Nef reduction and anticanonical bundles

Threefolds with big and nef anticanonical bundles I

We start the classification of smooth projective threefolds X whose anticanonical bundles -K_X are big and nef but not ample. In this paper we treat the case b_2(X) = 2 and the morphism associated with the base point free linear system |-mK_X|, m>>0, is divisorial.

On the canonical threefolds with strictly nef anticanonical divisors

Any ample Cartier divisor D on a projective variety X is strictly nef (i.e. D.C>0 for any effective curve C on X). In general, the converse statement does not hold. But this is conjectured to be true for anticanonical divisors. The present paper establishes this fact for normal complex projective threefolds with canonical singularities. This result extends several previously known special cases. The proof rests mainly on sophisticated techniques of three dimensional birational geometry developed in the last two decades.

正特征下具有 nef 反典范除数及相对维数为一的 Albanese 态射的簇

Varieties with nef anticanonical divisors and Albanese morphisms of relative dimension one in positive characteristic

本文研究正特征代数闭域上具有 nef 反典范除数的光滑射影簇，并在其到 Albanese 簇的态射相对维数为一时给出精确结构。

具有nef反典范丛的代数簇的非零问题

The Nonvanishing problem for varieties with nef anticanonical bundle

该研究针对具有nef反典范丛的代数簇非零问题，证明了带温和奇点的三维配对中，反典范除子的nef数值类必为有效类，推进了代数几何相关理论。

Cox环的nef反典范有理曲面

Cox rings of nef anticanonical rational surfaces

本文研究具有nef反典范类的光滑射影有理曲面的Cox环生成集，刻画生成元次数并应用于计算Halphen指数大于1的有理椭圆曲面的极小生成集。

A decomposition theorem for projective manifolds with nef anticanonical bundle

Let X be a simply connected projective manifold with nef anticanonical bundle. We prove that X is a product of a rationally connected manifold and a manifold with trivial canonical bundle. As an application we describe the MRC fibration of any projective manifold with nef anticanonical bundle.

Threefolds with nef anticanonical bundles

We prove that the Albanese map of a smooth projective threefold, whose anticanonical bundle is nef, is a surjective submersion. We also investigate morphisms of threefolds to curves and surfaces whose relative anticanonical bundle are nef.

Albanese maps of projective manifolds with nef anticanonical bundles

Let $X$ be a projective manifold such that the anticanonical bundle $-K_X$ is nef. We prove that the Albanese map $p: X \rightarrow Y$ is locally isotrivial. In particular, $p$ is a submersion.

Manifolds with nef anticanonical bundle

Let X be a compact Kähler manifold such that the anticanonical bundle $-K_X$ is nef. A classical conjecture claims that the Albanese map is submersive. We prove this conjecture if the general fibre is a weak Fano manifold. If X is projective we prove the conjecture also for fibres of dimension at most two.

具有 nef 反典范除子的射影簇的自同构群与齐次纤维化

Automorphism groups and Homogeneous fibrations on projective varieties with nef anticanonical divisors

本文研究具有 nef 反典范除子的代数簇的 Albanese 态射与极大有理链连通纤维化的齐次结构，构造自然群同态诱导 Aut^0 的 Chevalley 分解（相差等源），并类比 Nishi-Matsumura 定理，用现代语言重证之。

弱Fano簇的川又-宫冈型不等式

Kawamata--Miyaoka type inequality for varieties with nef anticanonical divisor

针对n≥2的Q-因子ε-lc弱Fano簇，证明了满足特定系数约束的川又-宫冈型不等式，丰富了代数几何中Fano簇的相关理论。

On the fundamental groups of effective compact Kähler orbifolds with nef anticanonical bundle

We show that the orbifold fundamental group of an effective compact K{ä}hler orbifold with nef anticanonical bundle has polynomial growth, which generalizes M.P ăun's results for manifolds [P ău97, Theorem 1,Theorem 2]

Albanese morphism of log smooth klt compact Kähler manifold with nef log anticanonical divisor

Let $(X, ω)$ be an n-dimensional compact Kähler manifold. Let $D=\sum (1-β_j) Y_j=\sum (1-β_j) [s_j=0]$ a divisor with simple normal crossings with $β_j \in ]0,1[$ such that $-(K_X+D)$ is nef. We show that its Albanese map is submersion outside an analytic set of codimension larger than two with connected fibres.

