On threefolds covered by lines

The curve of lines on a prime Fano threefold of genus 8

We show that a general prime Fano threefold X of genus 8 can be reconstructed from the pair $(Γ,L)$, where $Γ$ is its Fano curve of lines and $L=O_Γ(1)$ is the theta-characteristic which gives a natural embedding $Γ\subset \matbb{P}^5$.

$\mathbb{A}^1_*$-fibrations on affine threefolds

The affine line minus one point is the underlying space of the algebraic torus of dimension one. However the fibration of an affine algebraic threefold by the affine line minus one point is not always the quotient morphism of the threefold by the algebraic torus. We investigate the structure of affine algebraic threefolds having fibrations by the affine line minus one point. Our results cover not only algebro-geometric but also topological properties of the threefolds. We also consider when the fibration by the affine line minus one point becomes the quotient morphism by an algebraic torus of …

A maximally-graded invertible cubic threefold that does not admit a full exceptional collection of line bundles

We show that there exists a cubic threefold defined by an invertible polynomial that, when quotiented by the maximal diagonal symmetry group, has a derived category which does not have a full exceptional collection consisting of line bundles. This provides a counterexample to a conjecture of Lekili and Ueda.

Smooth complete toric threefolds with no nontrivial nef line bundles

We describe all of the smooth complete toric threefolds of Picard number 5 with no nontrivial nef line bundles, and show that no such examples exist with Picard number less than 5.

三次三维流形上一对异面直线的希尔伯特概型

Hilbert Scheme of a Pair of Skew Lines on Cubic Threefolds

本文证明光滑三次三维流形上由一对不相交直线决定的希尔伯特概型不可约分支光滑且同构于对角线上直线法诺簇对称积的爆破，并研究其与超平面截面几何及布里奇兰德模空间的关系。

诺特线附近的典范三维簇

On canonical threefolds near the Noether line

本文是关于典范体积接近下界4/3p_g - 10/3的典范三维簇的研讨会扩展摘要，为与S. Coughlan等学者合作的研究项目。

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。

Hilbert schemes of lines and conics and automorphism groups of Fano threefolds

We discuss various results on Hilbert schemes of lines and conics and automorphism groups of smooth Fano threefolds with Picard rank 1. Besides a general review of facts well known to experts, the paper contains some new results, for instance, we give a description of the Hilbert scheme of conics on any smooth Fano threefold of index 1 and genus 10. We also show that the action of the automorphism group of a Fano threefold $X$ of index 2 (respectively, 1) on an irreducible component of its Hilbert scheme of lines (respectively, conics) is faithful if the anticanonical class of $X$ is very ampl…

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.

Pfaffian Lines and Vector Bundles on Fano Threefolds of Genus 8

Let X be a general complex Fano threefold of genus 8. We prove that the moduli space of rank two semistable sheaves on X with Chern numbers c_1=1, c_2=6 and c_3=0 is isomorphic to the Fano surface F(X) of conics on X. Inside F(X), the non-locally free sheaves are parameterized by a smooth curve of genus 26 isomorphic to the base of the family of lines on X.

Linear systems on a class of anticanonical rational threefolds

Let X be the blow-up of the three dimensional complex projective space along r general points of a smooth elliptic quartic curve B of P^3 and let L be any line bundle of X. The aim of this paper is to provide an explicit algorithm for determining the dimension of H^0(X,L).

亏格9和10的Fano三维流形中的柱体

Cylinders in Fano threefolds of genus 9 and 10

本文证明了亏格9和10的任意Fano三维流形包含一个柱体，即一个开子集同构于拟射影簇与仿射直线的乘积，并进一步证明了亏格10的Fano三维流形存在一点，使得通过该点的直线的Hilbert概形长度为3。

On integral points of some Fano Threefolds and their Hilbert schemes of lines and conics

Let $X^o=\mathbb P^3\setminus D$ where $D$ is the union of two quadrics such that their intersection contains a smooth conic, or the union of a smooth quadric surface and two planes, or the union of a smooth cubic surface $V$ and a plane $Π$ such that the intersection $V\capΠ$ contains a line. In all these cases we show that the set of integral points of $X^o$ is potentially dense. We apply the above results to prove that integral points are potentially dense in some log-Fano or in some log-Calabi-Yau threefold.

二次型与五次三维流形上直线的动态欧拉数

Quadratic types and the dynamic Euler number of lines on a quintic threefold

本文为五次三维流形上直线的局部贡献提供几何解释，定义动态欧拉数计算 A^1-欧拉数，并通过 Fermat 五次三维流形上 2875 条特殊直线的二次计数，得出一般五次三维流形上直线型的总和在 GW(k) 中为 1445<1> + 1430<-1>。

Threefolds of P^5 with one apparent quadruple point

In this article we classify all the smooth threefolds of P^5 with an apparent quadruple point provided that the family of its 4-secant lines is an irreducible (first order) congruence. This is sufficient to conclude the classification of all the smooth codimension two varieties of P^n with one apparent (n-1)-point and with irreducible family of (n-1)-secant lines.

