Scalar--Flat Lorentzian Einstein--Weyl Spaces

Ricci flat left invariant Lorentzian metrics on 2-step nilpotent Lie groups

We determine all Ricci flat left invariant Lorentzian metrics on simply connected 2-step nilpotent Lie groups. We show that the $2k+1$-dimensional Heisenberg Lie group $H_{2k+1}$ carries a Ricci flat left invariant Lorentzian metric if and only if $k=1$. We show also that for any $2\leq q\leq k$, $H_{2k+1}$ carries a Ricci flat left invariant pseudo-Riemannian metric of signature $(q,2k+1-q)$ and we give explicite examples of such metrics.

Curvature Estimates in Asymptotically Flat Lorentzian Manifolds

We consider an asymptotically flat Lorentzian manifold of dimension (1,3). An inequality is derived which bounds the Riemannian curvature tensor in terms of the ADM energy in the general case with second fundamental form. The inequality quantifies in which sense the Lorentzian manifold becomes flat in the limit when the ADM energy tends to zero.

Minimal flat Lorentzian surfaces in Lorentzian complex space forms

In this article we study minimal flat Lorentzian surfaces in Lorentzian complex space forms. First we prove that, for minimal flat Lorentzian surfaces in a Lorentzian complex form, the equation of Ricci is a consequence of the equations of Gauss and Codazzi. Then we classify minimal flat Lorentzian surfaces in the Lorentzian complex plane ${\bf C}^2_1$. Finally, we classify minimal flat slant surfaces in Lorentzian complex projective plane $CP^2_1$ and in Lorentzian complex hyperbolic plane $CH^2_1$.

递归洛伦兹外尔空间

Recurrent Lorentzian Weyl spaces

本研究针对具有递归曲率张量的非闭洛伦兹外尔流形，分别推导了维数大于3和3维情形下的局部结构形式、等价变换群、微分不变量生成元，还刻画了齐次、不同上同调度及爱因斯坦-外尔型的相关流形性质。

Killing magnetic curves in non-flat Lorentzian-Heisenberg spaces

We obtain some explicit formulas for Killing magnetic curves in non-flat Lorentzian-Heisenberg spaces.

Causal Classification of Conformally Flat Lorentzian Cylinders

We derive all possible causality conditions for conformally flat Lorentzian metrics on the two-dimensional cylinder.

Nonunimodular Lorentzian flat Lie algebras

A Lorentzian flat Lie group is a Lie group $G$ with a flat left invariant metric $μ$ with signature $(1,n-1)=(-,+,\ldots,+)$. The Lie algebra $\mathfrak{g}=T_eG$ of $G$ endowed with $\langle\;,\;\rangle=μ(e)$ is called flat Lorentzian Lie algebra. It is known that the metric of a flat Lorentzian Lie group is geodesically complete if and only if its Lie algebra is unimodular. In this paper, we characterise nonunimodular Lorentzian flat Lie algebras as double extensions (in the sense of Aubert-Medina \cite{Aub-Med}) of Riemannian flat Lie algebras. As application of this result, we give all nonu…

Conformally flat Lorentzian manifolds with special holonomy groups

The local classification of conformally flat Lorentzian manifolds with special holonomy groups is obtained. The corresponding local metrics are certain extensions of Riemannian spaces of constant sectional curvature to Walker metrics.

Point invariants of third-order ODEs and hyper-CR Einstein-Weyl structures

We characterize Lorentzian three-dimensional hyper-CR Einstein-Weyl structures in terms of invariants of the associated third order ordinary differential equations.

Locally conformally flat Lorentzian quasi-Einstein manifolds

We show that locally conformally flat quasi-Einstein manifolds are globally conformally equivalent to a space form or locally isometric to a $pp$-wave or a warped product.