Algebraic Surfaces with Log-Terminal Singularities and nef Anticanonical Class and Reflection Groups in Lobachevsky Spaces. I (Basics of the Diagram Method)

In our preprint: "Algebraic surfaces with log-terminal singularities and nef anticanonical class and reflection groups in Lobachevsky spaces", Preprint Max-Planck-Institut für Mathematik, Bonn, (1989) MPI/89-28 (Russian), we had extended the diagram method to algebraic surfaces with nef anticanonical class (before, it was applied to Del Pezzo surfaces). The main problem here is that Mori polyhedron may not be finite polyhedral in this case. In the §4 of this preprint we had shown that this method does work for algebraic surfaces with nef anticanonical class and log terminal singularities. Rece…

Varieties with generically nef tangent bundles

We study the geometry of projective manifolds whose tangent bundles are nef on sufficiently general curves (i.e. the tangent bundle is generically nef) and show that manifolds whose anticanonical bundles are semi-ample have this property. Furthermore we introduce a notion of sufficient nefness and investigate the relation with manifolds whose anticanonical bundles are nef.

具有nef反典范除子的有理曲面的切丛的积极性

Positivity of the tangent bundle of rational surfaces with nef anticanonical divisor

本文研究了具有nef反典范除子的光滑射影有理曲面切丛的bigness性质，证明了有理椭圆曲面的切丛不是大的，并利用$(-2)$-曲线配置完全或部分确定了弱del Pezzo曲面切丛的bigness。

基本群与余正则性：具有平凡且数值有效反典范类的klt对

Fundamental Groups and Coregularity: Klt Pairs with Trivial and Nef Log-Anticanonical Class

该研究探讨射影klt对数对的绝对余正则性与基本群的关系，证明其基本群存在有限指数正规阿贝尔子群，结合相关定理得到任意维数下的余正则性界，还给出反典范类数值有效时orbifold基本群的性质。

On generic nefness of tangent sheaves

We show that the tangent bundle of a projective manifold with nef anticanonical class is generically nef. That is, its restriction to a curve cut out by general sufficiently ample divisors is a nef vector bundle. This confirms a conjecture of Peternell. As a consequence, the second Chern class of such a manifold has non-negative intersections with ample divisors. We also investigate under which conditions these positivities are strict, and answer a question of Yau.

Finiteness results for 3-folds with semiample anticanonical bundle

We prove some finiteness results on the movable cone for mildly singular 3-folds with semiample anticanonical bundle, giving some evidence for the Morrison--Kawamata cone conjecture for klt pairs.

Nef and semiample divisors on rational surfaces

In this paper we study smooth projective rational surfaces, defined over an algebraically closed field of any characteristic, with pseudo-effective anticanonical divisor. We provide a necessary and sufficient condition in order for any nef divisor to be semiample. We adopt our criterion to investigate Mori dream surfaces in the complex case.

Positivity of anticanonical divisors from the viewpoint of Fano conic bundles

We give the first examples of flat fiber type contractions of Fano manifolds onto varieties that are not weak Fano, and we prove that these morphisms are Fano conic bundles. We also review some known results about the interaction between the positivity properties of anticanonical divisors of varieties of contractions.

Exceptional and Rigid Sheaves on Surfaces with Anticanonical Class without Base Components

The paper consists of three parts. In the first of them different kinds stability are discussed. In particular, the stability concept with respect to nef divisor is introduced. A structure of rigid and superrigid vector bundles on smooth projective surfaces with nef anticanonical class is studied in the second part. We prove that any superrigid bundle has a unique exceptional filtration. In the last part we give constructive description of exceptional bundles on these surfaces.

具有 nef 反正则丛的 Kähler 流形的 Albanese 映射

Albanese map for Kähler manifolds with nef anticanonical bundle

本文研究具有 nef 反正则丛的 Kähler 流形的 Albanese 映射结构，证明一般纤维为 Calabi-Yau 时流形本身为 Calabi-Yau，并对 Cao–Höring 关于射影空间纤维情形的结果给出更拓扑的证明。

Structure of projective varieties with nef anticanonical divisor: the case of log terminal singularities

In this article we study the structure of klt projective varieties with nef anticanonical divisor (and more generally, varieties of semi-Fano type), especially the canonical fibrations associated to them. We show that: 1. the Albanese map for such variety is a locally constant fibration (that is, an analytic fibre bundle with connected fibres that $X$ is equal to the product of the universal cover of the Albanese torus by the fibre of the Albanese map quotient by a diagonal action of the fundamenatl group of the Albanese torus); 2. if the smooth locus is simply connected, the MRC fibration of …

Ulrich bundles on rational surfaces with an anticanonical pencil

Ulrich bundles are the simplest sheaves from the viewpoint of cohomology tables. Eisenbud and Schreyer conjectured that every projective variety carries an Ulrich bundle, which means it has the same cone of cohomology table as the projective space of same dimension. In this paper we show the existence of stable rank 2 Ulrich bundle on rational surfaces with an anticanonical pencil, under a mild Brill-Noether assumption by using Lazarsfeld-Mukai bundles. Also we see that each of those surfaces carries its Chow form given by the Pfaffian of skew-symmetric morphism coming from an Ulrich bundle.