On Fano threefolds with $\mathbb{C}^*$-action

We study three-dimensional Fano varieties with $\mathbb{C}^*$-action. Complementing recent results [13], we give classification results in the canonical case, where the maximal orbit quotient is $\mathbb{P}_2$ having a line arrangement of five lines in special position as its critical values.

The zeta functions of the Fano surfaces of cubic threefolds

We give an algorithm to compute the zeta function of the Fano surface of lines of a smooth cubic threefold $F$ into $\mathbb{P}^4$ defined over a finite field. We obtain some examples of Fano surfaces with supersingular reduction.

位于Noether线上的三维流的模空间

Moduli spaces of threefolds on the Noether line

本文研究了位于Noether线上典范三维流的模空间，通过显式分层与计算维度，实现了该类三维流的完整分类，并发现其不可约分支数量随几何亏格线性增长的新现象。

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.

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

Instanton bundles on the Segre threefold with Picard number three

We study instanton bundles $E$ on $\mathbb{P}^1\times \mathbb{P}^1 \times \mathbb{P}^1$. We construct two different monads which are the analog of the monads for instanton bundles on $\mathbb P^3$ and on the flag threefold $F(0,1,2)$. We characterize the Gieseker semistable cases and we prove the existence of $μ$-stable instanton bundles generically trivial on the lines for any possible $c_2(E)$. We also study the locus of jumping lines.

A lower bound on Seshadri constants of hyperplane bundles on threefolds

We give the lower bound on Seshadri constants for the case of very ample line bundles on threefolds. We consider the situation when the Seshadri constant is strictly less than 2 and give a version of Bauer's theorem \cite[Theorem 2.1]{B1} for singular surfaces so we can prove the same result for smooth threefolds.

On some Fano--Enriques threefolds

We give a classification of Fano threefolds $X$ with canonical Gorenstein singularities such that $X$ possess a regular involution, which acts freely on some smooth surface in $|-K_X|$, and the linear system $|-K_X|$ gives a morphism which is not an embedding. From this classification one gets, in particular, a description of some natural class of Fano--Enriques threefolds.

Explicit Noether-Lefschetz for arbitrary threefolds

We study the Noether-Lefschetz locus of a very ample line bundle L on an arbitrary smooth threefold Y. Building on results of Green, Voisin and Otwinowska, we give explicit bounds, depending only on the Castelnuovo-Mumford regularity properties of L, on the codimension of the components of the Noether-Lefschetz locus of |L|.

Projective normality of Abelian threefolds

In this article, we show that very ample line bundles of type (1,1,d) on an abelian threefold A, with NS(A)\simeq \ze, give projectively normal embeddings, if d=22,26 or when d\geq 28.

Rational curves and instantons on the Fano threefold $Y_5$

This thesis is an investigation of the moduli spaces of instanton bundles on the Fano threefold $Y_5$ (a linear section of $\mathbb{G}r(2,5)$). It contains new proofs of classical facts about lines, conics and cubics on $Y_5$, and about linear sections of $Y_5$. The main original results are a Grauert-Mülich theorem for the splitting type of instantons on conics, a bound to the splitting type of instantons on lines and an $SL_2$-equivariant description of the moduli space in charge 2 and 3. Using these results we prove the existence of a unique $SL_2$-equivariant instanton of minimal charge an…

Geometry of lines on certain Moishezon threefolds. I. Explicit description of families of twistor lines

We study real lines on certain Moishezon threefolds which are potentially twistor spaces of 3CP^2. Here, line means a smooth rational curve whose normal bundle is O(1)^2 and the reality implies the invariance under an anti-holomorphic involution on the threefolds. Our threefolds are birational to double coverings of CP^3 branched along singular quartic surfaces. On these threefolds we find families of real lines in explicit form and prove that which families have to be chosen as twistor lines depend on how we take small resolutions of the double covering. This is a first step for determining t…

从素Fano三维流形沿两条线得到的初等链接

Elementary links from prime Fano threefolds along two lines

本文研究了素Fano三维流形沿两条不相交直线的吹胀操作，建立了与Fano三维流形的初等链接，并探讨了不同亏格下的几何构造与反向链接。

Deformation classification of real non-singular cubic threefolds with a marked line

We prove that the space of pairs $(X,l)$ formed by a real non-singular cubic hypersurface $X\subset P^4$ with a real line $l\subset X$ has 18 connected components and give for them several quite explicit interpretations. The first one relates these components to the orbits of the monodromy action on the set of connected components of the Fano surface $F_\mathbb{R}(X)$ formed by real lines on $X$. For another interpretation we associate with each of the 18 components a well defined real deformation class of real non-singular plane quintic curves and show that this deformation class together wit…

Pfaffian representations of cubic threefolds

Given a cubic hypersurface $X\subset \mathbb{P}^4$, we study the existence of Pfaffian representations of $X$, namely of $6\times 6$ skew-symmetric matrices of linear forms $M$ such that $X$ is defined by the equation $Pf(M)=0$. It was known that such a matrix always exists whenever $X$ is smooth. Here we prove that the same holds whenever $X$ is singular, hence that every cubic threefold is Pfaffian.

