Uniruled varieties with split tangent bundle

非实性的一类单直纹簇

Non-solidity of uniruled varieties

本文研究维数至少为2的单直纹簇的非实性条件，为Abban-Okada关于Fano三维簇实性的猜想提供新证据，并构造了加权射影空间中高余维Fano三维簇的显式双有理链。

Symplectic Criteria on Stratified Uniruledness of Affine Varieties and Applications

We develop criteria for affine varieties to admit uniruled subvarieties of certain dimensions. The measurements are from long exact sequences of versions of symplectic cohomology, which is a Hamiltonian Floer theory for some open symplectic manifolds including affine varieties. Symplectic cohomology is hard to compute, in general. However, certain vanishing and invariance properties of symplectic cohomology can be used to prove that our criteria for finding uniruled subvarieties hold in some cases. We provide applications of the criteria in birational geometry of log pairs in the direction of …

On varieties which are uniruled by lines

Using the $\sharp$-minimal model program of uniruled varieties we show that for any pair $(X, \H)$ consisting of a reduced and irreducible variety $X$ of dimension $k \geq 3$ and a globally generated big line bundle $\H$ on $X$ with $d:= \H^k$ and $n:= h^0(X, \H)-1$ such that $d<2(n-k)-4$, then $X$ is uniruled of $\H$-degree one, except if $(k,d,n)=(3,27,19)$ and a ${\sharp}$-minimal model of $(X, \H)$ is $(\PP^3,Ø_{\PP^3}(3))$. We also show that the bound is optimal for threefolds.

Polarized endomorphisms of uniruled varieties (with Appendix by Y. Fujimoto and N. Nakayama)

We show that polarized endomorphisms of rationally connected threefolds with at worst terminal singularities are equivariantly built up from those on Q-Fano threefolds, Gorenstein log del Pezzo surfaces and P^1. Similar results are obtained for polarized endomorphisms of uniruled threefolds and fourfolds. As a consequence, we show that every smooth Fano threefold with a polarized endomorphism of degree > 1, is rational.

Finite groups of bimeromorphic selfmaps of non-uniruled Kähler threefolds

We prove the Jordan property for groups of bimeromorphic selfmaps of three-dimensional compact Kähler varieties of non-negative Kodaira dimension and positive irregularity.

On Nonvanishing for uniruled log canonical pairs

We prove the Nonvanishing conjecture for uniruled projective log canonical pairs of dimension $n$, assuming the Nonvanishing conjecture for smooth projective varieties in dimension $n-1$. We also show that the existence of good minimal models for non-uniruled projective klt pairs in dimension $n$ implies the existence of good minimal models for projective log canonical pairs in dimension $n$.

Birationality of the tangent map for minimal rational curves

For a uniruled projective manifold, we prove that a general rational curve of minimal degree through a general point is uniquely determined by its tangent vector. As applications, among other things we give a new proof, using no Lie theory, of our earlier result that a holomorphic map from a rational homogeneous space of Picard number 1 onto a projective manifold different from the projective space must be a biholomorphic map.

Birational geometry of defective varieties

Here we investigate the birational geometry of projective varieties of arbitrary dimension having defective higher secant varieties. We apply the classical tool of tangential projections and we determine natural conditions for uniruledness, rational connectivity, and rationality.

On the uniformity of the Iitaka fibration

We study pluricanonical systems on smooth projective varieties of positive Kodaira dimension, following the approach of Hacon-McKernan, Takayama and Tsuji succesfully used in the case of varieties of general type. We prove a uniformity result for the Iitaka fibration of smooth projective varieties of positive Kodaira dimension, provided that the base of the Iitaka fibration is not uniruled, the variation of the fibration is maximal, and the generic fiber has a good minimal model.

Algebraic geometry and projective differential geometry, Seoul National University concentrated lecture series, 1997

This is an expanded and updated version of a lecture series I gave at Seoul National University in September 1997. It is in some sense an update of the 1979 Griffiths and Harris paper with a similar title. I discuss: Homogeneous varieties, Topology and consequences Projective differential invariants, Varieties with degenerate Gauss images, When can a uniruled variety be smooth?, Dual varieties, Linear systems of bounded and constant rank, Secant and tangential varieties, Systems of quadrics with tangential defects, Recognizing uniruled varieties, Recognizing intersections of quadrics, Recogniz…

Classes de Chern des variétés uniréglées

We prove a Miyaoka-Yau type inequality for uniruled projective varieties.

