Boundedness for some rationally connected threefolds in $\mathbb P^6$

Codimension one regular foliations on rationally connected threefolds

In his work on birational classification of foliations on projective surfaces, Brunella showed that every regular foliation on a rational surface is algebraically integrable with rational leaves. This led Touzet to conjecture that every regular foliation on a rationally connected manifold is algebraically integrable with rationally connected leaves. Druel proved this conjecture for the case of weak Fano manifolds. In this paper, we extend this result showing that Touzet's conjecture is true for codimension one foliations on threefolds with nef anti-canonical bundle.

Finite quasisimple groups acting on rationally connected threefolds

We show that the only finite quasi-simple non-abelian groups that can faithfully act on rationally connected threefolds are the following groups: $\mathfrak{A}_5$, $\operatorname{PSL}_2(\mathbf{F}_7)$, $\mathfrak{A}_6$, $\operatorname{SL}_2(\mathbf{F}_8)$, $\mathfrak{A}_7$, $\operatorname{PSp}_4(\mathbf{F}_3)$, $\operatorname{SL}_2(\mathbf{F}_{7})$, $2.\mathfrak{A}_5$, $2.\mathfrak{A}_6$, $3.\mathfrak{A}_6$ or $6.\mathfrak{A}_6$. All of these groups with a possible exception of $2.\mathfrak{A}_6$ and $6.\mathfrak{A}_6$ indeed act on some rationally connected threefolds.

有理连通三维流形上1-闭链的几何可表示性

Geometric representability of 1-cycles on rationally connected threefolds

本文证明任意有理连通三维流形上都存在由光滑射影曲面参数化的1-闭链族，使其诱导Abel-Jacobi同构，从而将原先仅对部分光滑Fano三维流形成立的几何可表示性推广到一般情形。

包含所有曲线的三维簇是有理连通的

Threefolds containing all curves are rationally connected

本文证明：若三维簇$X$包含所有可嵌入的曲线，则$X$存在维数至少为3的有理连通子簇，且“所有曲线可嵌入”是三维簇的双有理性质。

$(\mathbb{Z}/4)^4$对有理连通三维流形的作用

Actions of $(\mathbb{Z}/4)^4$ on rationally connected threefolds

研究$(\mathbb{Z}/4)^4$对有理连通三维流形的作用，证明相关双有理结论，得出该群不嵌入$\operatorname{Cr}_3(\mathbb{C})$并结合早期结果，实现对$(\mathbb{Z}/m)^r$嵌入情况的完整分类。

有限交换群作用于有理连通三维流形 II：K3 类型群

Finite abelian groups acting on rationally connected threefolds II: groups of K3 type

本文研究有限交换群作用于三维有理连通流形的性质，重点分析K3类型群的分类及其与K3表面的关系。

Codimension two integral points on some rationally connected threefolds are potentially dense

Let $V$ be a smooth, projective, rationally connected variety, defined over a number field $k$, and let $Z\subset V$ be a closed subset of codimension at least two. In this paper, for certain choices of $V$, we prove that the set of $Z$-integral points is potentially Zariski dense, in the sense that there is a finite extension $K$ of $k$ such that the set of points $P\in V(K)$ that are $Z$-integral is Zariski dense in $V$. This gives a positive answer to a question of Hassett and Tschinkel from 2001.

Birational boundedness of rationally connected Calabi-Yau 3-folds

We prove that rationally connected Calabi--Yau 3-folds with kawamata log terminal (klt) singularities form a birationally bounded family, or more generally, rationally connected $3$-folds of $ε$-CY type form a birationally bounded family for $ε>0$. Moreover, we show that the set of $ε$-lc log Calabi--Yau pairs $(X, B)$ with coefficients of $B$ bounded away from zero is log bounded modulo flops. As a consequence, we deduce that rationally connected klt Calabi--Yau $3$-folds with mld bounded away from $1$ are bounded modulo flops.

