Qihong Xie
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.
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We give a reduction of the conjecture that for terminal projective threefolds whose anticanonical divisors are nef, the second Chern classes are pseudo-effective. On the other hand, some effective non-vanishing results are obtained as applications of the pseudo-effectivity of the second Chern classes.
In this note we show that given a lc pair $(X, Δ)$, a large enough multiple of the bundle $K_X+ Δ$ is effective provided that its Chern class contains an effective $\bQ$-divisor.
On Pseudo-Effectivity and Volumes of Adjoint Classes in Kähler Families with Projective Central Fiber
本文研究了Kähler族中伴随类的伪有效性与体积的变形行为，证明了体积的局部不变性并验证了Siu的不变性猜想。
Hodge-Chern classes and strata-effectivity in tautological rings
本文研究zip周期映射下重言环中类的层效应性，提出Griffiths-Hodge丛的Chern类均为层有效的猜想，推广了Ekedahl-van der Geer关于Hodge向量丛的结果，并证明了若干实例。
We obtain all linear Chern number inequalities satisfied by any smooth complete intersection threefold with ample canonical bundle.
We prove the Bogomolov-Gieseker type inequality conjectured by Bayer, Macri and Toda for threefolds with semistable tangent bundles and vanishing Chern classes in any characteristic, which was originally proved by Bayer, Macri and Stellari in characteristic zero. This gives the existence of Bridgeland stability conditions on such threefolds. As applications, we obtain Reider type theorem and confirm Fujita's conjecture for such threefolds in any characteristic.
This paper aims to investigate effectivity problems of pluricanonical systems on varieties of general type in positive characteristic. In practice, we will consider a sub-linear system $|S^0_{-}(X, K_X + nK_X)| \subseteq |H^0(X, K_X +nK_X)|$ generated by certain Frobenius stable sections, and prove that for a minimal terminal threefold $X$ of general type with either $q(X)>0$ or Gorenstein singularities, if $n\geq 28$ then $|S^0_{-}(X, K_X + nK_X)| \neq \emptyset$; if $n\geq 42$ then the linear system $|S^0_{-}(X, K_X + nK_X)|$ defines a birational map.
Rank Two Sheaves With Maximal Third Chern Class on the Fano Threefold of Index 2 and Degree 5
针对指数2、次数5的Fano三维簇$X_5$，通过倾斜稳定性与Bridgeland稳定性理论，完成具极大第三陈类的秩2半稳定层分类并明确其模空间性质。
Nef vector bundles on a quadric threefold with first Chern class two
本文分类了特征零代数闭域上三维光滑超二次簇上第一陈类为二的 nef 向量丛，并证明它们都是整体生成的。
Automorphisms of Calabi-Yau threefolds from algebraic dynamics and the second Chern class
本文是作者在MFO的报告记录，基于代数动力学视角，对严格Calabi-Yau三维流形的自同构及第二陈类问题进行了推测性探讨。
We revisit the nef curve cone theorem and use it to reprove the pseudo-effectivity of the second Chern classes for terminal weak Q-Fano varieties.
Chern numbers of terminal threefolds
本文证明在光滑三维簇的K_X-MMP中，每次翻转引起的第一陈数三次方之差仅由第二贝蒂数界定，给出终端三维簇陈数的有界性。
We study the behaviour of Chern numbers of three dimensional terminal varieties under divisorial contractions.
In this paper we show that the Chern numbers of a smooth Mori fibre space in dimension three are bounded in terms of the underlying topological manifold. We also generalise a theorem of Cascini and the second named author on the boundedness of Chern numbers of certain threefolds to the case of negative Kodaira dimension.
We investigate properties and describe examples of tilt-stable objects on a smooth complex projective threefold. We give a structure theorem on slope semistable sheaves of vanishing discriminant, and describe certain Chern classes for which every slope semistable sheaf yields a Bridgeland semistable object of maximal phase. Then, we study tilt stability as the polarisation $ω$ gets large, and give sufficient conditions for tilt-stability of sheaves of the following two forms: 1) twists of ideal sheaves or 2) torsion-free sheaves whose first Chern class is twice a minimum possible value.
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.
We prove a case of the conjecture of Douglas, Reinbacher and Yau about the existence of stable vector bundles with prescribed Chern classes on a Calabi-Yau threefold. For this purpose we prove the existence of certain stable vector bundle extensions over elliptically fibered Calabi-Yau threefolds.