Jordan groups, conic bundles and abelian varieties

A group $G$ is called Jordan if there is a positive integer $J=J_G$ such that every finite subgroup $\mathcal{B}$ of $G$ contains a commutative subgroup $\mathcal{A}\subset \mathcal{B}$ such that $\mathcal{A}$ is normal in $\mathcal{B}$ and the index $[\mathcal{B}:\mathcal{A}] \le J$ (V.L. Popov). In this paper we deal with Jordaness properties of the groups $Bir(X)$ of birational automorphisms of irreducible smooth projective varieties $X$ over an algebraically closed field of characteristic zero. It is known (Yu. Prokhorov - C. Shramov) that $Bir(X)$ is Jordan if $X$ is non-uniruled. On the …

On Varieties with trivial logarithmic tangent bundle

We characterize Kaehler manifolds with trivial logarithmic tangent bundle (with respect to a divisor D) as a class of certain compatifications of complex semi-tori.

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.

具有伪有效典范除子的簇及其小平维数

Varieties with pseudoeffective canonical divisor and their Kodaira dimension

本文研究复数域上光滑射影簇，围绕Shigefumi Mori提出的“簇为单直纹的当且仅当其小平维数为负”的猜想展开探讨，属于代数几何领域的理论研究。

On the symplectic invariance of log Kodaira dimension

Suppose that A and B are symplectomorphic smooth affine varieties. If A is acylic of dimension 2 then B has the same log Kodaira dimension as A. If the dimension of A is 3, has log Kodaira dimension 2 and satisfies some other conditions then B cannot be of log general type. We also show that if A and B are symplectomorphic affine varieties of any dimension then any compactification of A by a projective variety is uniruled if and only if any such compactification of B is uniruled.

Comment on: Pseudo-effectivity of the relative canonical divisor ... by Zsolt Patakfalvi

We discuss an elementary approach to the existence of non-uniruled ramified covers of varieties.

具有伪有效切丛的光滑射影曲面

Smooth projective surfaces with pseudo-effective tangent bundles

该数学代数几何研究刻画了非单直纹光滑射影曲面切丛伪有效的等价条件，并探讨了非有理直纹曲面吹胀的相关性质。

极小有理曲线邻域的几何

Geometry of Neighborhoods of Minimal Rational Curves

本文综述了单有理射影流形上极小有理曲线的芽等价问题，重点研究了当极小有理切线的相伴簇构成射影簇的等平凡族时，Zariski开子集上自然出现的G-结构及其与几何结构的微分几何的相互作用，并讨论了极小有理曲线芽等价的形式原理。

Existence of rational curves on algebraic varieties, minimal rational tangents, and applications

This survey paper discusses some of the recent progress in the study of rational curves on algebraic varieties. It was written for the survey volume of the priority programme "Global Methods in Complex Geometry", supported by the DFG. To start, we discuss existence results for rational curves on foliated manifolds. We construct the tangent morphism, define the variety of minimal rational tangents (VMRT) and name a number of results that show that many of the geometry properies of a uniruled variety are encoded in the projective geometry of the VMRT. The results are then applied in two differen…

关于具有伪有效切层的射影簇的极小模型计划

On the minimal model program for projective varieties with pseudo-effective tangent sheaf

本文发展正规射影簇上伪有效层的理论，并通过运行极小模型计划，证明具有伪有效切层的射影klt簇可分解为Fano簇和Q-阿贝尔簇。

Smooth projective horospherical varieties with nef tangent bundles

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

Toric varieties with ample tangent bundle

We give a simple combinatorial proof of the toric version of Mori's theorem that the only $n$-dimensional smooth projective varieties with ample tangent bundle are the projective spaces $\mathbb{P}^n$.

Semi-stability of the tangent sheaf of singular varieties

The main goal of this paper is to prove the polystability of the logarithmic tangent sheaf $\mathscr T_X(-D)$ of a log canonical pair $(X,D)$ whose canonical bundle $K_X+D$ is ample, generalizing in a significant way a theorem of Enoki. We apply this result and the techniques involved in its proof to get a version of this theorem for stable varieties (the higher dimensional analogue of Deligne-Mumford's stable curves) and to prove the polystability with respect to any polarization of the tangent sheaf of a singular Calabi-Yau variety.