Some remarks on Fano threefolds of index two and stability conditions

We prove that ideal sheaves of lines in a Fano threefold $X$ of Picard rank one and index two are stable objects in the Kuznetsov component $\mathsf{Ku}(X)$, with respect to the stability conditions constructed by Bayer, Lahoz, Macrì and Stellari, giving a modular description to the Hilbert scheme of lines in $X$. When $X$ is a cubic threefold, we show that the Serre functor of $\mathsf{Ku}(X)$ preserves these stability conditions. As an application, we obtain the smoothness of non-empty moduli spaces of stable objects in $\mathsf{Ku}(X)$. When $X$ is a quartic double solid, we describe a conn…

Calabi-Yau complete intersections with infinitely many lines

We give two new examples of families of Calabi-Yau complete intersection threefolds whose generic element contains infinitely many lines. We get some results about the normal bundles of these lines and the Hilbert scheme of lines on the threefolds.

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 base point free theorem of threefolds in positive characteristic

Let $(X, Δ)$ be a projective klt three dimensional pair defined over an algebraically closed field characteristic larger than 5. Let $L$ be a nef and big line bundle on $X$ such that $L-K_X-Δ$ is big and nef. We show that $L$ is indeed semi-ample.

Covered by Lines and Conic Connected Varieties

We study some properties of an embedded variety covered by lines and give a numerical criterion ensuring the existence of a singular conic through two of its general points. We show that our criterion is sharp. Conic-connected, covered by lines, QEL, LQEL, prime Fano, defective, and dual defective varieties are closely related. We study some relations between the above mentioned classes of objects using celebrated results by Ein and Zak.

On the base point free theorem for klt threefolds in large characteristic

In this article we present a refinement of the base point free theorem for threefolds in positive characteristic. If $L$ is a nef Cartier divisor of numerical dimension at least one on a projective Kawamata log terminal threefold $(X,Δ)$ over a perfect field $k$ of characteristic $p \gg 0$ such that $L-(K_X+Δ)$ is big and nef, then we show that the linear system $|mL|$ is base point free for all sufficiently large integer $m>0$.

On base point free theorem for log canonical threefolds over the algebraic closure of a finite field

We prove the base point free theorem for big line bundles on a three-dimensional log canonical projective pair defined over the algebraic closure of a finite field.

One base point free theorem for weak log Fano threefolds

Let $(X,D)$ be log canonical pair such $\dim X = 3$ and the divisor $-(K_X + D)$ is nef and big. For a special class of such $(X,D)$'s we prove that the linear system $|-n(K_{X}+D)|$ is free for $n \gg 0$.

On the period map for prime Fano threefolds of degree 10

We prove that the deformations of a smooth complex Fano threefold X with Picard number 1, index 1, and degree 10, are unobstructed. The differential of the period map has two-dimensional kernel. We construct two two-dimensional components of the fiber of the period map through X: one is isomorphic to the variety of conics in X, modulo an involution, another is birationally isomorphic to a moduli space of semistable rank-2 torsion-free sheaves on X, modulo an involution. The threefolds corresponding to points of these components are obtained from X via conic and line (birational) transformation…

通过中间 Jacobian 研究具有非 Eckardt 型对合的三次三维簇的模空间

The moduli space of cubic threefolds with a non-Eckardt type involution via intermediate Jacobians

本文研究带非 Eckardt 型对合的三次三维簇，将其周期映射到中间 Jacobian 不变部分，并利用 Prym 簇与 bigonal construction 证明全局 Torelli 定理。

Manifolds covered by lines and extremal rays

Let $X$ be a smooth complex projective variety and let $H \in \pic(X)$ be an ample line bundle. Assume that $X$ is covered by rational curves with degree one with respect to $H$ and with anticanonical degree greater than or equal to $(\dim X -1)/2$. We prove that there is a covering family of such curves whose numerical class spans an extremal ray in the cone of curves $\cone(X)$.

