arXiv subjects

Frederic Campana

Publications and source records attributed to Frederic Campana.

29 records · Page 2Linked to original sources

arXiv2006

Fibrations meromorphes sur certaines varietes de classe canonique triviale

Motivated by an example, due to Voisin, of a smooth simply-connected projective variety with trivial canonical class and cyclic Picard group, admitting a meromorphic endomorphism of high degree, we study meromorphic fibrations on certain varieties with trivial canonical class. We show (theorem 2.1) that any nontrivial meromorphic fibration of a variety with trivial canonical class and cyclic Picard group is in varieties of general type. Therefore, Voisin's endomorphism cannot preserve a fibration, at least in the generic case. As this example is symplectic, we study meromorphic fibrations on symplectic varieties. In dimension 4 (and in all dimensions modulo minimal model program) we get some results similar to Matsushita's. The main result here is theorem 3.6. We also study the relations between meromorphic endomorphisms and meromorphic fibrations. In particular, we prove that if an endomorphism does not preserve a fibration, then its general iterated orbit is Zariski-dense. We show that some power of an endomorphism preserves the "core" (a natural fibration constructed by the second author), and ask the similar question about the Iitaka fibration.

math.AG↗

arXiv2006

Varietes faiblement speciales a courbes entieres degenerees

We show that certain non-special but weakly special threefolds $X$ constructed by Bogomolov-Tschinkel enjoy strong complex hyperbolicity properties: their entire curves are algebraically degenerate and lie either on a fixed divisor or on the fiber of the unique elliptic fibration on $X$. These properties are consistent with the conjectural link between hyperbolicity and arithmetics of projective manifolds. The arithmetic counterpart of this hyperbolicity property (the non-potential density of $X$, object of two conflicting conjectures) remains however open.

math.AG↗

arXiv2006

A Brody theorem for orbifolds

We study the Kobayashi pseudodistance for orbifolds, proving an orbifold version of Brody's theorem and classifying which one-dimensional orbifolds are hyperbolic.

math.CV↗

arXiv2005

Une généralisation du théorème de Kobayashi-Ochiai

Let $ϕ:\Bbb C^n\to X$ a holomorphic map to an $n$-dimensional connected compact complex manifold $X$. We establish links between the positivity properties of the canonical bundle of $X$ and the rate of growth of $ϕ$ which extend results of Kodaira and Kobayashi-Ochiai. For example: if the average degree of $ϕ$ on balls of radius $r$ grows slowlier than $r^{2n}$, then $K_X$ is not pseudo-effective. If $X$ is moreover projective, it is uniruled. Assuming now that $K_X$ is pseudoeffective, of numerical dimension $ν$, we show that the characteristic function of $ϕ$ grows at least as fast as $r^{((2n)/(n-ν))}$.

math.AG↗

arXiv2004

Special Varieties and classification Theory

A new class of compact Kähler manifolds, called special, is defined, which are the ones having no surjective meromorphic map to an orbifold of general type. The special manifolds are in many respect higher-dimensional generalisations of rational and elliptic curves. For example, we show that being rationally connected or having vanishing Kodaira dimension implies being special. Moreover, for any compact Kähler $X$ we define a fibration $c_X:X\to C(X)$, which we call its core, such that the general fibres of $c_X$ are special, and every special subvariety of $X$ containing a general point of $X$ is contained in the corresponding fibre of $c_X$. We then conjecture and prove in low dimensions and some cases that: 1) Special manifolds have an almost abelian fundamental group. 2) Special manifolds are exactly the ones having a vanishing Kobayashi pseudometric. 3) The core is a fibration of general type, which means that so is its base $C(X)$,when equipped with its orbifold structure coming from the multiple fibres of $c_X$. 4) The Kobayashi pseudometric of $X$ is obtained as the pull-back of the orbifold Kobayashi pseudo-metric on $C(X)$, which is a metric outside some proper algebraic subset. 5) If $X$ is projective,defined over some finitely generated (over $\Bbb Q$) subfield $K$ of the complex number field, the set of $K$-rational points of $X$ is mapped by the core into a proper algebraic subset of $C(X)$. These two last conjectures are the natural generalisations to arbitrary $X$ of Lang's conjectures formulated when $X$ is of general type.

math.AG↗

arXiv2004

Orbifoldes à premi\ere classe de Chern nulle

An orbifold version of Bogomolov decomposition theorem is established for compact Kähler spaces with quotient singularities and first Chern class zero.The proof is a direct adaptation of the classical smooth case, using Ricci-flat Kähler metrics, and Cheeger-Gromoll splitting theorem. It implies that normal K3 surfaces are uniformised in the orbifold sense either by normal K3 surfaces or by a complex torus, as conjectured by D.Q. Zhang. It also implies some conjectures on "special" threefolds raised in math.AG/0110051

math.AG↗

arXiv2004

Orbifoldes, variétés spéciales et classification des variétés K\" ahlériennes compactes

This text is an introduction to math.AG/0110051 (to appear in Ann. Inst. Fourier), and describes a canonical decomposition of compact Kähler manifolds $X$ first by means of their "core", the unique fibration on $X$ with fibres special, and orbifold base of general type. Being special means that there does not exist a fibration onto an orbifold of general type. The core is next decomposed into a tower of fibrations with all orbifold fibres either with $κ=0$, or rationally generated (a weak version of rationally connected).

math.AG↗

arXiv2002

Generalized Tsen theorem and rationally connected Fano fibrations

We prove that a fibration X \to \Bbb P_1, the general fiber of which is a smooth Fano threefold, is rationally connected. The proof is based on a generalization of Tsen's classical theorem: a fibration X/C over a curve the general fiber of which is a Fano complete intersection in a product of weighted projective spaces has a section.

math.AG↗

arXiv2001

A reduction map for nef line bundles

In a recent preprint, H. Tsuji gave a number of interesting assertions on the structure of pseudo-effective line bundles on projective manifolds. In particular, he postulated the existence of an almost-holomorphic "reduction map", whose fibers are maximal subvarieties on which the line bundle is numerically trivial. The purpose of this note is to give a simple, algebraic proof of the existence of a reduction map in the case where the line bundle is nef.

math.AG↗

arXiv2001

Projective manifolds with splitting tangent bundles, I

We investigate the universal cover of projective threefolds whose tangent bundle is a direct sum of subbundles in case the Kodaira dimension is not 1 and 2. We also prove results on Fano manifolds with splitting tangent bundles in any dimension.

math.AG↗

arXiv1998

Holomorphic 2-forms on Complex Threefolds

We study holomorphic 2-forms on projective (or compact Kaehler) threefolds not of general type and prove that in almost all cases the 2-form is created by some standard process. This means roughly that every 2-form is induced by a meromorphic map to a surface, a torus or a symplectic manifold. If the 2-form has only finitely many zeroes, more precise results hold. Finally we prove that compact Kaehler threefolds with negative Kodaira dimension are uniruled unles they are simple, i.e. there is no positive dimensional compact subvariety through the general point of the manifold (these simple threefolds are expected not to exist). This generalises the fundamental result of Miyaoka and Mori to the Kaehler (non-simple) case.

math.AG↗