Rational curves of minimal degree and characterizations of Pn{\mathbb P}^n

In this paper we investigate complex uniruled varieties $X$ whose rational curves of minimal degree satisfy a special property. Namely, we assume that the tangent directions to such curves at a general point $x\in X$ form a linear subspace of $T_xX$. As an application of our main result, we give a unified geometric proof of Mori's, Wahl's, Campana-Peternell's and Andreatta-Wi\'sniewski's characterizations of ${\mathbb P}^n$.Comment: 14 page

K\"ahler-Einstein Metrics for some quasi smooth log del Pezzo Surfaces

Recently Johnson and Koll\'ar determined the complete list of anticanonically embedded quasi smooth log del Pezzo surfaces in weighted projective 3-spaces. They also proved that many of those surfaces admit a K\"ahler-Einstein metric, and that some of them do not have tigers. The aim of this paper is to settle the question of the existence of K\"ahler-Einstein metrics and tigers for those surfaces for which the question was still open. In order to do so, we will use techniques developed earlier by Nadel and Demailly and Koll\'ar.Comment: 10 page

Codimension 1 Mukai foliations on complex projective manifolds

In this paper we classify codimension 1 Mukai foliations on complex projective manifold

On Fano foliations 2

In this paper we study mildly singular del Pezzo foliations on complex projective manifolds with Picard number oneComment: 13 pages, several minor clarifications and correction

On Fano foliations

In this paper we address Fano foliations on complex projective varieties. These are foliations whose anti-canonical class is ample. We focus our attention on a special class of Fano foliations, namely del Pezzo foliations on complex projective manifolds. We show that these foliations are algebraically integrable, with one exceptional case when the ambient space is a projective space. We also provide a classification of del Pezzo foliations with mild singularities.Comment: 38 pages, several minor clarifications and correction

On smooth lattice polytopes with small degree

Toric geometry provides a bridge between the theory of polytopes and algebraic geometry: one can associate to each lattice polytope a polarized toric variety. In this paper we explore this correspondence to classify smooth lattice polytopes having small degree, extending a classification provided by Dickenstein, Di Rocco and Piene. Our approach consists in interpreting the degree of a polytope as a geometric invariant of the corresponding polarized variety, and then applying techniques from Adjunction Theory and Mori Theory.Comment: 12 pages. v2: attributions fixed, reference adde

Explicit log Fano structures on blow-ups of projective spaces

In this paper we determine which blow-ups $X$ of $\mathbb{P}^n$ at general points are log Fano, that is, when there exists an effective $\mathbb{Q}$-divisor $\Delta$ such that $-(K_X+\Delta)$ is ample and the pair $(X,\Delta)$ is klt. For these blow-ups, we produce explicit boundary divisors $\Delta$ making $X$ log Fano.Comment: 30 page

On the Fano variety of linear spaces contained in two odd-dimensional quadrics

In this paper we describe the geometry of the 2m-dimensional Fano manifold G parametrizing (m-1)-planes in a smooth complete intersection Z of two quadric hypersurfaces in the complex projective space P^{2m+2}, for m>0. We show that there are exactly 2^{2m+2} distinct isomorphisms in codimension one between G and the blow-up of P^{2m} at 2m+3 general points, parametrized by the 2^{2m+2} distinct m-planes contained in Z, and describe these rational maps explicitly. We also describe the cones of nef, movable and effective divisors of G, as well as their dual cones of curves. Finally, we determine the automorphism group of G. These results generalize to arbitrary even dimension the classical description of quartic del Pezzo surfaces (m=1).Comment: 31 pages. Minor revision, to appear in Geometry & Topolog

On codimension 1 del Pezzo foliations on varieties with mild singularities

In this paper we extend to the singular setting the theory of Fano foliations developed in our previous paper. A Q-Fano foliation on a complex projective variety X is a foliation F whose anti-canonical class is an ample Q-Cartier divisor. In the spirit of Kobayashi-Ochiai Theorem, we prove that under some conditions the index i of a Q-Fano foliation is bounded by the rank r of F, and classify the cases in which i=r. Next we consider Q-Fano foliations F for which i=r-1. These are called del Pezzo foliations. We classify codimension 1 del Pezzo foliations on mildly singular varieties.Comment: 20 pages, to appear in Mathematische Annale

Rational Curves on Varieties

The aim of these notes is to give a introduction to the ideas and techniques of handling rational curves on varieties. The main emphasis is on varieties with many rational curves which are the higher dimensional analogs of rational curves and surfaces. The first six sections closely follow the lectures of J. Koll\'ar given at the summer school ``Higher dimensional varieties and rational points''. The notes were written up by C. Araujo and substantially edited later. Revised version with 2 new sections