We show that the limit points of (x,y) for all 3-folds in P^5 with the Chern ratios $x=c_1^3/c_1c_2$, $y=c_3/c_1c_2$ must lie on the line segment $x+y=2$, $1\le x\le 2$. (Note that the determinantal ones already give x+y=2, $1\le x\le 17/12$.)
The purpose of this paper is to study the bimeromorphic invariants of compact complex manifolds in terms of Bott-Chern cohomology. We prove a blow-up formula for Bott-Chern cohomology. As an application, we show that for compact complex threefolds the non-Kählerness degrees, introduced by Angella-Tomassini [Invent. Math. 192, (2013), 71-81], are bimeromorphic invariants. Consequently, the $\partial\bar{\partial}$-Lemma on threefolds admits the bimeromorphic invariance.
Kawamata proposed a conjecture predicting that every nef and big line bundle on a smooth projective variety with trivial first Chern class has nontrivial global sections. We verify this conjecture for several cases, including (i) all hyperkähler varieties of dimension $\leq 6$; (ii) all known hyperkähler varieties except for O'Grady's 10-dimensional example; (iii) general complete intersection Calabi-Yau varieties in certain Fano manifolds (e.g. toric ones). Moreover, we investigate the effectivity of Todd classes of hyperkähler varieties and Calabi-Yau varieties. We prove that the fourth Todd…
Chern-Ricci flat balanced metrics on small resolutions of Calabi-Yau threefolds
本文通过粘合构造证明，射影节点 Kähler Calabi-Yau 三流形的所有小分解均容许 Chern-Ricci 平衡度量，从而解决了 Hull-Strominger 系统的伸缩子方程。
In this paper we prove that given a pair $(X,D)$ of a threefold $X$ and a boundary divisor $D$ with mild singularities, if $(K_X+D)$ is movable, then the orbifold second Chern class $c_2$ of $(X,D)$ is pseudo-effective. This generalizes the classical result of Miyaoka on the pseudo-effectivity of $c_2$ for minimal models. As an application we give a simple solution to Kawamata's effective non-vanishing conjecture in dimension $3$, where we prove that $H^0(X, K_X+H)\neq 0$, whenever $K_X+H$ is nef and $H$ is an ample, effective, reduced Cartier divisor. Furthermore, we study Lang-Vojta's conjec…
We investigate the existence of globally generated vector bundles of rank 2 with $c_1\leq 3$ on a smooth quadric threefold and determine their Chern classes. As an automatic consequence, every rank 2 globally generated vector bundle on $Q$ with $c_1=3$ is an odd instanton up to twist.
In this paper, we give a construction of the global Chern-Simons functions for toric Calabi-Yau stacks of dimension three using strong exceptional collections. The moduli spaces of sheaves on such stacks can be identified with critical loci of these functions. We give two applications of these functions. First, we prove Joyce's integrality conjecture of generalized DT invariants on local surfaces. Second, we prove a dimension reduction formula for virtual motives, which leads to two recursion formulas for motivic Donaldson-Thomas invariants.
We prove the conjectural Bogomolov-Gieseker type inequality for tilt slope stable objects on each Fano threefold X of Picard number one. Based on the previous works on Bridgeland stability conditions, this induces an open subset of geometric stability conditions on D^b(X). We also get a new stronger bound for the Chern characters of slope semistable sheaves on X.
Integral Bott--Chern cohomology of Generalized Calabi--Eckmann threefolds
本文计算了复维3的广义Calabi--Eckmann流形的整Bott--Chern上同调，证明非双全纯流形间该上同调环同构，并用特征映射区分不同上同调。
We study the Clifford type inequality for a particular type of curves $C_{2,2,5}$, which are contained in smooth quintic threefolds. This allows us to prove some stronger Bogomolov-Gieseker type inequalities for Chern characters of stable sheaves and tilt-stable objects on smooth quintic threefolds. Employing the previous framework by Bayer, Bertram, Macrì, Stellari and Toda, we construct an open subset of stability conditions on every smooth quintic threefold in $\mathbf{P}^4_{\mathbb C}$.
