Singularities of Rational Curves on K3 surfaces

Rationality of Fano threefolds with terminal Gorenstein singularities, I

We classify some special classes of non-rational Fano threefolds with terminal singularities. In particular, all such hyperelliptic and trigonal varieties are found.

On log canonical rational singularities

We prove that the class of log canonical rational singularities is closed under the basic operations of the minimal model program. We also give some supplementary results on the minimal model program for log canonical surfaces.

On the cotangent cohomology of rational surface singularities with almost reduced fundamental cycle

We prove dimension formulas for the cotangent spaces $T^{1}$ and $T^{2}$ for a class of rational surface singularities by calculating a correction term in the general dimension formulas. We get that it is zero if the dual graph of the rational surface singularity $X$ does not contain a particular type of configurations, and this generalizes a result of Theo de Jong stating that the correction term $c(X)$ is zero for rational determinantal surface singularities. In particular our result implies that $c(X)$ is zero for Riemenschneiders quasi-determinantal rational surface singularities, and this…

Nonisolated forms of rational triple point singularities of surfaces and their resolutions

The work is a detailed study of rational singularities of multiplicity 3 (RTP-singularities, for short). We give a list of nonisolated hypersurface singularities of which normalisations are the RTP-singularities, and construct their minimal resolution graphs by means of a subdivision of Newton polygons of those -- a method introduced by M. Oka for isolated complete intersection singularities. We show that nonisolated forms of RTP-singularities and their normalisations are both Newton non-degenerate.

Jet schemes and minimal toric embedded resolutions of rational double point singularities

Using the structure of the jet schemes of rational double point singularities, we construct "minimal embedded toric resolutions" of these singularities. We also establish, for these singularities, a correspondence between a natural class of irreducible components of the jet schemes centered at the singular locus and the set of divisors which appear on every "minimal embedded toric resolution". We prove that this correspondence is bijective except for the $E_8$ singulartiy. This can be thought as an embedded Nash correspondence for rational double point singularities.

有理奇点

Rational singularities

本文提出有理奇点的无消解定义，证明其与通常定义等价，并建立新的Kempf型判别准则，进而证明Cohen-Macaulay dlt奇点在任意特征下均为有理奇点。

有理奇点与 $q$-双有理态射

Rational singularities and $q$-birational morphism

本文推广了任意秩为1的反射层的奇点概念，将其与[Kov11]中的有理奇点联系起来，并证明了有理奇点标准事实的推广；通过定义非有理点集，引入$(B_{q+1})$作为Serre的$(S_{q+1})$的对偶概念，并证明了关于$q$-双有理态射的定理。

Hodge-Witt cohomology and Witt-rational singularities

We prove the vanishing modulo torsion of the higher direct images of the sheaf of Witt vectors (and the Witt canonical sheaf) for a purely inseparable projective alteration between normal finite quotients over a perfect field. For this, we show that the relative Hodge-Witt cohomology admits an action of correspondences. As an application we define Witt-rational singularities which form a broader class than rational singularities. In particular, finite quotients have Witt-rational singularities. In addition, we prove that the torsion part of the Witt vector cohomology of a smooth, proper scheme…

On infinitesimal deformations and obstructions for rational surface singularities

The purpose of this paper is to prove dimension formulas for $T^1$ and $T^2$ for rational surface singularities. These modules play an important role in the deformation theory of isolated singularities in analytic and algebraic geometry. The first may be identified as the Zariski tangent space of the versal deformation of the singularity; i.e. it is the space of infinitesimal deformations. The second contains the obstruction space -- in all known cases it is the whole obstruction space for rational surface singularities.

Rational and Semi-Rational Singularities

It is a conjecture of Kollár that a variety $X$ with rational singularities in some open subvariety $U$ has a rationalification; that is, a proper, birational morphism $f: Y \rightarrow X$ such that $Y$ has rational singularities, and which is an isomorphism over $U$. Whether this is true is already unknown in the case of a (normal) threefold with rational singularities along a curve $C$ except at a single point $p \in C$. There is an analogous conjecture for demi-normal varieties $X$, where we must insist that $Y$ has only semi-rational singularities. Our main result is that if a stronger ver…

Rationality Of the Locus Of Singularities Of the General Gough-Stewart Platform

We prove that the set of singular configurations of a general Gough Stewart platform has a rational parametrization. We introduce a reciprocal twist mapping which, for a general orientation of the platform, realizes the cubic surface of singularities as the blowing up of a quadric surface in five points.

