P^2的铅笔和无限二面角覆盖

Albanese varieties of cyclic covers of the projective plane and orbifold pencils

The paper studies a relation between fundamental group of the complement to a plane singular curve and the orbifold pencils containing it. The main tool is the use of Albanese varieties of cyclic covers ramified along such curves. Our results give sufficient conditions for a plane singular curve to belong to an orbifold pencil, i.e. a pencil of plane curves with multiple fibers inducing a map onto an orbifold curve whose orbifold fundamental group is non trivial. We construct an example of a cyclic cover of the projective plane which is an abelian surface isomorphic to the Jacobian of a curve …

Dihedral Galois covers of algebraic varieties and the simple cases

In this article we investigate the algebra and geometry of dihedral covers of smooth algebraic varieties. To this aim we first describe the Weil divisors and the Picard group of divisorial sheaves on normal double covers. Then we provide a structure theorem for dihedral covers, that is, given a smooth variety Y, we describe the algebraic "building data" on Y which are equivalent to the existence of such covers X --> Y. We introduce then two special very explicit classes of dihedral covers: the simple and the almost simple dihedral covers, and we determine their basic invariants. For the simple…

具有一束Fano双覆盖的有理刚性簇。I

Birationally rigid varieties with a pencil of Fano double covers. I

我们证明了一般Fano纤维化$π\colon V\to {\mathbb P}^1$是双有理超刚性的，只要它的纤维是指数为1的双Fano超曲面.特别地，在$V$上，没有其他有理连接纤维化的结构。证明是基于极大奇点的方法。

具有一束Fano双覆盖的有理刚性簇。III

Birationally rigid varieties with a pencil of Fano double covers. III

研究了Fano纤维空间$π\colon V\to {\mathbb P}^1$的双有理几何，其中纤维是指数为1的Fano双超曲面。对于这些品种的每一个家庭，我们要么证明双有理刚性或生产明确的非平凡结构的Fano纤维空间。本文发展了一种研究Fano纤维空间$V/{\mathbb P}^1$上可动系统的新的线性方法。

Elliptic dihedral covers in dimension 2, geometry of sections of elliptic surfaces, and Zariski pairs for line-conic arrangements

In this article, examples of Zariski pairs $(B_1, B_2)$ satisfying the following condition are given: (i) $°B_1 = °B_2 = 7$. (ii) Irreducible components of $B_i$ $(i = 1, 2)$ are lines and conics. (iii) Singularities of $B_i$ $(i = 1, 2)$ are nodes, tacnodes and ordinary triple points. In order to construct $B_i$ ($i = 1, 2$), we make use of geometry of sections of rational elliptic surfaces and their group structure. Dihedral covers play important roles to distinguish the topology of $({\mathbb P}^2, B_i)$ $(i = 1, 2)$.

The divisors of Prym semicanonical pencils

In the moduli space $\mathcal{R}_g$ of double étale covers of curves of a fixed genus $g$, the locus of covers of curves with a semicanonical pencil decomposes as the union of two divisors $\mathcal{T}^e_g$ and $\mathcal{T}^o_g$. Adapting arguments of Teixidor for the divisor of curves having a semicanonical pencil, we prove that both divisors are irreducible and compute their divisor classes in the Deligne-Mumford compactification $\overline{\mathcal{R}}_g$.

On the Waring rank of binary forms: The binomial formula and a dihedral cover of rank two forms

Waring problem for forms is important and classical in mathematics. It has been widely investigated because of its wide applications in several areas. In this paper, we consider the Waring problem for binary forms with complex coefficients. Firstly, we give an explicit formula for the Waring rank of any binary binomial and several examples to illustrating it. Secondly, we prove that, up to scalar multiplication, there are exactly $\binom{d-1}{2}$ binary forms of degree $d$ with Waring rank two and multiple of three fixed distinct linear forms.