反丰富丛、nef向量丛、big向量丛

Anti-ample bundle, nef vector bundle, big vector bundle

本文证明了反丰富向量丛的直像在有限平坦态射下保持反丰富性，并证明nef向量丛与nef且big向量丛的张量积仍为nef且big，推广了Schneider的结果。

Smooth projective horospherical varieties with nef tangent bundles

We show that smooth projective horospherical varieties with nef tangent bundles are rational homogeneous spaces.

Fano 5-folds with nef tangent bundles

We prove that Fano 5-folds with nef tangent bundles are rational homogeneous manifolds.

Arithmetically nef line bundles

Let $L$ be a line bundle on a scheme $X$, proper over a field. The property of $L$ being nef can sometimes be "thickened", allowing reductions to positive characteristic. We call such line bundles arithmetically nef. It is known that a line bundle $L$ may be nef, but not arithmetically nef. We show that $L$ is arithmetically nef if and only if its restriction to its stable base locus is arithmetically nef. Consequently, if $L$ is nef and its stable base locus has dimension $1$ or less, then $L$ is arithmetically nef.

Manifolds with nef cotangent bundle

Generalising a classical theorem by Ueno, we prove structure results for manifolds with nef or semiample cotangent bundle.

Ramified cover of varieties with nef cotangent bundle

We construct examples to show that having nef cotangent bundle is not preserved under finite ramified covers. Our examples also show that a projective manifold with Stein universal cover may not have nef cotangent bundle, disproving a conjecture of Liu-Maxim-Wang.

Fano n-folds with nef tangent bundle and Picard number greater than n-5

We prove that Fano n-folds with nef tangent bundle and Picard number greater than n-5 are rational homogeneous manifolds.

A smooth, complete threefold with no nontrivial nef line bundles

We present a smooth, complete toric threefold with no nontrivial nef line bundles. This is a counterexample to a recent conjecture of Fujino.

On projective space bundle with nef normalized tautological divisor

In this paper, we study the structure of projective space bundles whose relative anti-canonical line bundle is nef. As an application, we get a characterization of abelian varieties up to finite etale covering.

Seshadri constants on rational surfaces with anticanonical pencils

We study a Seshadri constant at a general point on a rational surface whose anticanonical linear system contains a pencil. First, we describe a Seshadri constant of an ample line bundle on such a rational surface explicitly by the numerical data of the ample line bundle. Secondly, we classify log del Pezzo surfaces which are special in terms of the Seshadri constants of the anticanonical divisors when the anticanonical degree is between 4 and 9.

Toric Fano manifolds with nef tangent bundles

In this note we prove that any toric Fano manifold with nef tangent bundle is a product of projective spaces. In particular, it implies that Campana-Peternell conjecture hold for toric manifolds.

Smooth projective toric varieties whose nontrivial nef line bundles are big

For any $n\geq 3$, we explicitly construct smooth projective toric $n$-folds of Picard number $\geq 5$, where any nontrivial nef line bundles are big.

Stability conditions on threefolds with nef tangent bundles

In this paper, we prove the Bogomolov-Gieseker type inequality conjecture for threefolds with nef tangent bundles. As a corollary, there exist Bridgeland stability conditions on these threefolds.

Strictly nef Bundles

In this short note we will show that every homogeneous strictly nef vector bundle on a complex flag variety is ample. Following this, we consider whether ampleness of a bundle on an abelian variety can be tested on curves.

Fano 5-folds with nef tangent bundles and Picard numbers greater than one

We prove that smooth Fano 5-folds with nef tangent bundles and Picard numbers greater than one are rational homogeneous manifolds.

Nef cone of flag bundles over a curve

Let $X$ be a smooth projective curve defined over an algebraically closed field $k$, and let $E$ be a vector bundle on $X$. We compute the nef cone of any flag bundle associated to $E$.

On foliations with nef anti-canonical bundle

In this paper we prove that the anti-canonical bundle of a holomorphic foliation $\mathcal{F}$ on a complex projective manifold cannot be nef and big if either $\mathcal{F}$ is regular, or $\mathcal{F}$ has a compact leaf. Then we address codimension one regular foliations whose anti-canonical bundle is nef with maximal Kodaira dimension.

Nefness of adjoint bundles for ample vector bundles of corank 3

Let $\mathcal{E}$ be an ample vector bundle of rank $r\geq 2$ on a smooth complex projective variety $X$ of dimension $n$. The aim of this paper is to describe the structure of pairs $(X,\mathcal{E})$ as above whose adjoint bundles $K_X+\det\mathcal{E}$ are not nef for $r=n-3$. Furthermore, we give some immediate consequences of this result in adjunction theory.