高次有理奇点的简单判别准则

Simple criteria for higher rational singularities

本文利用交Du Bois复形和非有理性复形，为复数域上正规簇的高次有理奇点提出了简单判别准则。

The embedded Nash problem of birational models of rational triple singularities

We consider the question whether one can construct an embedded resolution of singularities of a singular variety $X\subset \textbf{A}^n$ from the data of the irreducible components of the spaces of jets (of $X$) centered at the singular locus of $X.$ We show that the answer is no in general and that it is yes for some birational models of rational triple surface singularities.

更高F-理性奇点

Higher F-rational singularities

本文引入更高F-理性性概念，证明在特征零域上，正常变种为m-理性当且仅当在足够大的素数p下为m-F-理性，同时建立形式的对数扩展新结果。

孤立奇点的更高Du Bois与更高有理性质

The higher Du Bois and higher rational properties for isolated singularities

本文研究孤立奇点的更高Du Bois与更高有理性质，在局部完全交情形下给出完整刻画，证明k-Du Bois蕴含(k-1)-有理，并建立不变量间新关系及伴随定理反转的快速证明。

具有有理同伦圆盘光滑化的正规曲面奇点

Normal complex surface singularities with rational homology disk smoothings

本文证明极小好预解图至少含两个节点的正规曲面奇点无有理同伦圆盘光滑化，结合已有分类结果给出此类奇点的完整分类，同时证明了Wahl猜想及其有理猜想。

有理奇点和Du Bois奇点重数的上界

Upper bound on the multiplicity of rational and Du Bois singularities

本文证明了Du Bois奇点重数的尖锐上界，并恢复了有理奇点的已知上界，解决了Huneke和Watanabe的问题。

关于正特征中有理奇点的 Steenbrink 消失

On Steenbrink vanishing for rational singularities in positive characteristic

本文证明正特征有理奇点的 Steenbrink 消失特殊情形，并由此推出强 F-正则三维簇及特征 p>41 的 klt 三维簇均满足 Steenbrink 消失。

Non-rational centers of log canonical singularities

We show that if $(X,B)$ is a log canonical pair with $\dim X\geq d+2$, whose non-klt centers have dimension $\geq d$, then $X$ is has depth $\ge d+2$ at every closed point.

Local vanishing and Hodge filtration for rational singularities

Given an n-dimensional variety Z with rational singularities, we conjecture that for a resolution of singularities whose reduced exceptional divisor E has simple normal crossings, the (n-1)-th higher direct image of the sheaf of differential forms with log poles along E vanishes. We prove this when Z has isolated singularities and when it is a toric variety. We deduce that for a divisor D with isolated rational singularities on a smooth complex n-dimensional variety X, the generation level of Saito's Hodge filtration on the localization of the structure sheaf along D is at most n-3.

The Cox Ring of a Del Pezzo Surface Has Rational Singularities

For a smooth Del Pezzo surface the direct sum of global sections of all isomorphism classes of invertible sheaves on it can be almost canonically endowed with a ring structure, called the Cox ring. We show that in characteristic 0 this ring is Gorenstein and its spectrum has rational singularities. Also a gap in math.AG/0309111 is pointed out.

射影超曲面的 Hirzebruch-Milnor 类检测更高 du Bois 或有理奇点的例子

Examples of Hirzebruch-Milnor classes of projective hypersurfaces detecting higher du Bois or rational singularities

本文表明经典 Hirzebruch-Milnor 类仅在特殊情形下能检测射影超曲面的更高 du Bois 或有理奇点，并澄清了作者早期论文中的若干要点。

超曲面的高阶Du Bois奇点与高阶有理奇点

Higher Du Bois and higher rational singularities of hypersurfaces

本研究针对全局定义的超曲面，利用消失循环混合霍奇模的Hodge滤过给出高阶Du Bois与高阶有理奇点的等价刻画，并提出基于谱Hirzebruch-Milnor特征类的同调检测准则。

具有Gorenstein终端奇点的Fano三维簇上的有理曲线

Rational curves on Fano threefolds with Gorenstein terminal singularities

本文研究具有Gorenstein终端奇点的Fano三维簇上的有理曲线，推广了几何Manin猜想至奇异情形，并证明了某些奇异del Pezzo三维簇的该猜想。

Classification of rational unicuspidal projective curves whose singularities have one Puiseux pair

It is a very old and interesting open problem to characterize those collections of embedded topological types of local plane curve singularities which may appear as singularities of a projective plane curve C of degree d. The goal of the present article is to give a complete (topological) classification of those cases when C is rational and it has a unique singularity which is locally irreducible (i.e. C is unicuspidal) with one Puiseux pair.