具有一束Fano双覆盖的有理刚性簇。II

Birationally rigid varieties with a pencil of Fano double covers. II

我们继续研究Fano纤维化$π\colon V\to {\mathbb P}^1$的双有理几何，其纤维是指数为1的Fano双超曲面。对于大多数家庭的这种类型，不满足条件的充分扭曲的基础上，我们证明了双理性刚性（特别是，这意味着没有其他结构的纤维化合理连接品种）和计算其群体的双理性自映射。我们大大改善了主成分的方法的最大奇异性，首先，技术的多重性的纤维化$V/{\mathbb P}^1$到Fano品种线。

辫子单值线和Lefschetz束的一个倍度公式

A degree doubling formula for braid monodromies and Lefschetz pencils

每个紧辛4-流形可以被实现为沿着具有尖点和节点奇点的辛曲线分支的复射影平面的分支覆盖;覆叠映射由“非常充裕”线丛的截面的三元组诱导。本文给出了一个描述线性系（从一个非常充分的丛$L^k$到$L^{2k}$）倍度时分支曲线的辫单值不变量的行为的显式公式.作为结果，我们得到了类似的度加倍公式的映射类群monodromy的唐纳森的辛Lefschetz铅笔。

On surfaces with a canonical pencil

We classify the minimal surfaces of general type with $K^2 \leq 4χ-8$ whose canonical map is composed with a pencil, up to a finite number of families. More precisely we prove that there is exactly one irreducible family for each value of $χ\gg 0$, $4χ-10 \leq K^2 \leq 4χ-8$. All these surfaces are complete intersections in a toric $4-$fold and bidouble covers of Hirzebruch surfaces. The surfaces with $K^2=4χ-8$ were previously constructed by Catanese as bidouble covers of $\PP^1 \times \PP^1$.

Dihedral Monodromy and Xiao Fibrations

We construct three new families of fibrations $π: S \to B$ where $S$ is an algebraic complex surface and $B$ a curve that violate Xiao's conjecture relating the relative irregularity and the genus of the general fiber. The fibers of $π$ are certain étale cyclic covers of hyperelliptic curves that give coverings of $P^1$ with dihedral monodromy. As an application, we also show the existence of big and nef effective divisors in the Brill-Noether range.

The irreducible components of the moduli space of dihedral covers of algebraic curves

In this paper we introduce a new invariant for the action of a finite group $G$ on a compact complex curve of genus $g$. With the aid of this invariant we achieve the classification of the components of the moduli space of curves with an effective action by the dihedral group $D_n$. This invariant has been used in the meanwhile by the authors in order to extend the genus stabilization result of Livingston and Dunfield and Thurston to the ramified case. This new version contains an appendix clarifying the correspondence between the above components and the image loci in the moduli space M_g (cl…

Prym半典范线束的几何及其在三次三维簇上的应用

Geometry of Prym semicanonical pencils and an application to cubic threefolds

本研究探讨模空间中半典范线束对应除子上的Prym映射，揭示其差异与低亏格几何特性，并通过对特定除子的分析得到三次三维簇直线的计数结果。

Geometry of Prym varieties for special bielliptic curves of genus three and five

We construct two pencils of bielliptic curves of genus three and genus five. The first pencil is associated with a general abelian surface with a polarization of type $(1,2)$. The second pencil is related to the first by an unramified double cover, the Prym variety of which is canonically isomorphic to the Jacobian of a very general curve of genus two. Our results are obtained by analyzing suitable elliptic fibrations on the associated Kummer surfaces and rational double covers among them.

