Luca Chiantini, Carlo Madonna
In this paper we show that on a general sextic hypersurface $X\subset \bf P^4$, a rank 2 vector bundle $E$ splits if and only if $h^1(E(n))=0$ for any $n \in \bf Z$. We get thus a characterization of complete intersection curves in $X$
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We give a simple proof of the non-rationality of the Fano threefold defined by the equations Σx_i = Σx_i^2 = Σx_i^3 = 0 in P^6 .
We refine the classification of weak Fano threefolds with sextic del Pezzo fibrations by considering the Hodge numbers of them. By the refined classification result, such threefolds are classified into 17 cases. The main result of this paper is to show that there exists an example for each of 17 cases, except for 2 cases.
Double EPW-sextics with actions of A7 and irrational GM threefolds
本文构造了两个带 A7 作用的 K3[2] 型超凯勒四重流形，并利用其双 EPW 六次超曲面结构给出显式的非有理 Gushel-Mukai 三重流形族。
On the birational geometry of sextic threefold hypersurface in $\mathbf{P}(1,1,2,2,3)$
研究六次三维超曲面在加权射影空间中的理性质，证明其非理性。
Let $X$ be a double cover of $\mathbb P^3$ branched along a sextic surface $Y$. In this paper, we show that, for general $X$, the Abel-Jacobi map associated to the normalization $\tilde F(X)$ of the surface $F(X)$ of curves contained in $X$ which are preimages of lines bitangent to $Y$, gives an isogeny between the Albanese variety of $\tilde F(X)$ and the intermediate Jacobian of $X$.
Threefolds Fibred by Mirror Sextic Double Planes
本文系统研究以镜像六次双平面K3曲面为纤维的三维簇，提出广义函数与同调不变量，构造Weierstrass形式及极小形式，并分析其奇异纤维、典范除子和Betti数。
K-stability and space sextic curves of genus three
本文研究沿亏格3光滑六次曲线吹张三维射影空间得到的法诺三维簇，构造了其新K-稳定实例，并刻画了所有可忠实作用于其上的有限群。
We construct two small resolutions of singularities of the Coble fourfold (the double cover of the four-dimensional projective space branched over the Igusa quartic). We use them to show that all $S_6$-invariant three-dimensional quartics are birational to conic bundles over the quintic del Pezzo surface with the discriminant curves from the Wiman-Edge pencil. As an application, we check that $S_6$-invariant three-dimensional quartics are unirational, obtain new proofs of rationality of four special quartics among them and irrationality of the others, and describe their Weil divisor class grou…
We study two rational Fano threefolds with an action of the icosahedral group $\mathfrak{A}_5$. The first one is the famous Burkhardt quartic threefold, and the second one is the double cover of the projective space branched in the Barth sextic surface. We prove that both of them are $\mathfrak{A}_5$-Fano varieties that are $\mathfrak{A}_5$-birationally superrigid. This gives two new embeddings of the group $\mathfrak{A}_5$ into the space Cremona group.
We investigate versions of the Shafarevich conjecture, as proved for curves and abelian varieties by Faltings, for other classes of varieties. We first obtain analogues for certain Fano threefolds. We use these results to prove the Shafarevich conjecture for smooth sextic surfaces, which appears to be the first non-trivial result in the literature on the arithmetic of such surfaces. Moreover, we exhibit certain moduli stacks of Fano varieties which are not hyperbolic, which allows us to show that the analogue of the Shafarevich conjecture does not always hold for Fano varieties. Our results al…
EPW varieties as moduli spaces on ordinary GM surfaces and special GM threefolds
本文证明与强光滑 Gushel-Mukai 曲面相关的双对偶 EPW 六次曲面可实现为其有界导出范畴上半稳定对象的模空间，并观察到与特殊 Gushel-Mukai 三维族相关的双对偶 EPW 曲面可实现为其 Kuznetsov 分量上半稳定对象的模空间，进而讨论了主要结果到双 EPW 六次曲面和双 EPW 曲面的推广，以及在温和假设下对 Bayer 和 Perry 关于具有等价 Kuznetsov 分量的 Gushel-Mukai 三维族的一个陈述的改进。
A rationality criterion for real Fano threefolds
该研究探讨光滑几何有理法诺三维簇实轨迹的连通性，提出并证明了一个实合理性的充分准则，为相关几何对象的合理性分析提供了理论依据。
Counterexample to Néron-Ogg-Shafarevich criterion for Calabi-Yau threefolds
本文构造了一个Calabi-Yau三维流形反例，证明其模型特殊纤维奇异而上同调Galois作用良好，推翻了三维情形下的Néron-Ogg-Shafarevich判据。
Let $W\subset \mathbb{P}^{13}$ be the image of the rational map defined by the linear system of the sextic surfaces of $\mathbb{P}^3$ having double points along the edges of a tetrahedron. Let $\mathcal{L}$ be the linear system of the hyperplane sections of $W$. It is known that a general $S\in \mathcal{L}$ is an Enriques surface. The aim of this paper is to study the sublinear system $\mathcal{L}_{\bullet}\subset \mathcal{L}$ of the hyperplane sections of $W$ having a triple point at a general point $w \in W$. We will show that a general element of $\mathcal{L}_{\bullet}$ is birational to an …
We give a criterion for a continuous family of curves on a nodal $K$-trivial threefold $X_0$ to contribute geometrically rigid curves to a general smoothing of $X_0$. As an application, we prove the existence of geometrically rigid curves of arbitrary degree and explicitly bounded genus on general complete intersection Calabi-Yau threefolds.