族 \textnumero 2.12 中 Fano 三维簇的等变有理性

Equivariant rationality of Fano threefolds in the family \textnumero 2.12

本文研究族 2.12 中 Fano 三维完全交的等变有理性，刻画了忠实群作用可线性化的充要条件。

Manifolds covered by lines and the Hartshorne Conjecture for quadratic manifolds

Small codimensional embedded manifolds defined byequations of small degree are Fano and covered by lines. They are complete intersections exactly when the variety of lines through a general point is so and has the right codimension. This allows us to prove the Hartshorne Conjecture for manifolds defined by quadratic equations and to obtain the list of such Hartshorne manifolds.

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.

Elliptic Calabi-Yau Threefolds with Z_3 x Z_3 Wilson Lines

A torus fibered Calabi-Yau threefold with first homotopy group Z_3 x Z_3 is constructed as a free quotient of a fiber product of two dP_9 surfaces. Calabi-Yau threefolds of this type admit Z_3 x Z_3 Wilson lines. In conjunction with SU(4) holomorphic vector bundles, such vacua lead to anomaly free, three generation models of particle physics with a right handed neutrino and a U(1)_{B-L} gauge factor, in addition to the SU(3)_C x SU(2)_L x U(1)_Y standard model gauge group. This factor helps to naturally suppress nucleon decay. The moduli space and Dolbeault cohomology of the threefold is also …

Towards a separation theorem of points by adjoint linear systems on polarized threefolds

Main Result: Let $(M,L)$ be a smooth complex polarized threefold. Then the linear system $| K+tL|$ separates any two different points on $M$ for any $t\ge 6$, where $K$ is the canonical bundle of $M$. The argument in the proof is a variant of Ein-Lazarsfeld method. Although it is less powerful, it is cheaper, i.e., needs fewer pages. Unfortunately, at present, I cannot show the very ampleness because of technical difficulties. Some related topics are also discussed. A hard copy is available on request to the author.

关于有限域代数闭包上对数典范三维簇的基点自由定理与Mori dream空间

On base point free theorem and Mori dream spaces for log canonical threefolds over the algebraic closure of a finite field

本文在特征大于5时去除了大性条件，将有限域代数闭包上三维对数典范射影对的基点自由定理推广至一般线丛，并讨论了此类域上的Mori dream空间。

Measures of irrationality of the Fano surface of a cubic threefold

For $X$ a smooth cubic threefold we study the Plücker embedding of the Fano surface of lines $S$ of $X$. We prove that if $X$ is general then the minimal gonality of a covering family of curves of $S$ is four and that this happens for a unique family of curves. The analysis also shows that there is a unique pentagonal connecting family of curves, which leads to the fact that the connecting gonality of $S$ is five whereas the degree of irrationality, i.e.\ the minimal degree of a rational map from $S$ to $\mathbb{P}^2$, is six.

q-双三次曲面及其线面

$q$-bic threefolds and their surface of lines

本文研究q-双三次曲面的线面S的几何性质，通过模性理论和滤子理论计算其结构sheaf的上同调。

丹尼尔·费兰德关于Fano三种重束的注记

A remark by Daniel Ferrand on bundles on Fano threefolds

本文研究Fano三种重上秩为二的μ-半稳定束，重点在于其第一 Chern类为零、非零零空间及特定上同调条件的分类。

Formulae for Line Bundle Cohomology on Calabi-Yau Threefolds

We present closed form expressions for the ranks of all cohomology groups of holomorphic line bundles on several Calabi-Yau threefolds realised as complete intersections in products of projective spaces. The formulae have been obtained by systematising and extrapolating concrete calculations and they have been checked computationally. Although the intermediate calculations often involve laborious computations of ranks of Leray maps in the Koszul spectral sequence, the final results for cohomology follow a simple pattern. The space of line bundles can be divided into several different regions, …

The Fermat cubic and monodromy of lines

In this paper we study properties of the locus of second type lines of a general cubic threefold and fourfold. By analysing the geometry of the Fano scheme of lines of the Fermat cubic fourfold and in particular giving an explicit description of the locus of second type lines, we deduce that the Voisin map is birational over the second type locus. For a general cubic threefold, by studying properties of the second type locus again, we compute that various natural geometric monodromy groups are the full symmetric group.

(1,2)-曲面中的简单纤维化

Simple fibrations in (1,2)-surfaces

本研究引入(1,2)-曲面中简单纤维化的概念，分类了射影直线上至多带典范奇点的Gorenstein简单纤维化，确定其对应Noether线上的典范三维簇，还发现几何亏格模8余6时存在模空间的新分支。

Rationally cubic connected manifolds I: manifolds covered by lines

In this paper we study smooth complex projective polarized varieties (X,H) of dimension n \ge 2 which admit a dominating family V of rational curves of H-degree 3, such that two general points of X may be joined by a curve parametrized by V, and such that there is a covering family of rational curves of H-degree one. Our main result is that the Picard number of these manifolds is at most three, and that, if equality holds, (X,H) has an adjuction theoretic scroll structure over a smooth variety.