The unirational components of the strata of genus $11$ curves with several pencils of degree $6$ in $\mathcal{M}_{11}$

We show that the strata $ \mathcal{M}_{11,6}(k) \subset \mathcal{M}_{11} $ of $ 6-$gonal curves of genus $ 11 $, equipped with $k$ mutually independent and type I pencils of degree six, have a unirational irreducible component for $5\leq k\leq 9$. The unirational families arise from degree $ 9 $ plane curves with $ 4 $ ordinary triple and $ 5 $ ordinary double points that dominate an irreducible component of expected dimension. We will further show that the family of degree $ 8 $ plane curves with $ 10 $ ordinary double points covers an irreducible component of excess dimension in $ \mathcal{M…

Geometry of the Winger Pencil

We investigate the moduli of genus 10 curves that are endowed with a faithful action of the icosahedral group $\mathcal{A}_5$. We show among other things that this has the structure of a Deligne-Mumford stack whose underlying coarse moduli space essentially consists of two copies of the pencil of plane sextics that was introduced by Winger in 1924, but with the unique unstable member (a triple conic) replaced by a smooth non-planar curve. The orbifold defined by any member has genus zero and comes with 4 orbifold points. We show that by numbering the points, we get a fine moduli space whose ba…

On Green and Green-Lazarfeld conjectures for simple coverings of algebraic curves

Let X be a smooth genus g curve equipped with a simple morphism f: X -> C, where C is either the projective line or more generally any smooth curve whose gonality is computed by finitely many pencils. Here we apply a method developed by Aprodu to prove that if g is big enough then X satisfies both Green and Green-Lazarsfeld conjectures. We also partially address the case in which the gonality of C is computed by infinitely many pencils.

On the existence of primitive pencils for smooth curves

Let $C$ be a smooth curve with gonality $k\ge 6$ and genus $g\ge 2k^2+5k-6$. We prove that $W^1_d({C})$ has the expected dimension and that the general element of any irreducible component of $W^1_d({C})$ is primitive if either $g-k+4\le d\le g-2$ or $d=g-k+3$ and either $k$ is odd or $C$ is not a double covering of a curve of gonality $k/2$ and genus $k-3$. Even in the latter case we prove the existence of a complete and primitive $g^1_{g-k+3}$.

Enumerating pencils with moving ramification on curves

We consider the general problem of enumerating branched covers of the projective line from a fixed general curve subject to ramification conditions at possibly moving points. Our main computations are in genus 1; the theory of limit linear series allows one to reduce to this case. We first obtain a simple formula for a weighted count of pencils on a fixed elliptic curve E, where base-points are allowed. We then deduce, using an inclusion-exclusion procedure, formulas for the numbers of maps E->P^1 with moving ramification conditions. A striking consequence is the invariance of these counts und…

Decomposition of Jacobian varieties of curves with dihedral actions via equisymmetric stratification

Given a compact Riemann surface $X$ with an action of a finite group $G$, the group algebra Q[G] provides an isogenous decomposition of its Jacobian variety $JX$, known as the group algebra decomposition of $JX$. We consider the set of equisymmetric Riemann surfaces $\mathcal{M}(2n-1, D_{2n}, θ)$ for all $n\geq 2$. We study the group algebra decomposition of the Jacobian $JX$ of every curve $X\in \mathcal{M}(2n-1, D_{2n},θ)$ for all admissible actions, and we provide affine models for them. We use the topological equivalence of actions on the curves to obtain facts regarding its Jacobians. We …

曲率2的曲面上的pencil，包络线以及与平面三次曲线相切的圆锥线

Genus $2$ pencils on surfaces with $p_g=K^2=1$, envelopes, and conics tangent to plane cubic curves

本文研究了具有p_g=K^2=1的曲面中曲率2 pencil的存在性，证明其在模空间中形成不可约子流形，并探讨了与平面三次曲线相切的圆锥线几何关系。

Lines on the Dwork Pencil of Quintic Threefolds

We present an explicit parametrization of the families of lines of the Dwork pencil of quintic threefolds. This gives rise to isomorphic curves which parametrize the lines. These curves are 125:1 covers of certain genus six curves. These genus six curves are first presented as curves in P^1*P^1 that have three nodes. It is natural to blow up P^1*P^1 in the three points corresponding to the nodes in order to produce smooth curves. The result of blowing up P^1*P^1 in three points is the quintic del Pezzo surface dP_5, whose automorphism group is the permutation group S_5, which is also a symmetr…

