Atsushi Fujioka, Jun-ichi Inoguchi
In this paper we introduce the notion of timelike surface with harmonic inverse mean curvature in 3-dimensional Lorentzian space forms, and study their fundamental properties.
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In the 3-dimensional Lorentz-Minkowski space we prove that the sign of the Gaussian curvature of any timelike minimal surface is determined by the degeneracy and the orientations of the two null curves that generate the surface. Moreover, we also investigate the behavior of the Gaussian curvature near singular points of a timelike minimal surface which admits some kind of singular points.
Framed null curves and timelike surfaces via Lorentzian harmonic maps into de-Sitter 2-space
本文基于框架零曲线构造了满足特定特征值方程的洛伦兹调和映射，并研究了两类类时间曲面的几何性质与奇异性特征。
It is shown that timelike surfaces of constant mean curvature 1 in anti-de Sitter 3-space can be constructed from a pair of Lorentz holomorphic and Lorentz antiholomorphic null curves in PSL(2,R) via Bryant type representation formulae. These formulae are used to investigate an explicit one-to-one correspondence, the so-called Lawson correspondence, between timelike surfaces of constant mean curvature 1 in anti-de Sitter 3-space and timelike minimal surfaces in Minkowski 3-space. The hyperbolic Gauss map of timelike surfaces in anti-de Sitter 3-space, which is a close analogue of the classical…
Split-harmonic maps and the interpolation problem for timelike minimal surfaces
本文利用分裂调和映射重新证明类时极小曲面的奇异 Björling 定理，并解决分裂 Fourier 曲线到点及另一条曲线的类时极小曲面插值问题。
In this paper, we study the timelike tubular Weingarten surfaces in 3-dimensional Minkowski space $IR_1^3 $.We have obtained some conditions for being $({K_{II},H})$, $({K_{II},K})$, timelike tubular Weingarten surfaces where are the second Gaussian curvature the Gaussian curvature and the mean curvature, respectively.
The equivalence of smooth and synthetic notions of timelike sectional curvature bounds
本文证明了在强因果时空中的时间平面曲率界与合成时间界之间的等价性，并建立了时间分离和带符号距离函数的Hessian比较作为工具。
This paper demonstrates existence for all time of mean curvature flow in Minkowski space with a perpendicular Neumann boundary condition, where the boundary manifold is a convex cone and the flowing manifold is initially spacelike. Using a blowdown argument, we show that under renormalisation this flow converges towards a homothetically expanding hyperbolic solution.
Timelike Meridian Surfaces of Elliptic type in the Minkowski 4-Space
本文研究 Minkowski 4-空间中位于类时轴旋转超曲面上的椭圆型类时子午面，分析了其常曲率与平行平均曲率向量场等基本类别，并为描述具有平行规范化平均曲率向量场类时曲面的自然 PDEs 背景系统提供了显式解。
Timelike Thomsen surfaces are timelike minimal surfaces that are also affine minimal. In this paper, we make use of both the Lorentz conformal coordinates and the null coordinates, and their respective representation theorems of timelike minimal surfaces, to obtain a complete global classification of these surfaces and to characterize them using a geometric invariant called lightlike curvatures. As a result, we reveal the relationship between timelike Thomsen surfaces, and timelike minimal surfaces with planar curvature lines. As an application, we give a deformation of null curves preserving …
Timelike meridian surfaces of hyperbolic type in the Minkowski 4-space
研究 Minkowski 4-空间中一类特殊的类时曲面（双曲型类时子午面），证明其具有平坦法向连接，并分类了常 Gauss 曲率、极小、非零常平均曲率以及具有平行归一化平均曲率向量场的曲面。
Timelike Surfaces with Parallel Normalized Mean Curvature Vector Field in the Minkowski 4-Space
本文研究 Minkowski 4-空间中具有平行归一化平均曲率向量场的类时曲面，通过引入规范参数并证明存在唯一性定理，将确定曲面的几何函数和偏微分方程个数降至最少，解决了该类曲面的 Lund-Regge 问题。
We use integrable systems techniques to study the singularities of timelike non-minimal constant mean curvature (CMC) surfaces in the Lorentz-Minkowski 3-space. The singularities arise at the boundary of the Birkhoff big cell of the loop group involved. We examine the behaviour of the surfaces at the big cell boundary, generalize the definition of CMC surfaces to include those with finite, generic singularities, and show how to construct surfaces with prescribed singularities by solving a singular geometric Cauchy problem. The solution shows that the generic singularities of the generalized su…
We classify (spacelike or timelike) surfaces of revolution with zero $f$-mean curvature in $\Bbb G^2\times\Bbb R_1,$ the Lorentz-Minkowski 3-space $\Bbb R^3_1$ endowed with the Gaussian-Euclidean density $e^{-f(x,y,z)}=\frac 1{2π}e^{-\frac{x^2+y^2}2}.$ It is proved that an $f$-maximal surface of revolution is either a horizontal plane or a spacelike $f$-Catenoid. For the timelike case, a timelike $f$-minimal surface is either a vertical plane containing $z$-axis, the cylinder $x^2+y^2=1,$ or a timelike $f$-Catenoid. Spacelike and timelike $f$-Catenoids are new examples of $f$-minimal surfaces …
On any timelike surface with zero mean curvature in the four-dimensional Minkowski space we introduce special geometric (canonical) parameters and prove that the Gauss curvature and the normal curvature of the surface satisfy a system of two natural partial differential equations. Conversely, any two solutions to this system determine a unique (up to a motion) timelike surface with zero mean curvature so that the given parameters are canonical. We find all timelike surfaces with zero mean curvature in the class of rotational surfaces of Moore type. These examples give rise to a one-parameter f…
In this study, we investigate the existence theorems for timelike ruled surfaces in Minkowski 3-space. We obtain a general system and give the existence theorems for a timelike ruled surface according to Gaussian curvature, distribution parameter and strictional distance. Moreover, we give some special cases such as the directirx of the surface is a geodesic, an asymptotic line, a line of curvature or a general helix.
