David Jensen
This is the second of two papers on the birational geometry of $\bar{M}_{g,1}$. We construct rational maps from $\bar{M}_{5,1}$ and $\bar{M}_{6,1}$ to lower-dimensional moduli spaces. As a consequence, we identify geometric divisors that generate extremal rays of the effective cones for these spaces.
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We prove that very general non-rational Fano threefolds which are not birational to cubic threefolds are not stably rational.
Rationality of moduli of elliptic fibrations with fixed monodromy
证明了椭圆K3曲面和椭圆有理曲面的模空间具有固定单值群的合理性结果。
Using deformation theory of rational curves, we prove a conjecture of Sommese on the extendability of morphisms from ample subvarieties when the morphism is a smooth (or mildly singular) fibration with rationally connected fibers. We apply this result in the context of Fano fibrations and prove a classification theorem for projective bundle and quadric fibration structures on ample subvarieties.
Fibrations by plane projective rational quartic curves in characteristic two
本文在特征二下对平面射影有理四次曲线纤维化进行了完整的双有理等价分类。
We consider threefold del Pezzo fibrations over a curve germ whose central fiber is non-rational. Under the additional assumption that the singularities of the total space are at worst ordinary double points, we apply a suitable base change and show that there is a 1-to-1 correpspondence between such fibrations and certain non-singular del Pezzo fibrations equipped with a cyclic group action.
We consider K3 surfaces which are double cover of rational elliptic surfaces. The former are endowed with a natural elliptic fibration, which is induced by the latter. There are also other elliptic fibrations on such K3 surfaces, which are necessarily induced by special linear systems on the rational elliptic surfaces. We describe these linear systems. In particular, we observe that every conic bundle on the rational surface induces a genus 1 fibration on the K3 surface and we classify the singular fibers of the genus 1 fibration on the K3 surface it terms of singular fibers and special curves…
In this paper we look for necessary and sufficient conditions for a genus one fibration to have rational curves. We show that a projective variety with log terminal singularities that admits a relatively minimal genus one fibration $X\rightarrow B$ does contain vertical rational curves if and only if it not isomorphic to a finite étale quotient of a product $\tilde{B}\times E$ over $B$. Many sufficient conditions for the existence of rational curves in a variety that admits a genus one fibration are proved in this paper.
The main aim of this article is to show that a very general 3-dimensional del Pezzo fibration of degree 1,2,3 is not stably rational except for a del Pezzo fibration of degree 3 belonging to explicitly described 2 families. Higher dimensional generalizations are also discussed and we prove that a very general del Pezzo fibration of degree 1,2,3 defined over the projective space is not stably rational provided that the anticanonical divisor is not ample.
We consider a rational surface with a relatively minimal fibration. Picard number of a such fibred surface is bounded in terms of the genus of a general fibre. When Picard number is the maximum for any given genus, we characterize a such fibred surface whose Mordell-Weil group is trivial by singular fibres. Furthermore, we describe the defining equation explicitly.
We study K3 surfaces over a number field $k$ which are double covers of extremal rational elliptic surfaces. We provide a list of all elliptic fibrations on certain K3 surfaces together with the degree of a field extension over which each genus one fibration is defined and admits a section. We show that the latter depends, in general, on the action of the cover involution on the fibers of the genus 1 fibration.
In this paper we complete the classification of the elliptic fibrations on K3 surfaces which admit a non-symplectic involution acting trivially on the Néron--Severi group. We use the geometric method introduced by Oguiso and moreover we provide a geometric construction of the fibrations classified. If the non-symplectic involution fixes at least one curve of genus 1, we relate all the elliptic fibrations on the K3 surface with either elliptic fibrations or generalized conic bundles on rational elliptic surfaces. This description allows us to write the Weierstrass equations of the elliptic fibr…
This paper has been withdrawn. A completely rewritten new version will be soon submitted. In order to obtain the inequality $6(g-1) \le K_f^2$ some additional conditions must be imposed on the fibration, involving the number on certain exceptional curves.
