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Mihai Paun

Publications and source records attributed to Mihai Paun.

29 records · Page 2Linked to original sources
arXiv2008

Relative critical exponents, non-vanishing and metrics with minimal singularities

In this article we prove a non-vanishing statement, as well as several properties of metrics with minimal singularities of adjoint bundles. Our arguments involve many ideas from Y.-T. Siu's analytic proof of the finite generation of the canonical ring. An important technical tool is the notion of relative critical exponent of two closed positive currents with respect to a measure.

math.CV↗
arXiv2008

A Bergman kernel proof of the Kawamata subadjunction theorem

The main purpose of the following article is to give a proof of Y. Kawamata's celebrated subadjunction theorem in the spirit of our previous work on Bergman kernels. We will use two main ingredients : an $\displaystyle L^{2\over m}$--extension theorem of Ohsawa-Takegoshi type (which is also a new result) and a more complete version of our former results.

math.AG↗
arXiv2008

Bergman kernels and the pseudoeffectivity of relative canonical bundles

The main result of the present article is a (practically optimal) criterium for the pseudoeffectivity of the twisted relative canonical bundles of surjective projective maps. Our theorem has several applications in algebraic geometry; to start with, we obtain the natural analytic generalization of some semipositivity results due to E. Viehweg and F. Campana. As a byproduct, we give a simple and direct proof of a recent result due to C. Hacon--J. McKernan, S. Takayama and H. Tsuji concerning the extension of twisted pluricanonical forms. More applications will be offered in the sequel of this article.

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↗
arXiv2005

Nondeformability of entire curves in projective hypersurfaces of high degree

In this article, we prove that there does not exist a family of entire curves in the universal family of hypersurfaces of degree $d\geq 2n$ in the complex projective space ${\mathbb P}^n$. This can be seen as a weak version of the Kobayashi conjecture asserting that a general projective hypersurface of high degree is hyperbolic in the sense of Kobayashi.

math.AG↗
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

The pseudo-effective cone of a compact Kähler manifold and varieties of negative Kodaira dimension

We prove that a holomorphic line bundle on a projective manifold is pseudo-effective if and only if its degree on any member of a covering family of curves is non-negative. This is a consequence of a duality statement between the cone of pseudo-effective divisors and the cone of ``movable curves'', which is obtained from a general theory of movable intersections and approximate Zariski decomposition for closed positive (1,1)-currents. As a corollary, a projective manifold has a pseudo-effective canonical bundle if and only if it is is not uniruled. We also prove that a 4-fold with a canonical bundle which is pseudo-effective and of numerical class zero in restriction to curves of a covering family, has non negative Kodaira dimension.

math.AG↗
arXiv2001

Numerical characterization of the Kähler cone of a compact Kähler manifold

The goal of this work is give a precise numerical description of the Kähler cone of a compact Kähler manifold. Our main result states that the Kähler cone depends only on the intersection form of the cohomology ring, the Hodge structure and the homology classes of analytic cycles: if $X$ is a compact Kähler manifold, the Kähler cone $\cK$ of $X$ is one of the connected components of the set $\cP$ of real $(1,1)$ cohomology classes $\{α\}$ which are numerically positive on analytic cycles, i.e. $\int_Yα^p>0$ for every irreducible analytic set $Y$ in $X$, \hbox{$p=\dim Y$}. This result is new even in the case of projective manifolds, where it can be seen as a generalization of the well-known Nakai-Moishezon criterion, and it also extends previous results by Campana-Peternell and Eyssidieux. The principal technical step is to show that every nef class $\{α\}$ which has positive highest self-intersection number $\int_Xα^n>0$ contains a Kähler current; this is done by using the Calabi-Yau theorem and a mass concentration technique for Monge-Ampère equations. The main result admits a number of variants and corollaries, including a description of the cone of numerically effective $(1,1)$ classes and their dual cone. Another important consequence is the fact that for an arbitrary deformation $\cX\to S$ of compact Kähler manifolds, the Kähler cone of a very general fibre $X_t$ is ``independent'' of $t$, i.e.\ invariant by parallel transport under the $(1,1)$-component of the Gauss-Manin connection.

math.AG↗