Lorentzian Flat Lie Groups Admitting a Timelike Left-Invariant Killing Vector Field

We call a connected Lie group endowed with a left-invariant Lorentzian flat metric Lorentzian flat Lie group. In this Note, we determine all Lorentzian flat Lie groups admitting a timelike left-invariant Killing vector field. We show that these Lie groups are 2-solvable and unimodular and hence geodesically complete. Moreover, we show that a Lorentzian flat Lie group $(\mathrm{G},μ)$ admits a timelike left-invariant Killing vector field if and only if $\mathrm{G}$ admits a left-invariant Riemannian metric which has the same Levi-Civita connection of $μ$. Finally, we give an useful characteriza…

Locally Conformally Flat Lorentzian Gradient Ricci Solitons

It is shown that locally conformally flat Lorentzian gradient Ricci solitons are locally isometric to a Robertson-Walker warped product, if the gradient of the potential function is non null, and to a plane wave, if the gradient of the potential function is null. The latter gradient Ricci solitons are necessarily steady.

Dispersionless BKP equation, the Manakov-Santini system and Einstein-Weyl structures

We construct a map of solutions of the dispersionless BKP (dBKP) equation to solutions of the Manakov-Santini (MS) system. This map defines an Einstein-Weyl structure corresponding to the dBKP equation through the general Lorentzian Einstein-Weyl structure corresponding to the MS system. We give a spectral characterisation of reduction of the MS system which singles out the image of the dBKP equation solutions and also consider more general reductions of this class. We define the BMS system and extend the map defined above to the map (Miura transformation) of solutions of the BMS system to sol…

Martinet 分布上的平坦亚洛伦兹结构

Flat sub-Lorentzian structures on Martinet distribution

本文研究 Martinet 分布上的两个平坦亚洛伦兹问题，刻画了可达集、最优轨迹、亚洛伦兹距离和球面，并揭示可达集与 Martinet 平面相交性质的差异。

Embeddings of non-positively curved compact surfaces in flat Lorentzian manifolds

We prove that any metric of non-positive curvature in the sense of Alexandrov on a compact surface can be isometrically embedded as a convex spacelike Cauchy surface in a flat spacetime of dimension (2+1). The proof follows from polyhedral approximation.

On supersymmetric Einstein-Weyl spaces

We consider weighted parallel spinors in Lorentzian Weyl geometry in arbitrary dimensions, choosing the weight such that the integrability condition for the existence of such a spinor, implies the geometry to be Einstein-Weyl. We then use techniques developed for the classification of supersymmetric solutions to supergravity theories to characterise those Lorentzian EW geometries that allow for a weighted parallel spinor, calling the resulting geometries supersymmetric. The overall result is that they are either conformally related to ordinary geometries admitting parallel spinors (w.r.t. the …

平直洛伦兹李群：完全描述

Flat Lorentzian Lie groups: A complete description

本文完整描述了平直洛伦兹李群的结构，解决了一个长期存在的伪黎曼李群理论问题。主要结果表明，任何平直洛伦兹李群要么具有时变平行左不变向量场，要么为Kundt类型，且对应的李代数属于六个明确类别。

On the $C^k$-embedding of Lorentzian manifolds in Ricci-flat spaces

In this paper we investigate the problem of non-analytic embeddings of Lorentzian manifolds in Ricci-flat semi-Riemannian spaces. In order to do this, we first review some relevant results in the area, and then motivate both the mathematical and physical interest in this problem. We show that any $n$-dimensional compact Lorentzian manifold $(M^{n},g)$, with $g$ in the Sobolev space $H_{s+3}$, $s>\frac{n}{2}$, admits an isometric embedding in an $(2n+2)$-dimensional Ricci-flat semi-Riemannian manifold. The sharpest result available for this type of embeddings, in the general setting, comes as a…

The space of Lorentzian flat tori in anti-de Sitter 3-space

We describe the space of isometric immersions from the Lorentz plane $L^2$ into the 3-dimensional anti-de Sitter space, and solve several open problems of this context raised by M. Dajczer and K. Nomizu in 1981. We also obtain from the above result a description of the space of Lorentzian flat tori isometrically immersed in the anti de Sitter space in terms of pairs of closed curves with wave front singularities in the hyperbolic plane satisfying some compatibility conditions.

On flat complete causal Lorentzian manifolds

We describe up to finite coverings causal flat affine complete Lorentzian manifolds such that the past and the future of any point are closed near this point. We say that these manifolds are strictly causal. In particular, we prove that their fundamental groups are virtually abelian. In dimension 4, there is only one, up to a scaling factor, strictly causal manifold which is not globally hyperbolic. For a generic point of this manifold, either the past or the future is not closed and contains a lightlike straight line.