Del Pezzo fibrations appear as minimal models of rationally connected varieties. The rationality of smooth del Pezzo fibrations is a well studied question but smooth fibrations are not dense in moduli. Little is known about the rationality of the singular models. We prove birational rigidity, hence non-rationality, of del Pezzo fibrations with simple non-Gorenstein singularities satisfying the famous $K^2$-condition. We then apply this result to study embeddings of $\operatorname{PSL}_2(7)$ into the Cremona group.
On the density of rational points on elliptic fibrations
设$V_1$是Fano三重曲面，作为$P(1,1,1,2,3)$中的6次超曲面（数域$K$上）。则存在有限扩张$K'/K$，使得$X$的$K'$ -有理点集是Zebraki稠密的.
Ample subvarieties and rationally connected fibrations
在一些正性假设下，研究了有理连通纤维化从子流形到其周围簇的扩张性质。给定复射影流形X上的一簇有理曲线诱导一个子流形Y上的覆盖族，该子流形Y具有X中的丰富法丛，在适当的条件下，主要结果涉及X和Y上相应的有理连通纤维结构.这些结果的应用包括纤维型Mori压缩的扩张定理和当Y具有投射丛或二次纤维化结构时的分类定理。
We consider fibrations by affine lines on smooth affine surfaces obtained as complements of smooth rational curves $B$ in smooth projective surfaces $X$ defined over an algebraically closed field of characteristic zero. We observe that except for two exceptions, these surfaces $X \setminus B$ admit infinitely many families of $\mathbb{A}^1$-fibrations over the projective line with irreducible fibers and a unique singular fiber of arbitrarily large multiplicity. For $\mathbb{A}^1$-fibrations over the affine line, we give a new and essentially self-contained proof that the set of equivalence cla…
Rational Lagrangian fibrations on punctual Hilbert schemes of K3 surfaces
不可约辛簇V上的有理Lagrange纤维化f是一个有理映射，它与一个具有Lagrange纤维的正则满射双有理等价。通过与K3曲面的类比，很自然地期望存在有理拉格朗日纤维化当且仅当V具有Bogomolov-Beauville平方为0的因子D。当V是一般代数K3曲面S的点Hilbert格式时，证明了这个猜想。f的构造使用扭曲的傅立叶-Mukai变换，该变换诱导V的双有理同构，该双有理同构具有另一个K 3表面M上的扭曲层的特定模空间，从S获得作为其傅立叶-Mukai伙伴。
Polarized rational surfaces $(X, \mathcal L)$ of sectional genus two ruled in conics are studied. When they are not minimal, they are described as the blow-up of $\mathbb F_1$ at some points lying on distinct fibers. Ampleness and very ampleness of $\mathcal L$ are studied in terms of their location. When $\mathcal L$ is very ample and there is a line contained in $X$ and transverse to the fibers, the conic fibrations $(X, \mathcal L)$ are classified and a related property concerned with the inflectional locus is discussed.
By studying the theory of rational curves, we introduce a notion of rational simple connectedness for projective homogeneous spaces. As an application, we prove that over a function field of an algebraic surface, a projective homogeneous space admits a rational point if the elementary obstruction vanishes.
Given a relatively minimal fibration $f: S \to \Bbb P^1$ on a rational surface $S$ with general fiber $C$ of genus $g$, we investigate under what conditions the inequality $6(g-1)\le K_f^2$ occurs, where $K_f$ is the canonical relative sheaf of $f$. We give sufficient conditions for having such inequality, depending on the genus and gonality of $C$ and the number of certain exceptional curves on $S$. We illustrate how these results can be used for constructing fibrations with the desired property. For fibrations of genus $11\le g\le 49$ we prove the inequality: $$ 6(g-1) +4 -4\sqrt g \le K_f^2…
Elliptic fibrations of some extremal K3 surfaces
本文研究了极值椭圆K3曲面的构造。它给出了一个完整的治疗这些纤维化，可以来自合理的椭圆曲面，通过简单的操作，他们的维尔斯特拉斯方程。特别是，这种方法使我们能够找到38个半稳定的极值椭圆K3纤维化，其中32个确实是定义在有理数明确的方程。它们被实现为非半稳定的极值有理椭圆曲面通过基变化的拉回。这与J. Top和N. Yui的方法，该方法对于半稳定的极值有理椭圆曲面具有相同的过程。
We investigate the existence, and lack of unicity, of a holomorphic fibration by discs transversal to a rational curve in a complex surface.