The $\ell$-adic Dualizing Complex on an Excellent Surface with Rational Singularities

In this article, we show that if $X$ is an excellent surface with rational singularities, the constant sheaf $\mathbb{Q}_{\ell}$ is a dualizing complex. In coefficient $\mathbb{Z}_{\ell}$, we also prove that the obstruction for $\mathbb{Z}_{\ell}$ to become a dualizing complex lying on the divisor class groups at singular points. As applications, we study the perverse sheaves and the weights of $\ell$-adic cohomology groups on such surfaces.

有理奇点族中总Cartier指数的有界性

Boundedness of total Cartier indices for rational singularities in families

本文研究了有理奇点族中变种的总Cartier指数有界性问题，通过分别处理二维和高维情况，解决了Han和Jiang提出的问题。

Rational singularities of normal T-varieties

A T-variety is an algebraic variety X with an effective regular action of an algebraic torus T. Altmann and Hausen gave a combinatorial description of an affine T-variety X by means of polyhedral divisors. In this paper we compute the higher direct images of the structure sheaf of a desingularization of X in terms of this combinatorial data. As a consequence, we give a criterion as to when a T-variety has rational singularities. We also provide a partial criterion for a T-variety to be Cohen-Macaulay. As an application we characterize in this terms quasihomogeneous elliptic singularities of su…

On tautness of two-dimensional $F$-regular and $F$-pure rational singularities

The weighted dual graph of a two-dimensional normal singularity $(X, x)$ represents the topological nature of the exceptional locus of its minimal log resolution. $(X, x)$ and its graph are said to be taut if the singularity can be uniquely determined by the graph. Laufer gave a complete list of taut singularities over $\mathbb{C}$. In positive characteristics, taut graphs over $\mathbb{C}$ are not necessarily taut and tautness have been studied only for special cases. In this paper, we prove the tautness of $F$-regular singularities. We also discuss the tautness of $F$-pure rational singulari…

Demi-Normal Surface Singularities

We prove semi-rationalification and semi-log-canonicalization for Gorenstein demi-normal surfaces. That is, given a Gorenstein demi-normal surface X with semi-rational (respectively, semi-log canonical) singularities in an open set U with complement a finite set of points, there is a proper birational morphism f : Y --> X such that f is an isomorphism over U and Y has only semi-rational (respectively, semi-log canonical) singularities. We proceed by passing to the normalization and then gluing along the conductor in an appropriate rationalification or log-canonicalization of the normalization …

1-rational singularities and quotients by reductive groups

We prove that good quotients of algebraic varieties with 1-rational singularities also have 1-rational singularities. This refines a result of Boutot on rational singularities of good quotients.

关于Gorenstein $\mathbb{Q}_p$-有理三维和四维奇点

On Gorenstein $\mathbb{Q}_p$-rational threefold and fourfold singularities

本文证明对于$n\leq 4$且$p>5$，拟Gorenstein $F$-纯且$\mathbb{Q}_p$-有理$n$维奇点是典范的，基于对对数典范奇点dlt修正的对偶复形的分析。

近Gorenstein有理表面奇点

Nearly Gorenstein rational surface singularities

本文研究了近Gorenstein有理表面奇点的性质，通过分析canonical trace理想和基本循环，给出了其判据并进行了分类。

Pushforwards of Measures on Real Varieties under Maps with Rational Singularities

Let $X,Y$ be algebraic varieties defined over $\Bbb R$. Assume $Y$ is smooth and $X$ is Gorenstein. Suppose $φ:X\to Y$ is a flat $\Bbb R$-morphism such that all the fibers have rational singularities. We show that the pushforward of any smooth, compactly supported measure on $X$ has a continuous density with respect to any smooth measure with non-vanishing density on $Y$. This extends a result of Aizenbud and Avni from the $p$-adic case to the archimedean case.

