On the derived category of a weighted projective threefold

关于具有二维消没循环空间的射影三维簇

On projective threefolds with two-dimensional space of vanishing cycles

本文完整分类了复数域上消没循环空间维数为2的光滑射影三维簇，以及光滑射影曲面上第二陈类为3的秩2非常充足向量束。

Derived category of V_{12} Fano threefolds

A V_{12} Fano threefold is a smooth Fano threefold X of index 1 with Pic X = Z and (-K_X)^3=12. We show that the bounded derived category of coherent sheaves on any V_{12} threefold X admits a semiorthogonal decomposition consisting of two exceptional bundles and of the derived category of a curve of genus 7. As an application we show that the Fano surface of X (the surface parameterizing conics on X) is canonically isomorphic to the symmetric square of the associated genus 7 curve.

Picard秩为2和3的光滑射影环面三维体中曲面的代数双曲性

Algebraic hyperbolicity for surfaces in smooth projective toric threefolds with Picard rank 2 and 3

本文研究Picard秩为2和3的光滑射影环面三维体中曲面的代数双曲性，利用组合描述与Haase和Ilten的方法，在这些簇中确定了众多代数双曲性曲面。

Effective non-vanishing conjectures for projective threefolds

Let X be a smooth projective threefold, and let A be an ample line bundle such that $K_X+A$ is nef. We show that if $K_X$ or $-K_X$ is pseudoeffective, the adjoint bundle $K_X+A$ has global sections. We also give a very short proof of the Beltrametti-Sommese conjecture in dimension three, recently proven by Fukuma: if A is an ample line bundle such that $K_X+2A$ is nef, the adjoint bundle $K_X+2A$ has global sections.

Complex projective threefolds with non-negative canonical Euler-Poincare characteristic

Let $V$ be a complex nonsingular projective 3-fold of general type with $χ(ω_V)\geq 0$ (resp. $>0$). We prove that the m-canonical map $Φ_{|mK_V|}$ is birational onto its image for all $m\ge 14$ (resp. $\geq 8$). Known examples show that the lower bound $r_3=14$ (resp. $=8$) is optimal.

Connecting certain rigid birational non-homeomorphic Calabi--Yau threefolds via Hilbert scheme

We shall give an explicit pair of birational projective Calabi--Yau threefolds which are rigid, non-homeomorphic, but are connected by projective flat deformation over some connected base scheme.

Projective Threefolds with Holomorphic Conformal Structure

The authors give a complete classification of projective threefolds admitting a holomorphic conformal structure. A Corollary is the complete list of projective threefolds, whose tangent bundle is a symmetric square.

Fujita-type freeness for quasi-log canonical threefolds

In this paper, we show that Fujita's basepoint-freeness conjecture for projective quasi-log canonical singularities holds true in dimension three. Immediately, we prove Fujita-type basepoint-freeness for projective semi-log canonical threefolds.

紧复三维流形上的全纯射影联络

Holomorphic projective connections on compact complex threefolds

本文分类复射影三维流形上的全纯射影联络，证明其若非平坦则必位于阿贝尔三维流形上，并排除平凡典范线丛的单连通紧复三维流形存在此类联络。

Calabi--Yau threefolds in $\mathbb{P}^6$

We study the geometry of $3$-codimensional smooth subvarieties of the complex projective space. In particular, we classify all quasi-Buchsbaum Calabi--Yau threefolds in projective $6$-space. Moreover, we prove that this classification includes all Calabi--Yau threefolds contained in a possibly singular 5-dimensional quadric as well as all Calabi--Yau threefolds of degree at most $14$ in $\mathbb{P}^6$.

The dimension of the Hilbert scheme of special threefolds

The Hilbert scheme of projective 3-folds of codimension 3 or more that are linear scrolls over the projective plane or over a smooth quadric surface or that are quadric or cubic fibrations over the projective line is studied. All known such threefolds of degree from 7 to 11 are shown to correspond to smooth points of an irreducible component of their Hilbert scheme, whose dimension is computed. A relationship with the locus of good determinantal subschemes is investigated

通过一维壁穿越得到光滑射影法诺三维流形上潘德哈里潘德 - 托马斯稳定对的后代生成级数的例子

Examples of descendent generating series for Pandharipande--Thomas stable pairs on smooth projective Fano threefolds via one-dimensional wall-crossing

研究光滑射影法诺三维流形上潘德哈里潘德 - 托马斯稳定对的后代生成级数，利用壁穿越设置，通过在特定李代数及多项式实现中计算，得出法诺三维流形上一维唐纳森 - 托马斯不变量等。

On the extendability of projective surfaces and a genus bound for Enriques-Fano threefolds