We study weak approximation on rationally connected varieties under an assumption of strong approximation for a "simple" variety or under Schinzel's hypothesis. We also get some unconditional results.
Let X be a smooth, complex Fano variety. For every prime divisor D in X, we set c(D):=dim ker(r:H^2(X,R)->H^2(D,R)), where r is the natural restriction map, and we define an invariant of X as c_X:=max{c(D)|D is a prime divisor in X}. In a previous paper we showed that c_X<9, and that if c_X>2, then either X is a product, or X has a flat fibration in Del Pezzo surfaces. In this paper we study the case c_X=2. We show that up to a birational modification given by a sequence of flips, X has a conic bundle structure, or an equidimensional fibration in Del Pezzo surfaces. We also show a weaker prope…
Enumerating curves on rational surfaces: the rational fibration method
本文提出了一种新的、简单的方法来处理有理曲面上有理曲线的计数问题。应用包括平面曲线的Kontsevich公式的简短证明和Hirzebruch曲面F_3的类似问题的解决方案。
We develop technics of birational geometry to study automorphisms of affine surfaces admitting many distinct rational fibrations, with a particular focus on the interactions between automorphisms and these fibrations. In particular, we associate to each surface $S$ of this type a graph encoding equivalence classes of rational fibrations from which it is possible to decide for instance if the automorphism group of $S$ is generated by automorphisms preserving these fibrations.
On C-fibrations over projective curves
本文的目标是提出一个修改后的版本（GML）的ML不变，它应该考虑到在一个投影基地的裁决，并允许进一步分层光滑仿射有理曲面。我们提供了一个非平凡的例子，GML不变计算光滑仿射有理表面承认没有C-行动。我们将GML不变量应用于某些三重ML不变量的计算。
We study (smooth, complex) Fano 4-folds X having a rational contraction of fiber type, that is, a rational map X-->Y that factors as a sequence of flips followed by a contraction of fiber type. The existence of such a map is equivalent to the existence of a non-zero, non-big movable divisor on X. Our main result is that if Y is not P^1 or P^2, then the Picard number rho(X) of X is at most 18, with equality only if X is a product of surfaces. We also show that if a Fano 4-fold X has a dominant rational map X-->Z, regular and proper on an open subset of X, with dim(Z)=3, then either X is a produ…
To classify complex rational cuspidal curves $E\subseteq \mathbb{P}^2$ it remains to classify the ones with complement of log general type, i.e. the ones for which $κ(K_X+D)=2$, where $(X,D)$ is a log resolution of $(\mathbb{P}^2,E)$. It is conjectured that $κ(K_X+\frac{1}{2}D)=-\infty$ and hence $\mathbb{P}^2\setminus E$ is $\mathbb{C}^{**}$-fibered, where $\mathbb{C}^{**}=\mathbb{C}^1\setminus\{0,1\}$, or $-(K_X+\frac{1}{2}D)$ is ample on some minimal model of $(X,\frac{1}{2}D)$. Here we classify, up to a projective equivalence, those rational cuspidal curves for which the complement is $\ma…
Let X -> Y be a fibration whose fibers are complete intersections of two quadrics. We develop new categorical and algebraic tools---a theory of relative homological projective duality and the Morita invariance of the even Clifford algebra under quadric reduction by hyperbolic splitting---to study semiorthogonal decompositions of the bounded derived category of X. Together with new results in the theory of quadratic forms, we apply these tools in the case where X -> Y has relative dimension 1, 2, or 3, in which case the fibers are curves of genus 1, Del Pezzo surfaces of degree 4, or Fano three…
On rationality of nonsingular threefolds with a pencil of Del Pezzo surfaces of degree 4
本文证明了$P^1$上两个纤维二次曲面的完全交的非奇异性准则。作为推论，我们得到以下除了Alexeev定理的合理性标准Del Pezzo纤维化的4度$P^1$：我们证明，任何纤维化的这种拓扑欧拉示性数-4是合理的。
Conjectures on the existence of zero-cycles on arbitrary smooth projective varieties over number fields were proposed by Colliot-Thélène, Sansuc, Kato and Saito in the 1980's. We prove that these conjectures are compatible with fibrations, for fibrations into rationally connected varieties over a curve. In particular, they hold for the total space of families of homogeneous spaces of linear groups with connected geometric stabilisers. We prove the analogous result for rational points, conditionally on a conjecture on locally split values of polynomials which a recent work of Matthiesen establi…
Nonrational del Pezzo fibrations
设$X$是有理涡卷$\mathrm{Proj}(\oplus_{i=1}^{4}\mathcal{O}_{\mathbb{P}^{1}}(d_{i}))$上$|3M+nL|$中的一个通约数，其中$d_{i}$和$n$是整数，$M$是重言式线丛，$L$是到$\mathbb{P}^{1}$的自然投影的纤维，而$d_{1}\geqslant...\geqslant d_{4}=0$。证明了$X$是有理$\iff$ $d_{1}=0$和$n=1$ .