具有孤立有理奇点的射影簇的半正交分解

Semiorthogonal decompositions of projective varieties with isolated rational singularities

本文研究具有孤立有理奇点的射影簇，提出从小消解的半正交分解诱导目标分解的方法，并通过零范畴生成元与消解态射逆像的性质证明其存在性，推广了奇异曲面的已有结果。

A new method to compute the singularities of offsets to rational plane curves

Given a planar curve defined by means of a real rational parametrization, we prove that the affine values of the parameter generating the real singularities of the offset are real roots of a univariate polynomial that can be derived from the parametrization of the original curve, without computing or making use of the implicit equation of the offset. By using this result, a finite set containing all the real singularities of the offset, and in particular all the real self-intersections of the offset, can be computed. We also report on experiments carried out in the computer algebra system Mapl…

具有预k-有理奇点的代数簇上的微分形式

Differential forms on varieties with pre-$k$-rational singularities

本文针对具有预k-有理奇点的复代数簇，比较两类紧支上同调群的混合霍奇结构，研究其微分形式相关概念，重点考察Du Bois复形分次部分的深度。

On the rationality of Kawamata log terminal singularities in positive characteristic

We show that there exists a natural number $p_0$ such that any three-dimensional Kawamata log terminal singularity defined over an algebraically closed field of characteristic $p>p_0$ is rational and in particular Cohen-Macaulay.

Classification of 3-dimensional isolated rational hypersurface singularities with C*-action

In the paper "Algebraic classification of rational CR structures on topological 5-sphere with transversal holomorphic S^1-action in C^4" (Yau and Yu, Math. Nachrichten 246-247(2002), 207-233), we give algebraic classification of rational CR structures on the topological 5-sphere with transversal holomorphic S^1-action in C^4. Here, algebraic classification of compact strongly pseudoconvex CR manifolds X means classification up to algebraic equivalence, i.e. roughly up to isomorphism of the normalization of the complex analytic variety V which has X as boundary. The problem is intimately relate…

有理奇点和极小椭圆奇点上秩一自反模的分类

The classification of reflexive modules of rank one over rational and minimally elliptic singularities

本文分类了有理和极小椭圆奇点上的秩一自反模，确定了特殊与平坦模，证明Behnke猜想，并推广McKay对应及相关结果。

高 Du Bois 与高有理奇点

Higher Du Bois and higher rational singularities

本文证明平坦族中相对 Kähler 微分高次直接像的局部自由性，提出 $k$-有理奇点定义并证明其蕴含 $k$-Du Bois 性质，应用于 Hodge 数与形变理论。

A Castelnuovo-Mumford regularity bound for threefolds with rational singularities

The purpose of this paper is to establish a Castelnuovo-Mumford regularity bound for threefolds with mild singularities. Let $X$ be a non-degenerate normal projective threefold in $\mathbb{P}^r$ of degree $d$ and codimension $e$. We prove that if $X$ has rational singularities, then $\text{reg}(X) \leq d-e+2$. Our bound is very close to a sharp bound conjectured by Eisenbud-Goto. When $e=2$ and $X$ has Cohen-Macaulay Du Bois singularities, we obtain the conjectured bound $\text{reg}(X) \leq d-1$, and we also classify the extremal cases. To achieve these results, we bound the regularity of fibe…

Viehweg hyperbolicity for Whitney equisingular families with Gorenstein rational singularities

We prove the analogue of Viehweg's hyperbolicity conjecture for Whitney equisingular families of projective varieties with Gorenstein rational singularities whose geometric generic fiber has a good minimal model. Namely, for such families with maximal variation, the base spaces are of log general type. The main new ingredient is the use of intersection complexes as Hodge modules in the construction of logarithmic Higgs sheaves by Viehweg-Zuo and Popa-Schnell. This construction suggests an equisingular stratification of the moduli space of varieties of general type, with each stratum being hype…

On Rational Points of Varieties over Local Fields having a Model with Tame Quotient Singularities

We study rational points on a smooth variety X over a complete local field K with algebraically closed residue field, and models of X with tame quotient singularities. If a model of X is the quotient of a Galois action on a weak Néron model of the base change of X to a tame Galois extension of K, then we construct a canonical weak Néron model of X with a map to this model, and examine its special fiber. As an application we get examples of singular models of X such that X has K-rational points specializing to a singular point of this model. Moreover we obtain formulas for the motivic Serre inv…

Universal abelian covers of rational surface singularities and multi-index filtrations

In previous papers, there were computed the Poincare series of some (multi-index) filtrations on the ring of germs of functions on a rational surface singularity. These Poincare series were written as the integer parts of certain fractional power series, an interpretation of whom was not given. Here we show that, up to a simple change of variables, these fractional power series are specializations of the equivariant Poincare series for filtrations on the ring of germs of functions on the universal abelian cover of the surface singularity. We compute these equivariant Poincare series.