亏格为\(g\)的紧致黎曼曲面上双股全辫群的有限商群与一般型复曲面：循环、二面体和超特殊商群

Finite quotients of full surface braid groups and complex surfaces of general type: cyclic, dihedral, and extra-special quotients

研究亏格为\(g\)的紧致黎曼曲面上双股全辫群的有限循环、二面体和超特殊商群数量，在商映射特定条件下计算。并将代数结果用于构造一般型光滑曲面，得到两个三维族的极小曲面，其有相同双正则不变量和贝蒂数，但第一同调群挠部分不同。

A new family of isolated symplectic singularities with trivial local fundamental group

We construct a new infinite family of 4-dimensional isolated symplectic singularities with trivial local fundamental group, answering a question of Beauville raised in 2000. Three constructions are presented for this family: (1) as singularities in blowups of the quotient of $\mathbb{C}^4$ by the dihedral group of order $2d$, (2) as singular points of Calogero-Moser spaces associated with dihedral groups of order $2d$ at equal parameters, (3) as singularities of a certain Slodowy slice in the $d$-fold cover of the nilpotent cone in ${\mathfrak{sl}}_d$.

有理倍数平面的构造

Constructions for rational multiple planes

本文在适当一般条件下，分类了更简单的三重平面，且给出了无限多个非克雷莫纳等价的更高倍数平面家族的例子。

$k$-微分分层中的有限等剩余覆盖

Finite isoresidual covers in strata of $k$-differentials

本研究针对黎曼球面上本原$k$-微分分层的$k$-剩余映射分歧覆盖，推广阿贝尔微分的已知结果，通过多尺度紧化相交计算等方法给出纤维基数公式，并应用于锥球面度量研究。

一般覆盖的格林猜想

Green's conjecture for general covers

该研究证明了高克利福德维数曲线的几类覆盖满足格林合冲猜想，还发现皮卡数最小为9的Nikulin曲面截面曲线满足经典格林猜想但不满足Prym-格林猜想，给出了模空间R_g中格林猜想成立的明确轨迹。

Rational correspondences between moduli spaces of curves defined by Hurwitz spaces

By associating to a curve C of genus g=2k and a pencil of degree d=k+1 the so-called trace curve (resp. the reduced trace curve) we define a rational map from the Hurwitz space of admissible covers of genus g=2k and degree d=k+1 to a moduli space of stable curves. We study the induced map between the divisor class groups of these moduli spaces of curves.

关于曲线及其族的 Galois 覆盖

On Galois Coverings of curves and their Families

本文利用结构态射对层前推关联的局部系统描述代数曲线及其族的Galois覆盖，通过Galois群作用将层分解为不可约局部系统以刻画群在同调群上的作用，并详细研究了$¶^1$的二面体覆盖。

Theta对偶性与超椭圆曲线循环覆盖的Prym-Torelli定理

Theta-duality and Prym-Torelli for cyclic covers of hyperelliptic curves

本文通过theta对偶性重构方法，研究超椭圆曲线奇素数度循环覆盖的Prym映射的通用单射性，识别出额外剩余轨迹，并在不同度下给出单射性条件。

射影线上的十二点、分支覆盖和有理椭圆纤维化

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。

Doubly isogenous genus-2 curves with $D_4$-action

We study the extent to which curves over finite fields are characterized by their zeta functions and the zeta functions of certain of their covers. Suppose C and C' are curves over a finite field K, with K-rational base points P and P', and let D and D' be the pullbacks (via the Abel-Jacobi map) of the multiplication-by-2 maps on their Jacobians. We say that (C,P) and (C',P') are *doubly isogenous* if Jac(C) and Jac(C') are isogenous over K and Jac(D) and Jac(D') are isogenous over K. For curves of genus 2 whose automorphism groups contain the dihedral group of order eight, we show that the nu…