Let X be a Calabi-Yau threefold and μthe symmetric trilinear form on the second cohomology group H^{2}(X,\Z) defined by the cup product. We investigate the interplay between the Chern classes c_{2}(X), c_{3}(X) and the trilinear form μ, and demonstrate some numerical relations between them. When the cubic form μ(x,x,x) has a linear factor over \R, some properties of the linear form and the residual quadratic form are also obtained.
We apply a conjectured inequality on third chern classes of stable two-term complexes on threefolds to Fujita's conjecture. More precisely, the inequality is shown to imply a Reider-type theorem in dimension three which in turn implies that K_X + 6L is very ample when L is ample, and that 5L is very ample when K_X is trivial.
Supersymmetric heterotic string models, built from a Calabi-Yau threefold $X$ endowed with a stable vector bundle $V$, usually lead to an anomaly mismatch between $c_2(V)$ and $c_2(X)$; this leads to the question whether the difference can be realized by a further bundle in the hidden sector. In math.AG/0604597 a conjecture is stated which gives sufficient conditions on cohomology classes on $X$ to be realized as the Chern classes of a stable reflexive sheaf $V$; a weak version of this conjecture predicts the existence of such a $V$ if $c_2(V)$ is of a certain form. In this note we prove that …
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.
We prove that if $X---> X^+$ is a threefold terminal flip, then $c_1(X).c_2(X)\leq c_1(X^+).c_2(X^+)$ where $c_1(X)$ and $c_2(X)$ denote the Chern classes. This gives the affirmative answer to a Question by Xie \cite{Xie2}. We obtain the similar but weaker result in the case of divisorial contraction to curves.
$m$-Pseudo-effectivity and a Monge-Ampère-Type Equation for Forms of Positive Degree
本文在紧Kähler流形上推广了双次数(1,1)的伪有效与大Bott-Chern上同调类概念，提出解为正次数形式的Monge-Ampère型非线性偏微分方程，证明解的唯一性并给出几何应用。
Point Singularities and Local Third Chern Classes for Rank-Two Torsion-free Sheaves on Threefolds
本文研究复三维流形上秩二无挠层孤立点奇点携带的局部第三陈类贡献，证明其可由全族数据计算且形变不变，并给出相对K-理论解释。
We study semistable sheaves of rank $2$ with Chern classes $c_1=0$, $c_2=2$ and $c_3=0$ on the Fano 3-fold $V_5$ of Picard number $1$, degree $5$ and index $2$. We show that the moduli space of such sheaves has a component that is isomorphic to $\mathbb{P}^5$ by identifying it with the moduli space of semistable quiver representations. This provides a natural smooth compactification of the moduli space of minimal instantons, as well as Ulrich bundles of rank $2$ on $V_5$.
We prove that a given Calabi-Yau threefold with a stable holomorphic vector bundle can be perturbed to a solution of the Strominger system provided that the second Chern class of the vector bundle is equal to the second Chern class of the tangent bundle. If the Calabi-Yau threefold has strict SU(3) holonomy then the equations of motion derived from the heterotic string effective action are also satisfied by the solutions we obtain.