射影簇切层的正性——MRC纤维化的结构

Positivity of tangent sheaves of projective varieties -- the structure of MRC fibrations

本文将光滑射影簇nef切丛的结构定理推广到射影klt簇，证明其切层正曲率或几乎nef时，在有限拟平展覆盖下存在到阿贝尔簇的有理连通纤维化，并发展了凝聚层正性理论。

On tangent cones at infinity of algebraic varieties

In this paper, we establish the following version at infinity of Whitney's theorem [6, 7]: Geometric and algebraic tangent cones at infinity of complex algebraic varieties coincide. The proof of this fact is based on a geometric characterization of geometric tangent cones at infinity and using the global Łojasiewicz inequality with explicit exponents for complex algebraic varieties. We also show that tangent cones at infinity of complex algebraic varieties can be computed using Gröbner bases.

Equivalence problem for minimal rational curves with isotrivial varieties of minimal rational tangents

We formulate the equivalence problem, in the sense of E. Cartan, for families of minimal rational curves on uniruled projective manifolds. An important invariant of this equivalence problem is the variety of minimal rational tangents. We study the case when varieties of minimal rational tangents at general points form an isotrivial family. The main question in this case is for which projective variety $Z$, a family of minimal rational curves with $Z$-isotrivial varieties of minimal rational tangents is locally equivalent to the flat model. We show that this is the case when $Z$ satisfies certa…

On varieties with trivial tangent bundle

I prove a crystalline characterization of abelian varieties in characteristic $p>0$ amongst the class of varieties with trivial tangent bundle. I show using my characterization that a smooth, projective, ordinary variety with trivial tangent bundle is an abelian variety if and only if its second crystalline cohomology is torsion free. I also show that a conjecture of Ke-Zheng Li about characteristic $p>0$ varieties with trivial tangent bundles is true for surfaces. I give a new proof of a result of Li and prove a refinement of it. Based on my characterization of abelian varieties I propose mod…

关于具有伪有效切层的紧致Kähler流形的一个注记

A note on compact Kähler varieties with pseudo-effective tangent sheaf

本文证明具有商奇点且切层伪有效的紧致Kähler流形经有限拟平展覆盖后平坦纤维化为复环面，纤维有理连通，并在klt情形及射影、低维情形下推广。

Varieties with Ample Tangent Sheaves

This paper generalises Mori's famous theorem about "Projective manifolds with ample tangent bundles" to normal projective varieties in the following way: A normal projective variety over $\mathbb{C}$ with ample tangent sheaf is isomorphic to the complex projective space.

Simplicity of tangent bundles of smooth horospherical varieties of Picard number one

Recently, Kanemitsu has discovered a counterexample to the long-standing conjecture that the tangent bundle of a Fano manifold of Picard number one is (semi)stable. His counterexample is a smooth horospherical variety. There is a weaker conjecture that the tangent bundle of a Fano manifold of Picard number one is simple. We prove that this weaker conjecture is valid for smooth horospherical varieties of Picard number one. Our proof follows from the existence of an irreducible family of unbendable rational curves whose tangent vectors span the tangent spaces of the horospherical variety at gene…

具有nef切丛的簇的Hilbert不可约性

Hilbert irreducibility for varieties with a nef tangent bundle

本文证明了具有Hilbert型性质的簇的纤维化性质，并将其应用于研究具有nef切丛的簇上的有理点。

Tensor ranks on tangent developable of Segre varieties

We describe the stratification by tensor rank of the points belonging to the tangent developable of any Segre variety. We give algorithms to compute the rank and a decomposition of a tensor belonging to the secant variety of lines of any Segre variety. We prove Comon's conjecture on the rank of symmetric tensors for those tensors belonging to tangential varieties to Veronese varieties.

Fano manifolds and stability of tangent bundles

We determine the stability/instability of the tangent bundles of the Fano varieties in a certain class of two orbit varieties, which are classified by Pasquier in 2009. As a consequence, we show that some of these varieties admit unstable tangent bundles, which disproves a conjecture on stability of tangent bundles of Fano manifolds.