We consider a mixed function of type $H(z,\bar z)=f(z)\bar g(z)$ where $f,g$ are non-degenerate but they are not assumed to be convenient. We assume that $f=0$ and $g=0$ and $f=g=0$ are non-degenerate and locally tame. We will show that $H$ has a tubular Milnor fibration and a spherical Milnor fibration. We show also two fibrations are equivalent.
Let $X$ be a projective variety with log terminal singularities and vanishing augmented irregularity. In this paper we prove that if $X$ admits a relatively minimal genus one fibration then it does contain a subvariety of codimension one covered by rational curves contracted by the fibration. We then focus on the case of varieties with numerically trivial canonical bundle and we discuss several consequences of this result.
Twelve points on the projective line, branched covers, and rational elliptic fibrations
在P^1上的十二个点的空间Sym^{12} P^1中，下列因子实际上是相同的：（A）有理椭圆纤维化中十二个节点纤维的可能轨迹（B）可以表示为立方体加正方形的12阶二进制形式;（C）光滑平面四次曲线从平面一般点的12条切线的轨迹;（D）超椭圆亏格为3的曲线到P^1的4次映射的分支轨迹;（E）由θ-特征诱导的从亏格4曲线到P^1的3次映射的分支轨迹;以及其他几个。相应的模空间是光滑的，但它们并不都是同构的;有些是其他的有限余覆盖。我们描述了这些空间之间的互联网络，并给出了单值性，合理性和普林相关的后果。列举的后果包括：（i）该位点的度为3762（例如，有3762个有理椭圆纤维化的节点高于11个给定的一般点的基础）;（ii）如果C -> P^1是一个覆盖，如（D），那么有135个不同的覆盖分支在相同的点;（iii）在（C）中出现的12条切线的一般集合以120种本质上不同的方式出现。这个故事的某些部分是众所周知的，而其他一些部分则是经典的（Zeuthen，Zebraki，Coble，Mumford等）。统一的图像是令人惊讶的复杂和连接许多美丽的结构，包括Recillas的三角结构和Shioda的E8-Mordell-Weil晶格。
We prove some results on the fibers and images of rational maps from a hyper-Kähler manifold. We study in particular the minimal genus of fibers of a fibration into curves. The last section of this paper is devoted to the study of the rational map defined by a linear system on a hyper-Kähler fourfold satisfying numerical conditions similar to those considered by O'Grady in his study of fourfolds numerically equivalent to $K3^{[2]}$. We extend his results to this more general context.