洛伦兹度量在近阿贝尔李群上的研究

Lorentzian homogeneous Ricci-flat metrics on almost abelian Lie groups

本文研究四维及以上近阿贝尔李群上的洛伦兹左不变度量，构造了推广的皮托夫解，该解为 Ricci-flat 但非平坦，并具有封闭类时曲线。

洛伦兹凯勒时空几何中的Bochner平坦性与孤子动力学

Bochner Flatness and Soliton Dynamics in Lorentzian Kähler Spacetime Geometry

研究Bochner平坦洛伦兹凯勒时空流形上的Riemann孤子和η-双曲Ricci孤子，在爱因斯坦场方程和完美流体假设下导出孤子参数判据，并分析暗流体、刚性物质、尘埃和辐射等物理模型。

The Einstein-Weyl Equations, Scattering Maps, and Holomorphic Disks

We show that conformally compact, globally hyperbolic, Lorentzian Einstein-Weyl 3-manifolds are in natural one-to-one correspondence with orientation-reversing diffeomorphisms of the 2-sphere. The proof hinges on a holomorphic-disk analog of Hitchin's mini-twistor correspondence.

Conformal holonomy of C-spaces, Ricci-flat, and Lorentzian manifolds

The main result of this paper is that a Lorentzian manifold is locally conformally equivalent to a manifold with recurrent lightlike vector field and totally isotropic Ricci tensor if and only if its conformal tractor holonomy admits a 2-dimensional totally isotropic invariant subspace. Furthermore, for semi-Riemannian manifolds of arbitrary signature we prove that the conformal holonomy algebra of a $C$-space is a Berger algebra. For Ricci-flat spaces we show how the conformal holonomy can be obtained by the holonomy of the ambient metric and get results for Riemannian manifolds and plane wav…

Symmetries of Lorentzian Three-Manifolds with Recurrent Curvature

Locally homogeneous Lorentzian three-manifolds with recurrect curvature are special examples of Walker manifolds, that is, they admit a parallel null vector field. We obtain a full classification of the symmetries of these spaces, with particular regard to symmetries related to their curvature: Ricci and matter collineations, curvature and Weyl collineations. Several results are given for the broader class of three-dimensional Walker manifolds.

具有单值和乐群的闭共形平坦洛伦兹流形的分类

Classification of closed conformally flat Lorentzian manifolds with unipotent holonomy

本研究分类了n≥3维、PO(2,n)中单幂和乐的闭共形平坦洛伦兹流形，将其分为四类几何类型，明确了有限覆盖下的同胚形式，还进一步分类了其中容许本质共形流的两类流形。

On Codazzi tensors on a hyperbolic surface and flat Lorentzian geometry

Using global considerations, Mess proved that the moduli space of globally hyperbolic flat Lorentzian structures on $S\times\mathbb{R}$ is the tangent bundle of the Teichmüller space of $S$, if $S$ is a closed surface. One of the goals of this paper is to deepen this surprising occurrence and to make explicit the relation between the Mess parameters and the embedding data of any Cauchy surface. This relation is pointed out by using some specific properties of Codazzi tensors on hyperbolic surfaces. As a by-product we get a new Lorentzian proof of Goldman's celebrated result about the coinciden…

Isotropic quasi-Einstein manifolds

We investigate the local structure of four-dimensional Lorentzian quasi-Einstein manifolds under conditions on the Weyl tensor. We show that if the Weyl tensor is harmonic and the potential function preserves this harmonicity then, in the isotropic case, the manifold is necessarily a $pp$-wave. Using the quasi-Einstein equation, further conclusions are obtained for $pp$-waves.