A moduli space of stable sheaves on a cubic threefold
本文研究光滑三次三维簇X上特定陈示性类的Gieseker半稳定层的模空间，证明其非空、光滑、不可约且维数为8，还将其与X的Kuznetsov分支中的Bridgeland模空间联系并给出集合论描述。
We obtain a formula for the generating series of (the push-forward under the Hilbert-Chow morphism of) the Hirzebruch homology characteristic classes of the Hilbert schemes of points for a smooth quasi-projective variety of arbitrary pure dimension. This result is based on a geometric construction of a motivic exponentiation generalizing the notion of motivic power structure, as well as on a formula for the generating series of the Hirzebruch homology characteristic classes of symmetric products. We apply the same methods for the calculation of generating series formulae for the Hirzebruch cla…
Let $K$ be an algebraically closed field of characteristic $0$. For $m\geq n$, we define $τ_{m,n,k}$ to be the set of $m\times n$ matrices over $K$ with kernel dimension $\geq k$. This is a projective subvariety of $\bbP^{mn-1}$, and is called the (generic) determinantal variety. In most cases $τ_{m,n,k}$ is singular with singular locus $τ_{m,n,k+1}$. In this paper we give explicit formulas computing the Chern-Mather class ($c_M$) and the Chern-Schwartz-MacPherson class ($c_{SM}$) of $τ_{m,n,k}$, as classes in the projective space. We also obtain formulas for the conormal cycles and the charac…
The topology of Calabi-Yau threefolds with Picard number three
本文研究第二贝蒂数为3的Calabi-Yau三维流形的拓扑，利用Wall定理和三次型、第二陈类等不变量，证明在特定条件下实椭圆曲线有两个连通分支，并构造大量不支持Calabi-Yau结构的6维流形。
An orbifold version of Bogomolov decomposition theorem is established for compact Kähler spaces with quotient singularities and first Chern class zero.The proof is a direct adaptation of the classical smooth case, using Ricci-flat Kähler metrics, and Cheeger-Gromoll splitting theorem. It implies that normal K3 surfaces are uniformised in the orbifold sense either by normal K3 surfaces or by a complex torus, as conjectured by D.Q. Zhang. It also implies some conjectures on "special" threefolds raised in math.AG/0110051
Chern bounds and tangent geometry of polarized Calabi-Yau threefolds
本文通过第一jet丛和切几何，证明了极化Calabi-Yau三维簇的Chern数二次不等式，改进一致界，并研究切次数与切簇正规化态射。
The desingularization of the theta divisor of a cubic threefold as a moduli space
本文构造并研究三次三重曲面上特定 Gieseker 稳定层的四维光滑模空间，证明其经 Abel–Jacobi 映射双有理消解 theta 除子奇点，并由此给出范畴版与经典版 Torelli 定理的新证明。
Boundedness questions for Calabi-Yau threefolds
本文研究光滑 Calabi–Yau 三维簇由三次上积形式和第二陈类线性形式决定后的模空间有界性，借助刚性有理曲面分析，并在 Picard 数 2 时证明一般有界性。
On the cohomological classification of vector bundles on smooth real affine surfaces and threefolds
本文研究光滑实仿射曲面和三次曲面上的向量包的上同调分类，证明在适当的上同调条件下，这种分类与在代数闭基域上由Mohan Kumar和Murthy以及Asok和Fasel得到的分类相吻合，并给出了Kucharz关于三次曲面上秩3向量包的Chern类的代数循环三元组的定理的高效证明，同时回答了Kucharz留下的问题，给出了首个实仿射三次曲面上具有平凡Chern类但非稳定自由的射影模。
We prove that the moduli space of stable sheaves of rank 2 with a certain Chern classes on a smooth quadric $Q$ in $\PP_3$, is isomorphic to $\PP_3$. Using this identification, we give a new proof that a certain Brill-Noether locus on a non-hyperelliptic curve of genus 4, is isomorphic to the Donagi-Izadi cubic threefold.
Miyaoka type inequality for terminal threefolds with nef anti-canonical divisors
本文研究带nef反典范除子的终端射影3-簇的Miyaoka型不等式，给出陈类乘积的下界及紧性结果，通过部分分类与例子完成证明，还拓展了终端弱Fano簇的相关非零性与Miyaoka–Kawamata型不等式研究。
Positivity of Segre-MacPherson classes
本文证明复非奇异簇上具有有效特征圈的可构造函数的带符号Segre-MacPherson类有效，统一并推广了正性问题的多项结果。
We investigate compact complex manifolds of dimension three and second Betti number $b_2(X) = 0$. We are interested in the algebraic dimension $a(X)$, which is by definition the transcendence degree of the field of meromorphic functions over the field of complex numbers. The topological Euler characteristic $χ_{\mathrm{ top}}(X) $ equals the third Chern class $c_3(X)$ by a theorem of Hopf. Our main result is that, if $X$ is a compact 3-dimensional complex manifold with $b_2(X) = 0$ and $a(X) > 0$, then $c_3(X) = χ_{\rm top}(X) = 0$, that is, we either have $b_1(X) = 0, \ b_3(X) = 2$ or $b_1(X)…
Moduli spaces of sheaves on Fano threefolds and K3 surfaces of genus 9
本文利用射影对偶性研究亏格9的Fano三流形上秩2稳定层到反典范K3曲面的限制，构建了有理Lagrange纤维化，并通过Bridgeland稳定性条件展示了其所有K-平凡双有理模型及锥结构。
We prove that for a compact Kähler threefold with canonical singularities and vanishing first Chern class, the projective fibres are dense in the semiuniversal deformation space. This implies that every Kähler threefold of Kodaira dimension zero admits small projective deformations after a suitable bimeromorphic modification. As a corollary, we see that the fundamental group of any Kähler threefold is a quotient of an extension of fundamental groups of projective manifolds, up to subgroups of finite index. In the course of the proof, we show that for a canonical threefold with $c_1 = 0$, the A…
Let M(2;0,3) be the moduli space of rank-2 stable vector bundles with Chern classes c_1=0, c_2=3 on the Fano threefold X, the double solid of index two. We prove that the vector bundles obtained by Serre's construction from smooth elliptic quintic curves on X form an open part of an irreducible component M' of M(2;0,3) and that the Abel-Jacobi map F:M'-->J(X) into the intermediate Jacobian J(X) defined by the second Chern class is generically finite of degree 84 onto a translate of the theta divisor. We also prove that the family of elliptic quintics on a general X is irreducible and of dimens…
We discuss conjectures following from the attractor mechanism in type II string theory about the possible Chern classes of stable holomorphic vector bundles on Calabi-Yau threefolds. In particular, we give sufficient conditions for Chern classes to correspond to stable bundles.