Singular rationally connected threefolds with non-zero pluri-forms

This paper is concerned with singular projective rationally connected threefolds $X$ which carry non-zero pluri-forms, \textit{i.e.} $H^0(X,(Ω_X^1)^{[\otimes m]}) \neq \{0\}$ for some $m > 0$, where $(Ω_X^1)^{[\otimes m]}$ is the reflexive hull of $(Ω_X^1)^{\otimes m}$. If $X$ has $\mathbb{Q}$-factorial terminal singularities, then we show that there is a fibration $p$ from $X$ to $\mathbb{P}^1$. Moreover, there is a natural isomorphism from $H^0(X, (Ω_X^1)^{[\otimes m]})$ to $H^0(\mathbb{P}^1, \mathscr{O}_{\mathbb{P}^1}(-2m+\sum_{z\in \mathbb{P}^1} [\frac{(m(p,z)-1)m}{m(p,z)}]))$ for all $m>0…

具有固定指数的有理连通对数 Calabi–Yau 对的双有理有界性

Birational boundedness of rationally connected log Calabi-Yau pairs with fixed index

本文研究固定维数的有理连通 klt 对数 Calabi–Yau 对，证明在反对数典范类的固定 Cartier 指数与 nef 条件下，相应射影簇的集合模 flops 有界。

Symplectic geometry of rationally connected threefolds

We study symplectic geometry of rationally connected $3$-folds. The first result shows that rationally connectedness is a symplectic deformation invariant in dimension $3$. If a rationally connected $3$-fold $X$ is Fano or $b_2(X)=2$, we prove that it is symplectic rationally connected, i.e. there is a non-zero Gromov-Witten invariant with two insertions being the class of a point. Finally we prove that many rationally connected $3$-folds are birational to a symplectic rationally connected variety.

作用在有理连通三维簇上的有限阿贝尔群 I：乘积型群

Finite abelian groups acting on rationally connected threefolds I: Groups of product type

本文研究忠实作用在三维有理连通簇上的有限阿贝尔群，将其分为乘积型、K3型及第三类，完成乘积型群分类，建立K3型群有界性结果并提出第三类群猜想。

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。

An optimal boundedness on weak $\bQ$-Fano threefolds

Let $X$ be a terminal weak $\bQ$-Fano threefold. We prove that $P_{-6}(X)>0$ and $P_{-8}(X)>1$. We also prove that the anti-canonical volume has a universal lower bound $-K_X^3 \geq 1/330$. This lower bound is optimal.

Boundedness of threefolds of Fano type with Mori fibration structures

We show boundedness of $3$-folds of $ε$-Fano type with Mori fibration structures. The proof is based on the birational boundedness result in our previous work arXiv:1509.08722 combining with arguments in Kawamata \cite{K} and Kollár--Miyaoka--Mori--Takagi \cite{KMMT}.

Weak boundedness of Fano threefolds with large Seshadri constants in characteristic $p>5$

Given $ε>0$, we show that over an algebraically closed field of characteristic $p>5$, the anticanonical volume of a Fano threefold $X$ (with arbitrary singularities) whose anticanonical divisor has Seshadri constant $ε(-K_X,x)>2+ε$ at some smooth point $x\in X$ is bounded from above.

正特征域上纤维化Fano三维簇的补集有界性

Boundedness of complements for fibered Fano threefolds in positive characteristic

本文在正特征大特征情形下证明了Fano型纤维化的典范丛公式，并解决了具有纤维化结构的Fano型三维配对的Shokurov补集有界性猜想，特别覆盖了$-(K_X+B)$为nef且非big的情形。

Boundedness of anti-canonical volumes of singular log Fano threefolds

We prove Weak Borisov--Alexeev--Borisov Conjecture in dimension three which states that the anti-canonical volume of an $ε$-klt log Fano pair of dimension three is bounded from above.