第一陈类为$(2,2)$的二次曲面上的Nef向量丛

Nef vector bundles on a quadric surface with first Chern class $(2,2)$

本研究针对特征零代数闭域上光滑二次曲面，完成了第一陈类为$(2,2)$的nef向量丛的分类工作，填补了相关代数几何研究的内容。

抛物 ample 和抛物 nef 向量丛的逆像与直接像

Pullback and Direct Image of Parabolic Ample and Parabolic Nef Vector Bundles

本文研究了抛物 ample 和抛物 nef 向量丛在有限满射映射下的逆像与直接像性质，证明其保持抛物 ample 和抛物 nef 的特性。

第一陈类为2的超二次曲面上的Nef向量丛

Nef vector bundles on a hyperquadric with first Chern class two

本研究在特征零的代数闭域下，完成了维数不小于4的光滑超二次曲面上第一陈类为2的nef向量丛的分类工作。

Generically nef vector bundles on ruled surfaces

The present paper concerns the invariants of generically nef vector bundles on ruled surfaces. By Mehta - Ramanathan Restriction Theorem and by Miyaoka characterization of semistable vector bundles on a curve, the generic nefness can be considered as a weak form of semistability. We establish a Bogomolov type inequality for generically nef vector bundles with nef general fiber restriction on ruled surfaces with no negative section. This gives an affermative answer in this case to a problem posed by Th. Peternell. Concerning ruled surfaces with a negative section, we prove a a similar result fo…

Generically nef vector bundles and geometric applications

The cotangent bundle of a non-uniruled projective manifold is generically nef, due to a theorem of Miyaoka. We show that the cotangent bundle is actually generically ample, if the manifold is of general type and study in detail the case of intermediate Kodaira dimension. Moreover, manifolds with generically nef and ample tangent bundles are investigated as well as connections to classical theorems on vector fields on projective manifolds.

Nef vector bundles on a projective space with first Chern class three

We classify nef vector bundles on a projective space with first Chern class three over an algebraically closed field of characteristic zero; we see, in particular, that these nef vector bundles are globally generated if the second Chern class is less than eight, and that there exist nef but non-globally generated vector bundles with second Chern class eight and nine on a projective plane.

Nef line bundles on Calabi-Yau threefolds, I

We prove that a nef line bundle $\mathcal L$ with $c_1(\mathcal L)^2 \ne 0$ on a Calabi-Yau threefold $X$ with Picard number $2$ and with $c_3(X) \ne 0$ is semiample, that is, some multiple of $\mathcal L$ is generated by global sections.

Nef and abundant divisors, semiampleness and canonical bundle formula

In this paper, we use canonical bundle formulas to prove some generalizations of an old theorem of Kawamata on the semiampleness of nef and abundant log canonical divisors. In particular, we show that for klt pairs $(X,B)$ with $K_X+B$ effective, $L \in Pic (X)$ nef, nefness and abundance of $K_X+B+L$ implies semiampleness. This essentially generalizes Kawamata's theorem to the setting of generalized abundance.

Base divisors of big and nef line bundles on irreducible symplectic varieties

Under some conditions on the deformation type, which we expect to be satisfied for arbitrary irreducible symplectic varieties, we describe which big and nef line bundles on irreducible symplectic varieties have base divisors. In particular, we show that such base divisors are always irreducible and reduced. This is applied to understand the behaviour of divisorial base components of big and nef line bundles under deformations and for K3$^{[n]}$-type and Kum$^n$-type.

Diagram Method for 3-Folds and its Application to Kahler Cone and Picard Number of Calabi-Yau 3-Folds, I. with Appendix by Vyacheslav V. Shokurov: "Anticanonical boundedness for curves"

We prove the general diagram method theorem valid for the quite large class of 3-folds with Q-factorial singularities (see Basic Theorem 1.3.2 and also Theorem 2.2.6). This gives the generalization of our results about Fano 3-folds with Q-factorial terminal singularities (Preprint alg-geom/9311007). As an application, we get the following result about 3-dimensional Calabi-Yau manifolds X: Assume that the Picard number ρ(X) > 93. Then one of two cases (i) or (ii) holds: (i) there exists a small extremal ray on X; (ii) There exists a nef element h such that h^3=0 (thus, the nef cone NEF(X) and t…

Nef vector bundles on a projective space with first Chern class 3 and second Chern class 8

We describe nef vector bundles on a projective space with first Chern class three and second Chern class eight over an algebraically closed field of characteristic zero by giving them a minimal resolution in terms of a full strong exceptional collection of line bundles.