The equivalence between timelike Ricci curvature and the timelike Brunn Minkowski inequality on synthetic Lorentzian spaces
本文研究了合成洛伦兹空间中时序里奇曲率与时序Brunn-Minkowski不等式的关系，证明了在时序q-非分支设定下，q-时序曲率维度条件与时序Brunn-Minkowski条件等价，扩展了在非光滑设定下的研究成果。
Total curvature and length estimates for timelike curves in Lorentzian length spaces
本文在具有上曲率界的洛伦兹长度空间中引入类时曲线的合成总曲率概念，证明其与光滑情形一致，并给出类时曲线长度的尖锐下界。
Basic Classes of Timelike General Rotational Surfaces in the Four-dimensional Minkowski Space
本文研究 Minkowski 4-空间中两类类时一般旋转曲面，刻画平坦、法联络平坦及常平均曲率等基本几何类别，并显式分类极小曲面与具有平行归一化平均曲率向量场的曲面。
Soliton Solutions to the Curvature Flow on the 2-dimensional De Sitter Space and Applications
该研究建立了德西特空间与双曲空间上曲率流、逆曲率流及曲线缩短流解的对应关系，推导了孤子解的判定条件与存在性，明确了非平凡完全孤子的类型。
In this study, we deal with the local structure of curves and surfaces immersed in a pseudo-isotropic space I_{p}^{3} that is a particular Cayley-Klein space. We provide the formulas of curvature, torsion and Frenet trihedron in order for spacelike and timelike curves. The causal character of all admissible surfaces in I_{p}^{3} has to be timelike or lightlike up to its absolute. We introduce the formulas of Gaussian and mean curvature for timelike surfaces in I_{p}^{3}. As applications, we describe the surfaces of revolution which are the orbits of a plane curve under a hyperbolic rotation wi…
In this article we introduce a notion of normalized angle for Lorentzian pre-length spaces. This concept allows us to prove some equivalences to the definition of timelike curvature bounds from below for Lorentzian pre-length spaces. Specifically, we establish some comparison theorems known as the local Lorentzian version of the Toponogov theorem and the Alexandrov convexity property. Finally, as an application we obtain a first variation Formula for non-negatively curved globally hyperbolic Lorentzian length spaces.
Timelike surfaces in the three-dimensional Heisenberg group with left invariant semi-Riemannian metric are studied. In particular, non-vertical timelike minimal surfaces are characterized by the non-conformal Lorentz harmonic maps into the de Sitter two phere. On the basis of the characterization, the generalized Weierstrass type representation will be established through the loop group decompositions.
We derive a correspondence between (Lorentzian) harmonic maps into the pseudosphere $S_1^2$, with appropriate regularity conditions, and certain connection 1-forms. To these harmonic maps, we associate a representation of type Weierstrass, and we apply it to construct timelike surfaces with constant mean curvature.
In this paper, we consider non developable ruled surface with spacelike ruling, timelike ruling, respectively. We give the relations between the structure functions with the curvature and torsion of the striction line of the timelike and spacelike non developable ruled surfaces. Also, we have calculated the gaussian and mean curvatures of timelike and spacelike non developable ruled surfaces using the structure functions.
The scope of this survey is to give a self-contained introduction to synthetic timelike Ricci curvature bounds for (possibly non-smooth) Lorentzian spaces via optimal transport $\&$ entropy tools, including a synthetic version of Hawking's singularity theorem and a synthetic characterisation of Einstein's vacuum equations. We will also discuss some motivations arising from the smooth world and some possible directions for future research.