We present a combinatorial criterion on reflexive polytopes of dimension 3 which gives a local-to-global obstruction for the smoothability of the corresponding Fano toric threefolds. As a result, we show examples of singular Gorenstein Fano toric threefolds which have compound Du Val, hence smoothable, singularities but are not smoothable.
Let B be a nef and big line bundle on a smooth complex threefold X with canonical bundle K. Let x be a point on X and suppose that BC\ge3 for any curve C passing x, B^2S\ge7 for any surface S containing x, and B^3\ge51. Then K+B is spanned at x. (Ein-Lazarsfeld proved the assertion assuming B^3\ge92.) Corollary: K+3L is spanned if L is an ample line bundle with L^3>1.
We prove rationality criteria over algebraically non-closed fields of characteristic $0$ for five out of six types of geometrically rational Fano threefolds of Picard number $1$ and geometric Picard number bigger than $1$. For the last type of such threefolds we provide a unirationality criterion and prove stable non-rationality under additional assumptions.
Using an explicit resolution of the diagonal for the variety V_5, we provide cohomological characterizations of the universal and quotient bundles. A splitting criterion for bundles over V_5 is also proved. The presentation of semistable aCM (i.e. with no intermediate cohomology) bundles is shown, together with a resolution-theoretic classification of low rank aCM bundles.
Algebraic hyperbolicity of very general hypersurfaces in projective spaces
本文证明了射影空间中非常一般超曲面代数双曲性的最后未解情形，即四次三维簇，完成了分类，方法基于Coskun--Riedl构造及稳定映射模空间和对数几何。
We prove a criterion of nonsingularity of a complete intersection of two fiberwise quadrics in a scroll over $P^1$. As a corollary we derive the following addition to the Alexeev theorem on rationality of standard Del Pezzo fibrations of degree 4 over $P^1$: we prove that any fibration of this kind with the topological Euler characteristic -4 is rational.
We give a criterion for a nef divisor $D$ to be semiample on a Calabi--Yau threefold $X$ when $D^3=0=c_2(X)\cdot D$ and $c_3(X)\neq 0$. As a direct consequence, we show that on such a variety $X$, if $D$ is strictly nef and $ν(D)\neq 1$, then $D$ is ample; we also show that if there exists a nef non-ample divisor $D$ with $D\not\equiv 0$, then $X$ contains a rational curve when its topological Euler characteristic is not $0$.
According to Mukai, any prime Fano threefold X of genus 7 is a linear section of the spinor tenfold in the projectivized half-spinor space of Spin(10). The orthogonal linear section of the spinor tenfold is a canonical genus-7 curve G, and the intermediate Jacobian J(X) is isomorphic to the Jacobian of G. It is proven that, for a generic X, the Abel-Jacobi map of the family of elliptic sextics on X factors through the moduli space of rank-2 vector bundles with c_1=-K_X and deg c_2=6 and that the latter is birational to the singular locus of the theta divisor of J(X).