纯电和纯磁韦伊尔曲率张量的庞特里亚金类障碍

A Pontryagin class obstruction for purely electric and purely magnetic Weyl curvature tensors

研究纯电或纯磁韦伊尔曲率张量的洛伦兹度量存在性，通过庞特里亚金形式的乘积 vanish 推导 cohomological 障碍，关联到 Petrov 子类型和非退化脐超曲面的分类。

On the Einstein-Weyl and conformal self-duality equations

The equations governing anti-self-dual and Einstein-Weyl conformal geometries can be regarded as `master dispersionless systems' in four and three dimensions respectively. Their integrability by twistor methods has been established by Penrose and Hitchin. In this note we present, in specially adapted coordinate systems, explicit forms of the corresponding equations and their Lax pairs. In particular, we demonstrate that any Lorentzian Einstein-Weyl structure is locally given by a solution to the Manakov-Santini system, and we find a system of two coupled third-order scalar PDEs for a general a…

New Explicit Lorentzian Einstein-Weyl Structures in 3-Dimensions

On a $3$D manifold, a Weyl geometry consists of pairs $(g, A) =$ (metric, $1$-form) modulo gauge $\widehat{g} = {\rm e}^{2φ} g$, $\widehat{A} = A + {\rm d}φ$. In 1943, Cartan showed that every solution to the Einstein-Weyl equations $R_{(μν)} - \frac{1}{3} R g_{μν} = 0$ comes from an appropriate $3$D leaf space quotient of a $7$D connection bundle associated with a 3$^{\rm rd}$ order ODE $y''' = H(x,y,y',y'')$ modulo point transformations, provided $2$ among $3$ primary point invariants vanish $$ \text{Wünschmann}(H) \equiv 0\equiv \text{Cartan}(H). $$We find that point equivalence of a single…

Riemannian counterparts to Lorentzian space forms

On a smooth $n$-manifold $M$ with $n \geq 3$, we study pairs $(g,T)$ consisting of a Riemannian metric $g$ and a unit length closed vector field $T$. Motivated by how Ricci solitons generalize Einstein metrics via a distinguished vector field, we propose to generalize space forms by considering those pairs $(g,T)$ whose corresponding Lorentzian metric $g_{\scriptscriptstyle L} = g - 2T^{\flat} \otimes T^{\flat}$ has constant curvature. We show by examples that such pairs exist when $M$ is noncompact, and that complete metrics exist among them. When $M$ is compact, however, the situation is mor…

$\n\ngtGL(2,\mathbb{R})$的洛伦兹几何与平坦仿射几何导论

Introduction to Lorentzian and Flat Affine Geometry of $\mathsf{GL}(2,\mathbb{R})$

本文研究4维实一般线性李群G₀连通单位分支的洛伦兹与平坦仿射几何，用李代数定义双不变黑塞度量与平坦仿射结构，确定其曲率等性质并讨论因果结构与展开映射。

Dynamical properties of the space of Lorentzian metrics

We study the mechanisms of the non properness of the action of the group of diffeomorphisms on the space of Lorentzian metrics of a compact manifold. In particular, we prove that nonproperness entails the presence of lightlike geodesic foliations of codimension 1. On the 2-torus, we prove that a metric with constant curvature along one of its lightlike foliation is actually flat. This allows us to show that the restriction of the action to the set of non-flat metrics is proper and that on the set of flat metrics of volume 1 the action is ergodic. Finally, we show that, contrarily to the Rieman…

The Lorentzian Lichnerowicz Conjecture for real-analytic, three-dimensional manifolds

Given a compact, three-dimensional, real-analytic Lorentzian manifold $(M,g)$, we prove that the identity component of the conformal group preserves a metric in the conformal class $[g]$, or $(M,g)$ is conformally flat.

About the Schouten-Weyl tensor on 3-dimensional Lorenzian Lie groups

The main purpose of this paper is to investigate the Schouten-Weyl tensor on the three-dimensional Lie groups with left-invariant Lorenzian metrics. The left-invariant Lorentzian metrics on the three-dimensional Lie groups with squared length zero Schouten-Weyl tensor are studied. Moreover, the three-dimensional metric Lie groups with almost harmonic (i.e. with zero curl and divergence) Schouten-Weyl tensor are investigated. In addition, the question about the harmonicity of contraction of the Schouten-Weyl tensor is considered.

Conformally Einstein Lorentzian Lie groups with Heisenberg symmetry

We describe all Lorentzian semi-direct extensions of the Heisenberg group which are conformally Einstein. As a by side result, Bach-flat left-invariant Lorentzian metrics on semi-direct extensions of the Heisenberg group are classified, thus providing new background solutions in conformal gravity.

