieties are defined as the intersection of the Grassmannian Gr(2, 5) in its Plücker embedding, with a quadric and a linear space. They occur in dimension 6 (with a slighty modified construction), 5, 4, 3, 2 (where they are just K3 surfaces of degree 10), and 1 (where they are just genus 6 curves). Their theory parallels that of another important class of Fano varieties, cubic fourfolds, with many common features such as the presence of a canonically attached hyperkähler fourfold: the variety of lines for a cubic is replaced here with a double EPW sextic.
There is a big difference though: in dimension at least 3, GM varieties attached to a given EPW sextic form a family of positive dimension. However, we prove that the Hodge structure of any of these GM varieties can be reconstructed from that of the EPW sextic or of an associated surface of general type, depending on the parity of the dimension (for cubic fourfolds, the corresponding statement was proved in 1985 by Beauville and Donagi). This is joint work with Alexander Kuznetsov.[-]

Positivity of the tangent bundle and its top exterior power (namely, the anticanonical bundle) is the subject of extensive literature, and several open problems. Notably, Campana and Peternell predict that if $-K_{X}$ is strictly nef, then $X$ is a Fano variety: this conjecture is proven up to dimension 3 by the work of Maeda and Serrano. In this talk, we investigate positivity of the intermediate exterior powers of the tangent bundle. We prove that if $X$ is a smooth projective n-fold and the third, fourth or (n-1)-th exterior power of $T_{X}$ is strictly nef, then $X$ is a Fano variety. Moreover, we classify smooth projective varieties of Picard number at least two with third or fourth exterior power of $T_{X}$ strictly nef. This work is actually slightly more general, as it boils down to classifying rationally connected varieties such that the degree of the anticanonical bundle on rational curves is quite large.[-]

A famous conjecture of Kobayashi from the 1970s asserts that a generic algebraic hypersurface of sufficiently large degree $d\geq d_n$ in the complex projective space of dimension $n+1$ is hyperbolic. Yum-Tong Siu introduced several fundamental ideas that led recently to a proof of the conjecture. In 2016, Damian Brotbek gave a new geometric argument based on the use of Wronskian operators and on an analysis of the geometry of Semple jet bundles. Shortly afterwards, Ya Deng obtained effective degree bounds by means of a refined technique. Our goal here will be to explain a drastically simpler proof that yields an improved (though still non optimal) degree bound, e.g. $d_n=[(en)^{2n+2}/5]$. We will also present a more general approach that could possibly lead to optimal bounds.[-]

The general principally polarized abelian variety of dimension at most five is known to be a Prym variety. This reduces the study of abelian varieties of small dimension to the beautifully concrete theory of algebraic curves. I will discuss recent breakthrough on finding a structure theorem for principally polarized abelian varieties of dimension six as Prym-Tyurin varieties associated to covers with $E_6$-monodromy, and the implications this uniformization result has on the geometry of the moduli space $A_6$. This is joint work with Alexeev, Donagi, Izadi and Ortega.[-]