Let $X_0$ be a smooth projective threefold which is Fano or which has Picard number $1$. Let $π:X\rightarrow X_0$ be a finite composition of blowups along smooth centers. We show that for "almost all" of such $X$, if $f\in Aut(X)$ then its first and second dynamical degrees are the same. We also construct many examples of finite blowups $X\rightarrow X_0$, on which any automorphism is of zero entropy. The main idea is that because of the log-concavity of dynamical systems and the invariance of Chern classes under holomorphic automorphisms, there are some constraints on the nef cohomology class…
If a smooth projective threefold $X$ satisfies a certain Property A (see below for definition), then any automorphism of $X$ has zero entropy. Let $Y$ be a smooth projective threefold satisfying Property A. Let $π:X\rightarrow Y$ be a blowup at either a point or at a smooth curve $C\subset Y$ with the following two properties: i) $c_1(Y).C$ is an odd number, and ii) the normal vector bundle $N_{C/Y} $ is decomposable. Then we show that $X$ also satisfies Property A. As a further application of Property A we prove the following result. Let $X_1$ be the blowup of $X_0=\mathbb{P}^3$ at a finite n…
Moduli of stable sheaves on quadric threefold
本文研究光滑二次三维体 Kuznetsov 分量上的 Bridgeland 稳定性条件，证明投影摩天大楼层数值类对应的模空间非空且同构于某个光滑不可约的四维 Gieseker 模空间，并借此将二次三维体重新解释为 Brill–Noether 轨迹。
Enumerating Calabi-Yau Manifolds: Placing bounds on the number of diffeomorphism classes in the Kreuzer-Skarke list
该研究针对Picard数不超过6的Kreuzer-Skarke列表构造的卡拉比-丘三维流形，利用基无关不变量与线丛指数约束给出微分同胚类数量的上下界，并推测有利条目可产生约$10^{400}$个不同的微分同胚类。
H-Instanton Bundles on Three-Dimensional Smooth Toric Varieties with Picard Number Two
本文在具有 Picard 数为二的三维光滑 toric 变分上，研究 $H$-瞬子丛的构造与存在性，并推广了经典单调方法。
We have developed a mathematical theory of the topological vertex--a theory that was original proposed by M. Aganagic, A. Klemm, M. Marino, and C. Vafa in hep-th/0305132 on effectively computing Gromov-Witten invariants of smooth toric Calabi-Yau threefolds derived from duality between open string theory of smooth Calabi-Yau threefolds and Chern-Simons theory on three manifolds.
In this paper we deal with a particular class of rank two vector bundles (\emph{instanton} bundles) on the Fano threefold of index one $F:=\mathbb{F}_1 \times \mathbb{P}^1$. We show that every instanton bundle on $F$ can be described as the cohomology of a monad whose terms are free sheaves. Furthermore we prove the existence of instanton bundles for any admissible second Chern class and we construct a nice component of the moduli space where they sit. Finally we show that minimal instanton bundles (i.e. with the least possible degree of the second Chern class) are aCM and we describe their mo…
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