On exterior powers of the tangent bundle on toric varieties

We study the positivity of exterior powers of the tangent sheaf on toric varieties in order to generalize results by Campana and Peternell about 3-folds with nef second exterior power of the tangent bundle. Using the theory of equivariant vector bundles and the toric MMP, we establish in the smooth case a criterion for the positivity of $Λ^m\mathcal T_X$ in terms of wall relations. As an application, we classify smooth toric varieties of arbitrary dimension $n\ge3$ with $Λ^2\mathcal T_X$ nef and those of dimension $n\ge 4$ with $Λ^3\mathcal T_X$ ample.

具有分裂切 sheaf 的 Fano 变种

Fano varieties with split tangent sheaf

研究了具有分裂切 sheaf 的 Fano 变种，证明其直接因子代数可积并诱导乘积结构。

Combinatorial Tangent Space and Rational Smoothness of Schubert Varieties

Following Contou-Carrere [CC], we consider the Bott-Samelson resolution of a Schubert variety as a variety of galleries in the Tits building associated to the situation. We prove that the rational smoothness of a Schubert variety can be expressed in terms of a subspace of the Zariski tangent space called, the combinatorial tangent space. For this, we use a characterization of rational smoothness of a Schubert variety introduced by Carrell and Peterson [CP].

Tangential varieties of Segre-Veronese surfaces are never defective

We compute the dimensions of all the secant varieties to the tangential varieties of all Segre-Veronese surfaces. We exploit the typical approach of computing the Hilbert function of special 0-dimensional schemes on projective plane by using a new degeneration technique.

绝妙对称簇上极小有理切线簇的图表示

Diagrams for varieties of minimal rational tangents on the wonderful symmetric varieties

本研究针对绝妙对称簇上的极小有理切线簇的表示问题，通过带标记Dynkin图进行刻画，证明当限制根系非A型时，由极小有理切线簇不可约分支得到的带标记Dynkin图与由带标记Kac图标记相邻节点得到的图一致。

The varieties of tangent lines to hypersurfaces in projective spaces

For a hypersurface in a projective space, we consider the set of pairs of a point and a line in the projective space such that the line intersects the hypersurface at the point with a fixed multiplicity. We prove that this set of pairs forms a smooth variety for a general hypersurface.

Stability of tangent bundles on smooth toric Picard-rank-2 varieties and surfaces

We give a combinatorial criterion for the tangent bundle on a smooth toric variety to be stable with respect to a given polarisation in terms of the corresponding lattice polytope. Furthermore, we show that for a smooth toric surface and a smooth toric variety of Picard rank 2, there exists an ample line bundle with respect to which the tangent bundle is stable if and only if it is an iterated blow-up of projective space.

Projective varieties with nef tangent bundle in positive characteristic

Let $X$ be a smooth projective variety defined over an algebraically closed field of positive characteristic $p$ whose tangent bundle is nef. We prove that $X$ admits a smooth morphism $X \to M$ such that the fibers are Fano varieties with nef tangent bundle and $T_M$ is numerically flat. We also prove that extremal contractions exist as smooth morphisms. As an application, we prove that, if the Frobenius morphism can be lifted modulo $p^2$, then $X$ admits, up to a finite étale Galois cover, a smooth morphism onto an ordinary abelian variety whose fibers are products of projective spaces.

Mori geometry meets Cartan geometry: Varieties of minimal rational tangents

We give an introduction to the theory of varieties of minimal rational tangents, emphasizing its aspect as a fusion of algebraic geometry and differential geometry, more specifically, a fusion of Mori geometry of minimal rational curves and Cartan geometry of cone structures.

A 型弓形簇中的切权与不变曲线

Tangent Weights and Invariant Curves in Type A Bow Varieties

本文对 A 型 Cherkis 弓形簇中的环面不变曲线进行了完整分类，提出基于蝶形图、杨图和二元列联表的组合编码，并建立新的切权公式，进而用于计算弓形簇的 1-骨架。

On stability of tangent bundle of toric varieties

Let $X$ be a nonsingular complex projective toric variety. We address the question of semi-stability as well as stability for the tangent bundle $T{X}$. In particular, a complete answer is given when $X$ is a Fano toric variety of dimension four with Picard number at most two, complementing earlier work of Nakagawa. We also give an infinite set of examples of Fano toric varieties for which $TX$ is unstable; the dimensions of this collection of varieties are unbounded. Our method is based on the equivariant approach initiated by Klyachko and developed further by Perling and Kool.