关于 $k$-Du Bois 与 $k$-有理奇点

On $k$-Du Bois and $k$-rational singularities

本文引入 $k$-Du Bois 与 $k$-有理奇点的新定义，推广至非 lci 情形，证明 $k$-有理奇点蕴含 $k$-Du Bois 性质，并识别仅依赖高阶上同调消失的理论方面。

Noncommutative resolutions and rational singularities

Let k be an algebraically closed field of characteristic zero. We show that the centre of a homologically homogeneous, finitely generated k-algebra has rational singularities. In particular if a finitely generated normal commutative k-algebra has a noncommutative crepant resolution, as introduced by the second author, then it has rational singularities.

Jumping Numbers on Algebraic Surfaces with Rational Singularities

In this article, we study the jumping numbers of an ideal in the local ring at rational singularity on a complex algebraic surface. By understanding the contributions of reduced divisors on a fixed resolution, we are able to present an algorithm for finding of the jumping numbers of the ideal. This shows, in particular, how to compute the jumping numbers of a plane curve from the numerical data of its minimal resolution. In addition, the jumping numbers of the maximal ideal at the singular point in a Du Val or toric surface singularity are computed, and applications to the smooth case are expl…

On rationalizing divisors

Rational pairs generalize the notion of rational singularities to reduced pairs $(X,D)$. In this paper we deal with the problem of determining whether a normal variety $X$ has a rationalizing divisor, i.e. a reduced divisor $D$ such that $(X, D)$ is a rational pair. We give a criterion for cones to have a rationalizing divisor, and relate the existence of such a divisor to the locus of rational singularities of a variety.

On rational cuspidal curves, open surfaces and local singularities

Let $C$ be an irreducible projective plane curve in the complex projective space ${\mathbb{P}}^2$. The classification of such curves, up to the action of the automorphism group $PGL(3,{\mathbb{C}})$ on ${\mathbb{P}}^2$, is a very difficult open problem with many interesting connections. The main goal is to determine, for a given $d$, whether there exists a projective plane curve of degree $d$ having a fixed number of singularities of given topological type. In this note we are mainly interested in the case when $C$ is a rational curve. The aim of this article is to present some of the old conj…

Rational blow-downs and smoothings of surface singularities

In this paper we give a necessary combinatorial condition for a negative--definite plumbing tree to be suitable for rational blow--down, or to be the graph of a complex surface singularity which admits a rational homology disk smoothing. New examples of surface singularities with rational homology disk smoothings are also presented; these include singularities with resolution graph having valency four nodes.

有理二重点奇点的Gröbner扇与最小嵌入环面消解

Gröbner Fans and Minimal Embedded Toric Resolutions of Rational Double Point Singularities

本文通过显式构造ADE奇点的Gröbner扇并利用profile概念进行加细，重构了有理二重点奇点的嵌入环面消解。

Dimension of the singularity category of a variety with rational singularities

In this paper, we study the Rouquier dimension of the singularity category of a variety with rational singularity. We construct an upper bound for the dimension of $\mathrm{D}_{\mathrm{sg}}(X)$ if $X$ has at worst rational singularities and $\dim X_{\mathrm{sing}} \leq 1$.

A note on resolution of rational and hypersurface singularities

It is well known that the exceptional set in a resolution of a rational surface singularity is a tree of rational curves. We generalize the combinatoric part of this statement to higher dimensions and show that the highest cohomologies of the dual complex associated to a resolution of an isolated rational singularity vanish. We also prove that the dual complex associated to a resolution of an isolated hypersurface singularity is simply connected. As a consequence, we show that the dual complex associated to a resolution of a 3-dimensional Gorenstein terminal singularity has the homotopy type o…