We introduce a new technique, based on Gaussian maps, to study the possibility, for a given surface, to lie on a threefold as a very ample divisor with given normal bundle. We give several applications, among which one to surfaces of general type and another one to Enriques surfaces. For the latter we prove that any threefold (with no assumption on its singularities) having as hyperplane section a smooth Enriques surface (by definition an Enriques-Fano threefold) has genus g < 18 (where g is the genus of its smooth curve sections). The latter bound was also proved recently by Prokhorov, who al…

Birational self-maps of threefolds of (un)-bounded genus or gonality

We study the complexity of birational self-maps of a projective threefold $X$ by looking at the birational type of surfaces contracted. These surfaces are birational to the product of the projective line with a smooth projective curve. We prove that the genus of the curves occuring is unbounded if and only if $X$ is birational to a conic bundle or a fibration into cubic surfaces. Similarly, we prove that the gonality of the curves is unbounded if and only if $X$ is birational to a conic bundle.

On the canonical maps of nonsingular threefolds of general type

Let $S$ be a nonsingular minimal complex projective surface of general type and the canonical map of $S$ is generically finite. Beauville showed that the geometric genus of the image of the canonical map is vanishing or equals the geometric genus of $S$ and discussed the canonical degrees for these two cases. We generalize his results to nonsingular minimal complex projective threefolds.

Maximum number of $\mathbb{F}_q$-rational points on nonsingular threefolds in $\mathbb{P}^4$

We determine the maximum number of $\mathbb{F}_q$-rational points that a nonsingular threefold of degree $d$ in a projective space of dimension $4$ defined over $\mathbb{F}_q$ may contain. This settles a conjecture by Homma and Kim concerning the maximum number of points on a hypersurface in a projective space of even dimension in this particular case.

Rationality in families of threefolds

We prove that in a family of projective threefolds defined over an algebraically closed field, the locus of rational fibers is a countable union of closed subsets of the locus of separably rationally connected fibers. When the ground field has characteristic zero, this implies that the locus of rational fibers in a smooth family of projective threefolds is the union of at most countably many closed subfamilies.

Derived Categories of Twisted Sheaves on Elliptic Threefolds

We construct an equivalence between the derived category of sheaves on an elliptic threefold without a section and a derived category of twisted sheaves (modules over an Azumaya algebra) on any small resolution of its relative Jacobian.

Vector Fields on Smooth Threefolds Vanishing on Complete Intersections

The existence of a vector field on a compact Kaehler manifold with nonempty zero locus and the properties of this zero locus strongly influence the geometry of the manifold. For example, J. Wahl proved that the existence of a vector field vanishing on an ample divisor of a projective normal variety X implies that X is a cone over this divisor. If X is smooth, X will be isomorphic to the n-dimensional projective space. This paper is a first attempt to generalize Wahl's theorem to higher codimensions: Given a complex smooth projective threefold X and a vector field on X vanishing on an irreducib…

Noether--Lefschetz theorem for hypersurface sections of singular threefolds

We prove a Noether--Lefschetz-type result for certain linear systems on a projective threefold with isolated singularities.

Failure of the integral Hodge conjecture for threefolds of Kodaira dimension zero

We prove that the product of an Enriques surface and a very general curve of genus at least 1 does not satisfy the integral Hodge conjecture for 1-cycles. This provides the first examples of smooth projective complex threefolds of Kodaira dimension zero for which the integral Hodge conjecture fails, and the first examples of non-algebraic torsion cohomology classes of degree 4 on smooth projective complex threefolds.

Stable rationality in smooth families of threefolds

We exhibit families of smooth projective threefolds with both stably rational and non stably rational fibers.

允许具有Picard数一的Fano三维簇的$\mathbb Q$-Gorenstein光滑化的加权射影空间

Weighted projective spaces admitting $\mathbb Q$-Gorenstein smoothings to Fano threefolds with Picard number one

研究加权射影三维簇允许Q-戈伦斯坦光滑化为Picard数一的光滑Fano三维簇，推导必要数值与局部条件，计算机搜索确定数值候选，构造若干光滑化并证明一个非光滑化例子，给出消失环准则保持Picard数一。

关于投影三重曲面上的叶状结构的对数Sarkisov程序

On The Log Sarkisov Program For Foliations On Projective 3-Folds

本文证明了具有适当奇点的投影三重曲面上协秩为一的叶状结构的Sarkisov程序，并展示了两个具有秩一叶状结构的叶状Mori纤维空间之间的弱化理性刚性。

Rationally connected threefolds with nef and bad anticanonical divisor

Let $X$ be a smooth projective rationally connected threefold with nef anticanonical divisor. We give a classification for the case when $-K_X$ is not semi-ample.

On threefolds covered by lines

A classification theorem is given of projective threefolds that are covered by a two-dimensional family of lines, but not by a higher dimensional family.

