Volume 1

Premier volume de l'Épijournal de Géométrie Algébrique (2016-2017)

1. Conic bundles that are not birational to numerical Calabi--Yau pairs

Kollár, János.
Let $X$ be a general conic bundle over the projective plane with branch curve of degree at least 19. We prove that there is no normal projective variety $Y$ that is birational to $X$ and such that some multiple of its anticanonical divisor is effective. We also give such examples for 2-dimensional […]

2. Bridgeland Stability Conditions on Fano Threefolds

Bernardara, Marcello ; Macrì, Emanuele ; Schmidt, Benjamin ; Zhao, Xiaolei.
We show the existence of Bridgeland stability conditions on all Fano threefolds, by proving a modified version of a conjecture by Bayer, Toda, and the second author. The key technical ingredient is a strong Bogomolov inequality, proved recently by Chunyi Li. Additionally, we prove the […]

3. On complete reducibility in characteristic $p$

Balaji, V. ; Deligne, P. ; Parameswaran, A. J..
Let $G$ be a reductive group over a field $k$ which is algebraically closed of characteristic $p \neq 0$. We prove a structure theorem for a class of subgroup schemes of $G$, for $p$ bounded below by the Coxeter number of $G$. As applications, we derive semi-simplicity results, generalizing earlier […]

4. Some remarks on regular foliations with numerically trivial canonical class

Druel, Stéphane.
In this article, we first describe codimension two regular foliations with numerically trivial canonical class on complex projective manifolds whose canonical class is not numerically effective. Building on a recent algebraicity criterion for leaves of algebraic foliations, we then address […]

5. Finitude de l'espace des n-cycles pour un espace complexe (n − 2)-concave réduit

Barlet, Daniel.
Nous montrons que, pour n ≥ 2, l'espace des n-cycles fermés dans un espace complexe fortement (n − 2)-concave a une structure naturelle d'espace complexe réduit localement de dimension finie et que cet espace représente le foncteur " famille analytique de n-cycles " paramétrée par des […]

6. Correspondences between convex geometry and complex geometry

Lehmann, Brian ; Xiao, Jian.
We present several analogies between convex geometry and the theory of holomorphic line bundles on smooth projective varieties or K\"ahler manifolds. We study the relation between positive products and mixed volumes. We define and study a Blaschke addition for divisor classes and mixed divisor […]

7. Syzygies of Prym and paracanonical curves of genus 8

Colombo, Elisabetta ; Farkas, Gavril ; Verra, Alessandro ; Voisin, Claire.
By analogy with Green's Conjecture on syzygies of canonical curves, the Prym-Green conjecture predicts that the resolution of a general level p paracanonical curve of genus g is natural. The Prym-Green Conjecture is known to hold in odd genus for almost all levels. Probabilistic arguments […]

8. On a theorem of Campana and Păun

Schnell, Christian.
Let $X$ be a smooth projective variety over the complex numbers, and $\Delta \subseteq X$ a reduced divisor with normal crossings. We present a slightly simplified proof for the following theorem of Campana and Păun: If some tensor power of the bundle $\Omega_X^1(\log \Delta)$ contains a subsheaf […]

9. Haas' theorem revisited

Bertrand, Benoît ; Brugallé, Erwan ; Renaudineau, Arthur.
Haas' theorem describes all partchworkings of a given non-singular plane tropical curve $C$ giving rise to a maximal real algebraic curve. The space of such patchworkings is naturally a linear subspace $W_C$ of the $\mathbb{Z}/2\mathbb{Z}$-vector space $\overrightarrow \Pi_C$ generated by […]