We revisit the abstract framework underlying the fibration method for producing rational points on the total space of fibrations over the projective line. By fine-tuning its dependence on external arithmetic conjectures, we render the method unconditional when the degree of the non-split locus is $\leq 2$, as well as in various instances where it is $3$. We are also able to obtain improved results in the regime that is conditionally accessible under Schinzel's hypothesis, by incorporating into it, for the first time, a technique due to Harari for controlling the Brauer--Manin obstruction in fa…
Abelian fibrations and rational points on symmetric products
给定数域上的一个簇，它的有理点是否潜在稠密，即，是否存在一个有限扩张，在其上的有理点是Zebraki稠密的？本文研究了曲面的对称积的势密度问题。与曲线的情况相反，有理点在足够高的对称乘积上不一定是潜在密集的。我们的主要结果是，有理点是潜在的密集的第N次对称产品的K3曲面，其中N是明确确定的几何曲面。其基本构造是，对于一定的N，K3曲面的N次对称幂是P^N上阿贝尔纤维化的双有理曲面。寻找具有此性质的最小N是一个有趣的几何问题。
We consider a mixed function of type $H(\mathbf z,\bar {\mathbf z})=f(\mathbf z)\bar g(\mathbf z)$ where $f$ and $g$ are convenient holomorphic functions which have isolated critical points at the origin and we assume that the intersection $f=g=0$ is a complete intersection variety with an isolated singlarity at theorigin. We assume also that $H$ satisfies the multiplicity condition.We will show that $H$ has a tubular Milnor fibration and also a spherical Milnor fibration. We give examples which does not satisfy the Newton multiplicity condition where one does not have Milnor fibration and the…
We study the problem of counting the number of varieties in families which have a rational point. We give conditions on the singular fibres that force very few of the varieties in the family to contain a rational point, in a precise quantitative sense. This generalises and unifies existing results in the literature by Serre, Browning-Dietmann, Bright-Browning-Loughran, Graber-Harris-Mazur-Starr, et al.
We consider the problem of counting the number of varieties in a family over $\mathbb{Q}$ with a rational point. We obtain lower bounds for this counting problem for some families over $\mathbb{P}^1$, even if the Hasse principle fails. We also obtain sharp results for some multinorm equations and for specialisations of certain Brauer group elements on higher dimensional projective spaces, where we answer some cases of a question of Serre. Our techniques come from arithmetic geometry and additive combinatorics.
We give a criterion to determine when the degree growth of a birational map of the complex projective plane which fixes (the action on the basis of the fibration is trivial) a rational fibration is linear up to conjugacy. We also compute the centraliser of such maps. It allows us to describe the centraliser of the birational maps of the complex projective plane which preserve a rational fibration (the action on the basis of the fibration being not necessarily trivial); this question is related to some classical problems of difference equations.
We prove that every map-germ ${f \bar g}: (\C^n,\0) {\to}(\C,0)$ with an isolated critical value at 0 has the Thom $a_{f \bar g}$-property. This extends Hironaka's theorem for holomorphic mappings to the case of map-germs $f \bar g$ and it implies that every such map-germ has a Milnor-Lê fibration defined on a Milnor tube. One thus has a locally trivial fibration $ϕ: \mathbb S_\e \setminus K \to \mathbb S^1$ for every sufficiently small sphere around $\0$, where $K$ is the link of $f \bar g$ and in a neighbourhood of $K$ the projection map $ϕ$ is given by $f \bar g / | f \bar g|$.
We prove that the moduli spaces of twisted $\mathrm{SL}_n$ and $\mathrm{PGL}_n$-Higgs bundles on a smooth projective curve have the same (stringy) class in the Grothendieck ring of rational Chow motives. On the level of Hodge numbers this was conjectured by Hausel and Thaddeus, and recently proven by Groechenig, Ziegler and the second author. To adapt their argument, which relies on p-adic integration, we use a version of motivic integration with values in rational Chow motives and the geometry of Néron models to evaluate such integrals on Hitchin fibers.
Towards the classification of semistable fibrations having exactly five singular fibers
本文研究光滑有理曲线上恰有5个奇异纤维的半稳定非平凡纤维化，将一般非超椭圆纤维亏格上界由17改进至11，并刻画其纤维gonality及由极小有理曲面曲线束解消得到的结构。
We classify nets of quadrics in P^3 which give rise to elliptic fibrations of Mordell-Weil rank zero.