Serre-invariant stability conditions and Ulrich bundles on cubic threefolds
本文证明了立方三项式上乌里奇束模空间的不可约性，并展示了塞雷不变稳定性条件的唯一性。
A Finiteness Theorem for Quartic K3-Fibred Calabi--Yau Threefolds in Scrolls
本文针对堆卷中以四次K3曲面为纤维的Calabi-Yau三维簇，应用Reid-Shepherd-Barron-Tai准则分类了n=1,2,3的情况，共得到14个形变族，补充了相关显式分类数据。
In this note we show that Frobenius split smooth, projective threefolds are Hodge-Witt (using a criterion for the degeneration of the slope spectral sequence of smooth projective threefolds which we prove), and that smooth, projective Frobenius split varieties do not have exotic torsion in their slope spectral sequence. We also record a few simple observations on Frobenius split threefolds. This paper complements authors paper with C. S. Rajan ``On Frobenius splitting and ordinarity'' which is also available on this archive.
Deformations of toric pairs and the smoothing of Batyrev Calabi-Yau threefolds
本文构造环面四维流形的形变以光滑化 Calabi-Yau 超曲面的奇点，提出逐面准则，并指出其覆盖 Batyrev--Kreuzer 普查中约 12.5% 的光滑化，其余需跨面关系。
Resonance, syzygies, and rank-$3$ Ulrich bundles on the del Pezzo threefold $V_5$
本文研究了在del Pezzo三维流形V_5上，通过乌里奇向量包的截面零点描述亏格14、次数18的曲线的几何条件，并探讨了其与特殊秩2向量包的共振关系，以及曲线唯一性结论。
We show that codimension one distributions with at most isolated singularities on certain smooth projective threefolds with Picard rank one have stable tangent sheaves. The ideas in the proof of this fact are then applied to the characterization of certain irreducible components of the moduli space of stable rank 2 reflexive sheaves on $\mathbb{P}^3$, and to the construction of stable rank 2 reflexive sheaves with prescribed Chern classes on general threefolds. We also prove that if $\mathscr{G}$ is a subfoliation of a codimension one distribution $\mathscr{F}$ with isolated singularities, the…
We show that the family of 21-dimensional intermediate jacobians of cubic fivefolds containing a given cubic fourfold X is generically an algebraic integrable system. In the proof we apply an integrability criterion, introduced and used by Donagi and Markman to find a similar integrable system over the family of cubic threefolds in X. To enter in the conditions of this criterion, we write down explicitly the known by Beauville and Donagi symplectic structure on the family F(X) of lines on the general cubic fourfold X, and prove that the family of planes on a cubic fivefold containing X is embe…
Rigidity and Realizability for Tropical Curves in Dimension 3
本研究利用伪全纯圆盘边界的$H_1(L)$类提出拉格朗日三维流形的无障碍判据，证明具裤对分解的刚性热带曲线的拉格朗日提升无障碍，并在同调镜像对称成立的前提下推出镜像阿贝尔三维流形中存在对应实现。
We use homological methods to establish a formal criterion for Generic Vanishing, in the sense originated by Green and Lazarsfeld and pursued further by Hacon and the first author, but in the context of an arbitrary Fourier-Mukai correspondence. For smooth projective varieties we apply this to deduce a Kodaira-type generic vanishing theorem for adjoint bundles of the form $K_X + L$ with $L$ a nef line bundle, and in fact a more general generic Nadel-type vanishing theorem for multiplier ideal sheaves. Still in the context of the Picard variety, the same method generates various other generic v…
Tensor-generating line bundles on toric varieties
该研究在环簇上刻画了张量生成线丛的组合判据，证明了Kleiman型的生成性判据，给出了相关例子并研究了典范丛生成的阻碍子簇及其对导出范畴自等价的影响。
Jacobian algebras and variation of hyperplane sections
通过 Milnor 代数研究至多孤立奇点超曲面的超平面截面模空间的变分，给出超平面截面映射为有限态的无穷小商判据，并推广了光滑情形的 Beauville-Patel-Riedl-Tseng 理论。
Deformed Hermitian Yang-Mills equation on rational homogeneous varieties
本文证明了有理齐次簇上形变埃尔米特 Yang-Mills 方程在任意不变 Kähler 度量下解的存在性，利用 Lie 理论和 Cartan 矩阵刻画齐次解，并给出全纯向量丛斜率半稳定性的新判据及其在 Fano 三维簇上的应用。
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