On Einstein Lorentzian nilpotent Lie groups

In this paper, we study Lorentzian left invariant Einstein metrics on nilpotent Lie groups. We show that if the center of such Lie groups is degenerate then they are Ricci-flat and their Lie algebras can be obtained by the double extension process from an abelian Euclidean Lie algebra. We show that all nilpotent Lie groups up to dimension $5$ endowed with a Lorentzian Einstein left invariant metric have degenerate center and we use this fact to give a complete classification of these metrics. We show that if $\mathfrak{g}$ is the Lie algebra of a nilpotent Lie group endowed with a Lorentzian l…

Einstein-Weyl structures on lightike hypersurfaces

We study Weyl structures on lightlikes hypersurfaces endowed with a conformal structure of certain type and specific screen distribution: the Weyl screen structures. We investigate various differential geometric properties of Einstein-Weyl screen structures on lightlike hypersurfaces and show that, for ambiant Lorentzian space $\mathbb{R}^{n+2}_{1}$ and a totally umbilical screen foliation, there is a strong interplay with the induced (Riemannian) Weyl-structure on the leaves. Finally, we establish necessary and sufficient conditions for a Weyl structure defined by the $1-$form of an almost co…

共形齐性洛伦兹空间

Conformally homogeneous Lorentzian spaces

该研究证明了容许连通本质可递共形变换群的单连通非共形平坦共形洛伦兹流形必为完备齐性平面波，完成了相关流形的分类，并证明此类齐性平面波的共形变换群是等距群的1维位似扩张。

Einstein--Weyl geometry, dispersionless Hirota equation and Veronese webs

We exploit the correspondence between the three-dimensional Lorentzian Einstein-Weyl geometries of the hyper-CR type, and the Veronese webs to show that the former structures are locally given in terms of solutions to the dispersionless Hirota equation. We also demonstrate how to construct hyper-CR Einstein--Weyl structures by Kodaira deformations of the flat twistor space $T\CP^1$, and how to recover the pencil of Poisson structures in five dimensions illustrating the method by an example of the Veronese web on the Heisenberg group.

A classification of left-invariant Lorentzian metrics on some nilpotent Lie groups

It has been known that there exist exactly three left-invariant Lorentzian metrics up to scaling and automorphisms on the three dimensional Heisenberg group. In this paper, we classify left-invariant Lorentzian metrics on the direct product of three dimensional Heisenberg group and the Euclidean space of dimension $n-3$ with $n \geq 4$, and prove that there exist exactly six such metrics on this Lie group up to scaling and automorphisms. Moreover we show that only one of them is flat, and the other five metrics are Ricci solitons but not Einstein. We also characterize this flat metric as the u…

Weyl Tensor Classification in Four-dimensional Manifolds of All Signatures

It is well known that the classification of the Weyl tensor in Lorentzian manifolds of dimension four, the so called Petrov classification, was a great tool to the development of general relativity. Using the bivector approach it is shown in this article a classification for the Weyl tensor in all four-dimensional manifolds, including all signatures and the complex case, in an unified and simple way. The important Petrov classification then emerges just as a particular case in this scheme. The boost weight classification is also extended here to all signatures as well to complex manifolds. For…

局部共形同构的洛伦兹空间

Locally conformally homogeneous Lorentzian spaces

该研究证明了局部共形同构的洛伦兹流形要么共形平坦，要么局部等价于齐性平面波，并揭示了平面波结构与Penrose极限的关系。

标量平坦的凯勒表面及其威利张量 annihilates 里奇形式

Scalar-flat Kähler surfaces whose Weyl tensor annihilates the Ricci form

研究标量平坦凯勒表面中威利张量作用于2形式 annihilates 里奇形式的性质，证明其为里奇平坦或局部等距于两个曲率相反的实表面乘积。

Gravitational interpretation of the Hitchin equations

By referring to theorems of Donaldson and Hitchin, we exhibit a rigorous AdS/CFT-type correspondence between classical 2+1 dimensional vacuum general relativity theory on S x R and SO(3) Hitchin theory (regarded as a classical conformal field theory) on the spacelike past boundary S, a compact, oriented Riemann surface of genus greater than one. Within this framework we can interpret the 2+1 dimensional vacuum Einstein equation as a decoupled ``dual'' version of the 2 dimensional SO(3) Hitchin equations. More precisely, we prove that if over S with a fixed conformal class a real solution of th…

Twistor spinors with zero on Lorentzian 5-space

We present in this paper a $C^1$-metric on an open neighbourhood of the origin in $\RR^{5}$. The metric is of Lorentzian signature $(1,4)$ and admits a solution to the twistor equation for spinors with a unique isolated zero at the origin. The metric is not conformally flat in any neighbourhood of the origin. The construction is based on the Eguchi-Hanson metric with parallel spinors on Riemannian 4-space.