On hyperelliptic $C^\infty$-Lefschetz fibrations of four-manifolds
我们发现，超椭圆辛Lefschetz纤维化是辛双有理的两重覆盖的理性直纹面，分支在一个辛嵌入表面。这就把亏格2纤维化的分类简化为有理直纹曲面中某些辛子流形的分类。
Unirationality is the Same as Rational Connectedness in Characteristic Zero
本文通过构造类似 MRC 纤维化的 MU 纤维化，证明了特征零域上光滑簇的单有理性、有理连通性与有理链连通性等价，并指出 MRC 商非单有理覆盖。
Rational fibered cubic fourfolds
该研究在三次四折体模空间中探究多类已知有理三次除子的交，刻画其不可约分支，给出新的有理del Pezzo曲面或二次曲面纤维化三次四折体显式例子，并研究纤维化有理截面与Brauer类的对应关系。
Let $Σ$ be a smooth projective surface, let $f' : S' \to Σ$ be a double cover of $Σ$ and let $μ: S \to S'$ be the canonical resolution. Put $f = f'\circμ$. An irreducible curve $C$ on $Σ$ is said to be a splitting curve with respect to $f$ if $f^*C$ is of the form $C^+ + C^- + E$, where $C^- = σ_f^*C^+$, $σ_f$ being the covering transformation of $f$ and all irreducible components of $E$ are contained in the exceptional set of $μ$. In this article, we show that a kind of "reciprocity" of splitting curves holds for a certain pair of curves on rational ruled surfaces. As an application, we consi…
We define a cone curve to be a reduced sextic space curve which lies on a quadric cone and does not go through the vertex. We classify families of bitangent planes of cone curves. The methods we apply can be used for any space curve with ADE singularities, though in this paper we concentrate on cone curves. An embedded complex projective surface which is adjoint to a degree one weak Del Pezzo surface contains families of minimal degree rational curves, which cannot be defined by the fibers of a map. Such families are called minimal non-fibration families. Families of bitangent planes of cone c…
Negative Curves and Elliptic Fibrations on a Special Rational Surface
本文针对双重 Hesse 构型吹胀有理曲面上的 $(-1)$-曲线，通过构造修正的 Cremona 分岔图并借助椭圆束闭公式，给出了曲线次数的闭式表达，同时刻画了奇点与方程。
On the classification of fibrations by genus two singular curves via fibrations by elliptic curves on surfaces
本研究利用Shimada 1991年提出的光滑化过程，对一般纤维同构意义下、光滑化为有理曲面椭圆纤维化的亏格二奇异曲线纤维化进行分类，并描述了通过有理椭圆曲面商重构这类纤维化的向量场。
We prove that the number of rational points of bounded height on certain del Pezzo surfaces of degree 1 defined over Q grows linearly, as predicted by Manin's conjecture. Along the way, we investigate the average number of integral points of small naive height on quadratic twists of a fixed elliptic curve with full rational 2-torsion.
A Kodaira fibration is a compact, complex surface admitting a holomorphic submersion onto a complex curve, such that the fibers have nonconstant moduli. We consider Kodaira fibrations X with nontrivial invariant rational cohomology in degree 1, proving that if the dimension of the holomorphic invariants is 1 or 2, then X admits a branch covering over a product of curves inducing an isomorphism on rational cohomology in degree 1. We also study the class of Kodaira fibrations possessing a holomorphic section, and demonstrate that having a section imposes no restriction on possible monodromies.
In this paper, we study rational sections of the relative Picard scheme of a linear system on a smooth projective variety. We prove that if the linear system is basepoint-free and the locus of non-integral divisors has codimension at least two, then all rational sections of the relative Picard scheme come from restrictions of line bundles on the variety. As a consequence, we describe the group of sections of the Hitchin fibration for moduli spaces of Higgs bundles on curves.
Boundedness of polarized log Calabi-Yau fibrations
本文研究基与一般纤维有界的极化对数Calabi-Yau纤维化的有界性问题，改进arXiv:2504.05243的结果至实际有界性并证实相关猜想，还给出该文献双有理有界性结果的更具概念性的新证明。
Degenerations and Fibrations of K3 Surfaces: Lattice Polarisations and Mirror Symmetry
本文研究K3曲面Tyurin退化及椭圆纤维化的格极化理论，建立兼容现有定义的格极化概念，并探讨两种设置间的镜像对称对应。
Gurtas Lefschetz Fibrations, Rational Blowdowns, and Exotic Symplectic Four-Manifolds
本文通过Gurtas Lefschetz纤维化的例外截面构造负球面，利用有理爆破在奇偶性条件下生成单连通的奇异辛四流形。
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