Abel-Jacobi maps associated to smooth cubic threefolds

We pose some questions about spaces parametrizing rational curves on rationally connected varieties. We give a partial answer for cubic threefolds. Many of our results were previously proved by Iliev, Markushevich and Tikhimirov by analyzing vector bundles on cubic threefolds. Our proofs use residuation rather than vector bundles.

Generalized Tsen theorem and rationally connected Fano fibrations

We prove that a fibration X \to \Bbb P_1, the general fiber of which is a smooth Fano threefold, is rationally connected. The proof is based on a generalization of Tsen's classical theorem: a fibration X/C over a curve the general fiber of which is a Fano complete intersection in a product of weighted projective spaces has a section.

Boundedness of Fano threefolds with log-terminal singularities of given index

We prove the boundedness theorem for Fano threefolds with log-terminal singularities of any fixed index. This is an improvement of our earlier result, where we required additionally that the variety is Q-factorial, with Picard number 1. The new ideas of the paper include the following. 1. Using Alexeev Minimal Model program with suitable boundary to find horizontal extremal contractions. 2. Using Kollár's effective Base Point Freeness theorem. 3. Using Kawamata's result on the length of extremal curves with suitable boundary to avoid gluing curves in some cases.

On the Bimeromorphic Geometry of Compact Complex Contact Threefolds

We prove that a compact contact threefold which is bimeromorphically equivalent to a Kaehler manifold and not rationally connected is the projectivised tangent bundle of a Kaehler surface.

Boundedness theorem for Fano log-threefolds

The main purpose of this article is to prove that the family of all Fano threefolds with log-terminal singularities with bounded index is bounded.

正特征下具有固定Gorenstein指数的弱Fano三维簇的有界性

Boundedness of weak Fano threefolds with fixed Gorenstein index in positive characteristic

该研究在特征p>5的不可数代数闭域上，通过引入Gorenstein指数、补与Kodaira型零化条件，部分证明了三维簇的BAB猜想，同时给出了射影三维簇上nef大可逆层Seshadri常数的一致下界。

对数 Calabi-Yau 三维簇的补集有界性

Boundedness of complements for log Calabi-Yau threefolds

本研究针对系数集为[0,1]的对数Calabi-Yau型簇，证明三维簇对若存在局部klt的ℝ-补集，则对应有限正整数集中的某个n有n-补集，同时证得ℝ-互补曲面对的补集有界性。

Calabi–Yau 三维簇的有界性问题

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。

Picard groups of Siegel modular threefolds and theta lifting

We show that the Humbert surfaces rationally generate the Picard groups of Siegel modular threefolds.

椭圆Calabi-Yau三维簇的有界性

Boundedness of elliptic Calabi-Yau threefolds

该研究证明椭圆Calabi-Yau三维簇构成有界族，同时明确在固定多重典范形式增长率与Iitaka纤维化多截面次数的条件下，Kodaira维数为2的极小终端三维簇也满足该有界性结论，且两个假设均为必要条件。

Weak boundedness theorems for canonically fibered Gorenstein minimal threefolds

Let $X$ be a Gorenstein minimal projective 3-fold with at worst locally factorial terminal singularities. Suppose the canonical map is of fiber type. Denote by $F$ a smooth model of a generic irreducible component in fibers of the canonical map of $X$ and so $F$ is a smooth curve or a smooth surface. The main result of the paper is that there is a computable constant $K$ (independent of $X$) such that $g(F)\leq 647$ or $p_g(F)\leq 38$ whenever $p_g(X)\geq K$. The method heavily relies on both a Noether type of inequality and a Miyaoka-Yau inequality and that is the reason we only treat a Goren…