Derived equivalent threefolds, algebraic representatives, and the coniveau filtration

A conjecture of Orlov predicts that derived equivalent smooth projective varieties over a field have isomorphic Chow motives. The conjecture is known for curves, and was recently observed for surfaces by Huybrechts. In this paper we focus on threefolds over perfect fields, and unconditionally secure results, which are implied by Orlov's conjecture, concerning the geometric coniveau filtration, and abelian varieties attached to smooth projective varieties.

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.

Strictly nef divisors on singular threefolds

Let $X$ be a normal projective threefold with mild singularities, and $L_X$ a strictly nef $\mathbb{Q}$-divisor on $X$. First, we show the ampleness of $K_X+tL_X$ with sufficiently large $t$ if either the Kodaira dimension $κ(X)\neq 0$ or the augmented irregularity $q^{\circ}(X)\neq 0$. Second, we show that, if $(X,Δ)$ is a projective klt threefold pair with the anti-log canonical divisor $-(K_X+Δ)$ being strictly nef, then $X$ is rationally connected.

A p-adic proof of Hodge symmetry for threefolds

In this note we give a p-adic proof of Hodge symmetry for smooth, projective threefolds over complex numbers.

光滑环面法诺曲面与三维簇的线丛的特殊集合

Exceptional Collections of Line Bundles for Smooth Toric Fano Surfaces and Threefolds

该研究利用Bayer-Popescu-Sturmfels的对角胞腔分解，得到单模射影环面曲面上满的强例外线丛集合，又通过Hanlon-Hicks-Lazarev的对角分解，为18个光滑环面法诺三维簇中的16个构造了此类线丛集合。

关于具有int-amplified自同态的Picard数为2的光滑有理连通射影三维簇

On smooth rationally connected projective threefolds of Picard number two admitting int-amplified endomorphisms

该研究证明Picard数为2的光滑有理连通射影三维簇为环面簇的充要条件是存在int-amplified自同态，并得出爆破等变下$\u202e\bb{P}^3$非同构满自同态的全不变光滑曲线必为直线的推论。

On 6-canonical map of irregular threefolds of general type

We prove that, for any nonsingular projective irregular 3-fold of general type, the 6-canonical map is birational onto its image.

Holomorphic forms on threefolds

Two conjectures relating the Kodaira dimension of a smooth projective variety and existence of number of nowhere vanishing 1-forms on the variety are proposed and verified in dimension 3.

On quint-canonical birationality of irregular threefolds

Let $X$ be a complex smooth projective threefold of general type. Assume $q(X)>0$. We show that the $m$-canonical map of $X$ is birational for all $m\geq 5$.

$\mathbb{Z}_2^s$-覆盖的几何与变形：一般类型过加权投影三维流形

Geography and Deformations of $\mathbb{Z}_2^s$-Covers of General Type Over Weighted Projective Threefolds

研究一般类型三维流形作为$\mathbb{Z}_2^s$-覆盖的加权投影空间的构造，探讨其不变量、变形理论及$m$-canonical映射行为，提出新的模空间构造准则并分类$\mathbb{Z}_2^s$-覆盖的平滑性条件。

Stable pair invariants on Calabi-Yau threefolds containing P^2

We relate Pandharipande-Thomas stable pair invariants on Calabi-Yau 3-folds containing the projective plane with those on the derived equivalent orbifolds via wall-crossing method. The difference is described by generalized Donaldson-Thomas invariants counting semistable sheaves on the local projective plane, whose generating series form theta type series for indefinite lattices. Our result also derives non-trivial constraints among stable pair invariants on such Calabi-Yau 3-folds caused by Seidel-Thomas twist.

节点五次 del Pezzo 三维簇及其导出范畴

Nodal quintic del Pezzo threefolds and their derived categories

本文为节点五次 del Pezzo 三维簇构造川又型半正交分解，将其导出范畴分解为有限维代数的导出范畴，通过双有理映射到二次曲面纤维化实现。

On the classification of defective threefolds

We classify all irreducible projective threefolds $X$ which are $k$-defective, i.e. some $k$-secant variety of $X$ has dimension less than the expected value. This results extends the classical Scorza's classification of the case $k=1$.

Threefolds of Kodaira dimension one

We prove that for any smooth complex projective threefold of Kodaira dimension one, the $m$-th pluricanonical map is birational to the Iitaka fibration for every $m\geq5868$ and divisible by $12$.

On Pseudo-Effectivity of the Second Chern Classes for Smooth Threefolds

We prove that for smooth projective threefolds whose anticanonical divisors are nef, the second Chern classes are pseudo-effective under a weak assumption. As an application, the pseudo-effectivity of the second Chern classes implies that Kawamata's Effective Non-vanishing Conjecture holds for such threefolds.