某些紧致洛伦兹局部对称空间的完备性

Completeness of certain compact Lorentzian locally symmetric spaces

该研究证明了满足局部de Rham-Wu分解中洛伦兹因子为Cahen-Wallach型或极大平坦因子为一维类时这两个条件之一的紧致洛伦兹局部对称空间是测地完备的，证明基于Mehidi与Zeghib的最新结果及Romero与Sánchez的早期结果。

Isometry groups of Lorentzian manifolds of finite volume and The local geometry of compact homogeneous Lorentz spaces

Based on the work of Adams and Stuck as well as on the work of Zeghib, we classify the Lie groups which can act isometrically and locally effectively on Lorentzian manifolds of finite volume. In the case that the corresponding Lie algebra contains a direct summand isomorphic to the two-dimensional special linear algebra or to a twisted Heisenberg algebra, we also describe the geometric structure of the manifolds if they are compact. Using these results, we investigate the local geometry of compact homogeneous Lorentz spaces whose isometry groups have non-compact connected components. It turns …

A generalized Weierstrass representation of Lorentzian surfaces in $\mathbb{R}^{2,2}$ and applications

We give a generalized Weierstrass formula for a Lorentz surface conformally immersed in the four-dimensional space $\mathbb{R}^{2,2}$ using spinors and Lorentz numbers. We also study the immersions of a Lorentzian surface in {\bf the} Anti-de Sitter space (a pseudo-sphere in $\mathbb{R}^{2,2}$): we give a new spinor representation formula and deduce the conformal description of a flat Lorentzian surface in that space.

On the Einstein condition for Lorentzian 3-manifolds

It is well known that in Lorentzian geometry there are no compact spherical space forms; in dimension 3, this means there are no closed Einstein 3-manifolds with positive Einstein constant. We generalize this fact here, by proving that there are also no closed Lorentzian 3-manifolds $(M,g)$ whose Ricci tensor satisfies $$ \text{Ric} = fg+(f-λ)T^{\flat}\otimes T^{\flat}, $$ for any unit timelike vector field $T$, any positive constant $λ$, and any smooth function $f$ that never takes the values $0,λ$. (Observe that this reduces to the positive Einstein case when $f = λ$.) We show that there is …

A note on extended Lorentzian recurrent manifolds

Extended recurrent pseudo-Riemannian manifolds were introduced by Mileva Prvanovic'. We reconsider her work in the light of recent results and show that the manifold is conformally flat, and it is a space of quasi-constant curvature. We also show that an extended recurrent Lorentzian manifold, with time-like associated covector, is a perfect fluid Robertson-Walker space-time. We obtain the equation of state; in n = 4 and if the scalar curvature is zero, a model for incoherent radiation is obtained.

平面波的共形商空间，以及局部齐性背景下的 Lichnerowicz 猜想

Conformal quotients of plane waves, and Lichnerowicz conjecture in a locally homogeneous setting

本文研究平面波上的共形群作用，证明存在非等距的真余紧相似作用，并在局部齐性背景下证明了洛伦兹共形 Lichnerowicz 猜想。

Vacuum static spaces with vanishing of complete divergence of Bach tensor and Weyl tensor

In this paper, we study vacuum static spaces with the complete divergence of the Bach tensor and Weyl tensor. First, we prove that the vanishing of complete divergence of the Bach tensor and Weyl tensor implies the harmonicity of the metric, and we present examples in which these conditions do not imply Bach flatness. As an application, we prove the non-existence of multiple black holes in vacuum static spaces with zero scalar curvature. On the other hand, we prove the Besse conjecture under these conditions, which are weaker than harmonicity or Bach flatness. Moreover, we show a rigidity resu…