法诺三维簇的川又有界性与分次环数据库

Kawamata boundedness for Fano threefolds and the Graded Ring Database

本研究实现了法诺三维簇的川又有界性，给出39550个候选希尔伯特级数列表，为法诺三维簇分类提供初步近似，明确了相关数据的含义及与分类计划的关联。

Abel-Jacobi map, integral Hodge classes and decomposition of the diagonal

Given a smooth projective 3-fold Y, with $H^{3,0}(Y)=0$, the Abel-Jacobi map induces a morphism from each smooth variety parameterizing 1-cycles in Y to the intermediate Jacobian J(Y). We study in this paper the existence of families of 1-cycles in Y for which this induced morphism is surjective with rationally connected general fiber, and various applications of this property. When Y itself is rationally connected, we relate this property to the existence of an integral homological decomposition of the diagonal. We also study this property for cubic threefolds, completing the work of Iliev-Ma…

Abel-Jacobi map, integral Hodge classes and decomposition of the diagonal

Given a smooth projective 3-fold Y, with $H^{3,0}(Y)=0$, the Abel-Jacobi map induces a morphism from each smooth variety parameterizing 1-cycles in Y to the intermediate Jacobian J(Y). We study in this paper the existence of families of 1-cycles in Y for which this induced morphism is surjective with rationally connected general fiber, and various applications of this property. When Y itself is rationally connected, we relate this property to the existence of an integral homological decomposition of the diagonal. We also study this property for cubic threefolds, completing the work of Iliev-Ma…

Zero cycles on rationally connected varieties over Laurent fields

We study zero cycles on rationally connected varieties defined over characteristic zero Laurent fields with algebraically closed residue fields. We show that the degree map induces an isomorphism for rationally connected threefolds defined over such fields. In general, the degree map is an isomorphism if rationally connected varieties defined over algebraically closed fields of characteristic zero satisfy the integral Hodge/Tate conjecture for one cycles, or if the Tate conjecture is true for divisor classes on surfaces defined over finite fields. To prove these results, we introduce technique…

On the boundedness of singularities via normalized volume

In this article we study conjectures regarding normalized volume and boundedness of singularities. We focus on singularities with a torus action of complexity 1, threefold singularities, and hypersurface singularities. Given a real value v>0, we prove that the class of K-semistable threefold singularities with normalized volume at least v forms a bounded family. Analogous statements are proved in the case of n-dimensional complexity-1 and n-dimensional hypersurface singularities for arbitary n. In the general case of klt singularities, i.e. without the assumption on K-semistability, we show th…

Unirational threefolds with no universal codimension 2 cycle

We prove that the general quartic double solid with $k\leq 7$ nodes does not admit a Chow theoretic decomposition of the diagonal, or equivalently has a nontrivial universal ${\rm CH}_0$ group. The same holds if we replace in this statement "Chow theoretic" by "cohomological". In particular, it is not stably rational. We also prove that the general quartic double solid with seven nodes does not admit a universal codimension 2 cycle parameterized by its intermediate Jacobian, and even does not admit a parametrization with rationally connected fibres of its Jacobian by a family of 1-cycles. This…

On equivalent conjectures for minimal log discrepancies on smooth threefolds

On smooth threefolds, the ACC for minimal log discrepancies is equivalent to the boundedness of the log discrepancy of some divisor which computes the minimal log discrepancy. We reduce it to the case when the boundary is the product of a canonical part and the maximal ideal to some power. We prove the reduced assertion when the log canonical threshold of the maximal ideal is either at most one-half or at least one.

Boundedness of fibers for pluricanonical maps of varieties of general type

We prove that the r-th pluricanonical maps of threefolds of general type have birationally bounded fibers if $r\geqslant 2$. Similarly, we prove that the r-th pluricanonical maps of fourfolds of general type have birationally bounded fibers if $r\geqslant 4$. We extend these results to higher dimensions in terms of constants arising naturally from the birational geometry of varieties of general type.

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.