Elliptic Calabi-Yau threefolds over a del Pezzo surface

We consider certain elliptic threefolds over the projective plane (more generally over certain rational surfaces) with a section in Weierstrass normal form. In particular, over a del Pezzo surface of degree 8, these elliptic threefolds are Calabi-Yau threefolds. We will discuss especially the generating functions of Gromov-Witten and Gopakumar-Vafa invariants.

关于加权涡卷中 K3 纤维化 Calabi-Yau 三流形

On K3 fibred Calabi-Yau threefolds in weighted scrolls

本文分类了嵌入加权涡卷的低度加权 K3 曲面纤维化温和奇异 Calabi-Yau 三流形，推广 Mullet 结果，并研究射影退化与嵌入问题。

Q-Fano threefolds of large Fano index, I

We study Q-Fano threefolds of large Fano index. In particular, we prove that the maximum of Fano index is attained for the weighted projective space P(3,4,5,7).

Bridgeland Stability conditions on threefolds I: Bogomolov-Gieseker type inequalities

We construct new t-structures on the derived category of coherent sheaves on smooth projective threefolds. We conjecture that they give Bridgeland stability conditions near the large volume limit. We show that this conjecture is equivalent to a Bogomolov-Gieseker type inequality for the third Chern character of certain stable complexes. We also conjecture a stronger inequality, and prove it in the case of projective space, and for various examples. Finally, we prove a version of the classical Bogomolov-Gieseker inequality, not involving the third Chern character, for stable complexes.

Derived categories of Fano threefolds

We consider the structure of the derived categories of coherent sheaves on Fano threefolds with Picard number 1 and describe a strange relation between derived categories of different threefolds. In the Appendix we discuss how the ring of algebraic cycles of a smooth projective variety is related to the Grothendieck group of its derived category.

三维代数簇的邦达尔-奥尔洛夫局部化猜想成立

The Bondal-Orlov Localization Conjecture Holds for Threefolds

该研究证明了满足特定条件的射影态射下导出前推的等价关系，并由此得到拟射影三维代数簇的邦达尔-奥尔洛夫局部化猜想的无特征形式，推进了代数几何中导出范畴的相关猜想研究。

Families of nodal curves on projective threefolds and their regularity via postulation of nodes

The main purpose of this paper is to introduce a new approach to study families of nodal curves on projective threefolds. Precisely, given $X$ a smooth projective threefold, $\E$ a rank-two vector bundle on $X$, $L$ a very ample line bundle on $X$ and $k \geq 0$, $δ>0 $ integers and denoted by $V= {\V}_δ ({\E} \otimes L^{\otimes k})$ the subscheme of ${\Pp}(H^0({\E} \otimes L^{\otimes k}))$ parametrizing global sections of ${\E} \otimes L^{\otimes k}$ whose zero-loci are irreducible and $δ$-nodal curves on $X$, we present a new cohomological description of the tangent space $T_{[s]}({\V}_δ ({\…

Derived-equivalent rational threefolds

We describe an infinite set of smooth projective threefolds that have equivalent derived categories but are not isomorphic, contrary to a conjecture of Kawamata. These arise as blow-ups of $\mathbb P^3$ at various configurations of 8 points, which are related by Cremona transformations.

Automorphism groups of positive entropy on projective threefolds

We prove two results about the natural representation of a group G of automorphisms of a normal projective threefold X on its second cohomology. We show that if X is minimal then G, modulo a normal subgroup of null entropy, is embedded as a Zariski-dense subset in a semi-simple real linear algebraic group of real rank < 3. Next, we show that X is a complex torus if the image of G is an almost abelian group of positive rank and the kernel is infinite, unless X is equivariantly non-trivially fibred.

On non-projective small resolutions

We construct a large class of projective threefolds with one node (aka non-degenerate quadratic singularity) such that their small resolutions are not projective.

射影三维流形自同构的Kawaguchi-Silverman猜想

Kawaguchi-Silverman conjecture on automorphisms of projective threefolds

本文将射影三维流形自同构的Kawaguchi-Silverman猜想归结为弱Calabi-Yau与有理连通两种情形，并证明非正则性满足q(X)≥dim X−1时猜想成立，同时探讨了Picard数为2的情形。

The Cohomology of Reflexive Sheaves on Smooth Projective 3-folds

We study the cohomology of reflexive rank 2 sheaves on smooth projective threefolds. Applications are given to the moduli space of reflexive sheaves.

The canonical volume of threefolds of general type with $χ<1$

We prove that the canonical volume $K^3\geq {1/30}$ for all projective 3-folds of general type with $χ(\mathcal{O})\leq 0$. This bound is sharp.