标量包的点希尔伯特方案和区分光滑变种的乘积

Tangent bundle of punctual Hilbert scheme and distinguishing products of varieties

研究光滑项目表面点希尔伯特方案的切包不可分解成分，并证明关于光滑项目表面点希尔伯特方案乘积分类的猜想，同时确定两个对称幂乘积的同构条件。

Positivity of the exterior power of the tangent bundles

Let $X$ be a complex smooth projective variety such that the exterior power of the tangent bundle $\bigwedge^{r} T_X$ is nef for some $1\leq r<\dim X$. We prove that, up to an étale cover, $X$ is a Fano fiber space over an Abelian variety. This gives generalizations of the structure theorem of varieties with nef tangent bundle by Demailly, Peternell and Schneider and that of varieties with nef $\bigwedge^{2} T_X$ by the author. Our result also gives an answer to a question raised by Li, Ou and Yang for varieties with strictly nef $\bigwedge^{r} T_X$ when $r < \dim X$.

Normalized tangent bundle, varieties with small codegree and pseudoeffective threshold

We propose a conjectural list of Fano manifolds of Picard number $1$ with pseudoeffective normalized tangent bundles, which we prove in various situations by relating it to the complete divisibility conjecture of Russo and Zak on varieties with small codegree. Furthermore, the pseudoeffective thresholds and hence the pseudoeffective cones of the projectivized tangent bundles of rational homogeneous spaces of Picard number $1$ are explicitly determined by studying the total dual VMRT and the geometry of stratified Mukai flops. As a by-product, we obtain sharp vanishing theorems on the global tw…

Syzygies of tangent developable surfaces and K3 carpets via secant varieties

We give simple geometric proofs of Aprodu-Farkas-Papadima-Raicu-Weyman's theorem on syzygies of tangent developable surfaces of rational normal curves and Raicu-Sam's result on syzygies of K3 carpets. As a consequence, we obtain a quick proof of Green's conjecture for general curves of genus $g$ over an algebraically closed field $\mathbf{k}$ with $\operatorname{char}(\mathbf{k}) = 0$ or $\operatorname{char}(\mathbf{k}) \geq \lfloor (g-1)/2 \rfloor$. We also show the arithmetic normality of tangent developable surfaces of arbitrary smooth projective curves of large degree.

On tangent cones to Schubert varieties in type $E$

We consider tangent cones to Schubert subvarieties of the flag variety $G/B$, where $B$ is a Borel subgroup of a reductive complex algebraic group $G$ of type $E_6$, $E_7$ or $E_8$. We prove that if $w_1$ and $w_2$ form a good pair of involutions in the Weyl group $W$ of $G$ then the tangent cones $C_{w_1}$ and $C_{w_2}$ to the corresponding Schubert subvarieties of $G/B$ do not coincide as subschemes of the tangent space to $G/B$ at the neutral point.

Picard数为1的Fano簇上切丛典范扩张的稳定性

Stability of the canonical extension of tangent bundles on Picard-rank-1 Fano varieties

本文研究Picard数为1的Fano簇上切丛典范扩张的斜率稳定性，证明在指数整除维数或维数加一时切丛稳定性蕴含典范扩张的（半）稳定性，并给出其在曲线稳定向量丛模空间上的应用推论。

具有大量微分形式的代数簇的丰度

Abundance for varieties with many differential forms

假设低维极小模型纲领成立，证明具有温和奇点且存在大量带 pluricanonical 丛系数自反微分形式的代数簇$X$满足丰度猜想，推出埃尔米特半正典范除子几乎半丰富等结论，且$K_X$数值维数为1时结果无条件成立，还处理 Calabi-Yau 簇上 nef 线丛半丰富性问题。

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 …

Canonical heights on hyper-Kähler varieties and the Kawaguchi-Silverman conjecture

The Kawaguchi--Silverman conjecture predicts that if $f\colon X \dashrightarrow X$ is a dominant rational-self map of a projective variety over $\overline{\mathbb{Q}}$, and $P$ is a $\overline{\mathbb{Q}}$-point of $X$ with Zariski-dense orbit, then the dynamical and arithmetic degrees of $f$ coincide: $λ_1(f) = α_f(P)$. We prove this conjecture in several higher-dimensional settings, including all endomorphisms of non-uniruled smooth projective threefolds with degree larger than $1$, and all endomorphisms of hyper-Kähler varieties in any dimension. In the latter case, we construct a canonical…