ACC for minimal log discrepancies of terminal threefolds

We prove that the ACC conjecture for minimal log discrepancies holds for threefolds in $[1-δ,+\infty)$, where $δ>0$ only depends on the coefficient set. We also study Reid's general elephant for pairs, and show Shokurov's conjecture on the existence of $(ε,n)$-complements for threefolds for any $ε\geq 1$. As a key important step, we prove the uniform boundedness of divisors computing minimal log discrepancies for terminal threefolds. We show the ACC for threefold canonical thresholds, and that the set of accumulation points of threefold canonical thresholds is equal to $\{0\}\cup\{\frac{1}{n}\…

p-subgroups in the space Cremona group

We prove that if $X$ is a rationally connected threefold and $G$ is a $p$-subgroup in the group of birational selfmaps of $X$, then $G$ is an abelian group generated by at most $3$ elements provided that $p\ge 17$. We also prove a similar result for $p\ge 11$ under an assumption that $G$ acts on a (Gorenstein) $G$-Fano threefold, and show that the same holds for $p\ge 5$ under an assumption that $G$ acts on a $G$-Mori fiber space.

Effective log Iitaka fibrations for surfaces and threefolds

We prove an analogue of Fujino and Mori's ``bounding the denominators'' in the log canonical bundle formula (see also Prokhorov and Shokurov) for Kawamata log terminal pairs of relative dimension one. As an application we prove that for a klt pair $(X,Δ)$ of Kodaira codimension one and dimension at most three such that the coefficients of $Δ$ are in a DCC set $\mathcal{A}$, there is a natural number $N$ that depends only on $\mathcal{A}$ for which the round down of $\N(K_X+Δ)$ induces the Iitaka fibration. We also prove a birational boundedness result for klt surfaces of general type.

关于Cremona群的有限阿贝尔子群的阿贝尔扩张

On abelian extensions of finite abelian subgroups of Cremona groups

本文研究Cremona群有限阿贝尔子群的扩张性质，证明在维数至多4时扩张与直积一致（除一例外），从而对有限阿贝尔群情形下忠实作用提升至终端Fano簇的问题给出肯定回答。

具有极大秩交换群作用的紧 Kähler 三维簇上等变极小模型纲领的存在性

Existence of the equivariant minimal model program for compact Kähler threefolds with the action of an abelian group of maximal rank

本文针对容许极大秩、正熵自由交换群作用的 $\mathbb{Q}$-factorial 紧 Kähler klt 三维簇，建立 $G$-等变对数极小模型纲领，并据此给出其有理连通或双纯等价于 $Q$-复环面的分类结论。

平面上二次丛的二次有理多截面的不存在性

Nonexistence of degree two rational multisections of conic bundles over the plane

本文证明判别式次数至少18的标准复二次丛不存在二次有理多截面，为证明Iskovskikh猜想（存在非单有理的三维二次丛）迈出关键一步，有望得到首个非单有理的有理连通代数簇。

Polarized endomorphisms of normal projective threefolds in arbitrary characteristic

Let $X$ be a projective variety over an algebraically closed field $k$ of arbitrary characteristic $p \ge 0$. A surjective endomorphism $f$ of $X$ is $q$-polarized if $f^\ast H \sim qH$ for some ample Cartier divisor $H$ and integer $q > 1$. Suppose $f$ is separable and $X$ is $\mathbb{Q}$-Gorenstein and normal. We show that the anti-canonical divisor $-K_X$ is numerically equivalent to an effective $\mathbb{Q}$-Cartier divisor, strengthening slightly the conclusion of Boucksom, de Fernex and Favre (Theorem C) and also covering singular varieties over an algebraically closed field of arbitrary…

具有严格 nef 相对反 log 典范除子的代数纤维空间

Algebraic fibre spaces with strictly nef relative anti-log canonical divisor

本文证明具有严格 nef 相对反 log 典范除子的纤维化必为纤维有理连通的局部常值纤维化，底空间为典范极化双曲流形，并在三维情形下证实了 Campana-Peternell 猜想的奇异版本。