Almost-splitting of spacetimes under timelike sectional curvature bounds
该论文在类时截面曲率有下界且下界接近零的全局双曲时空中，证明了一个几乎分裂定理，表明长极大类时线段附近的因果钻石在洛伦兹Gromov--Hausdorff意义下接近乘积时空，为洛伦兹分裂定理提供了定量版本。
Using techniques of integrable systems, we study a Weierstrass representation formula for timelike surfaces with prescribed mean curvature in Minkowski 3-space. It is shown that timelike minimal surfaces are obtained by integrating a pair of Lorentz holomorphic and Lorentz antiholomorphic null curves in Minkowski 3-space. The relationship between timelike minimal surfaces and bosonic Nambu-Goto string worldsheets in spacetime is also discussed in the appendix.
Singularities on timelike minimal surfaces in Lorentzian Heisenberg group
研究了洛伦兹希格纳群中时状极小曲面的构造及其奇点特性，通过调和映射和具体例子探讨了不同奇点类型。
Timelike surfaces of constant mean and constant Gaussian curvature in anti-de Sitter 3-space via loop groups
本文通过圈群方法，在反德西特三维空间中构造常平均曲率和常高斯曲率的类时曲面，证明二者仅差一个归一化，并给出显式公式与闭式例子。
In this study we give definitions and characterizations of transversal surfaces of timelike ruled surfaces. We study some special cases such as the striction curve is a geodesic, an asymptotic line or a line of curvature. Moreover, we obtain developable conditions for transversal surfaces of a timelike ruled surface.
A Gauss-Bonnet-Chern type obstruction for Killing vector fields on Lorentzian manifolds
本文提出了一种新的曲率障碍，利用类时（零）截面曲率刻画偶数维（奇数维）Lorentzian 流形上 Killing 或同位向量场的存在性，并将紧致情形下的 Gauss-Bonnet-Chern 障碍推广至非零类时截面曲率。
Timelike Ollivier-Ricci curvature
本文在洛伦兹几何中引入余维数为1的粗Ricci曲率构造，通过1-Lorentz-Wasserstein距离和Moser型流恢复类时方向上的Ricci曲率及Bakry-Émery张量，呼应Raychaudhuri方程。
In this paper, we get the time evolution equations of the curvature and torsion of the evolving spacelike curves in the Minkowski space. Also, we give inextensible evolutions of timelike ruled surfaces that are produced by the timelike normal and spacelike binormal vector fields of spacelike curve and derive the necessary conditions for an inelastic surface evolution. Then, we compute the coefficients of the first and second fundamental forms, the Gauss and mean curvatures for timelike special ruled surfaces. As a result, we give applications of the evolution equations for the curvatures of th…
Let $(M,\mathsf{d},\mathfrak{m},\ll,\leq,τ)$ be a causally closed, $\mathscr{K}$-globally hyperbolic, regular measured Lorentzian geodesic space satisfying the weak timelike curvature-dimension condition $\smash{\mathrm{wTCD}_p^e(K,N)}$ in the sense of Cavalletti and Mondino. We prove the existence of geodesics of probability measures on $M$ which satisfy the entropic semiconvexity inequality defining $\smash{\mathrm{wTCD}_p^e(K,N)}$ and whose densities with respect to $\mathfrak{m}$ are additionally uniformly $L^\infty$ in time. This holds apart from any nonbranching assumption. We also discu…
Hyperbolic angles in Lorentzian length spaces and timelike curvature bounds
本文在洛伦兹长度空间中引入双曲角概念，用于刻画类时曲率界并通过角度单调性条件实现，同时改进了测地线非分支结果。
In [8] Gerhardt proves longtime existence for the inverse mean curvature flow in globally hyperbolic Lorentzian manifolds with compact Cauchy hypersurface, which satisfy three main structural assumptions: a strong volume decay condition, a mean curvature barrier condition and the timelike convergence condition. Furthermore, it is shown in [8] that the leaves of the inverse mean curvature flow provide a foliation of the future of the initial hypersurface. We show that this result persists, if we generalize the setting by leaving the mean curvature barrier assumption out. For initial hypersurfac…
The splitting theorem for globally hyperbolic Lorentzian length spaces with non-negative timelike curvature
该研究针对含完整类时线、非负类时曲率的全局双曲洛伦兹长度空间，通过构造类时渐近线、引入类时线平行性概念，证明了综合分裂定理，并将局部分裂结果推广至全局。
For a regular surface in Euclidean space $\mathbb{R}^3$, umbilic points are precisely the points where the Gauss and mean curvatures $K$ and $H$ satisfy $H^2=K$; moreover, it is well-known that the only totally umbilic surfaces in $\mathbb{R}^3$ are planes and spheres. But for timelike surfaces in Minkowski space $\mathbb{R}^{1,2}$, it is possible to have $H^2=K$ at a non-umbilic point; we call such points {\em quasi-umbilic}, and we give a complete classification of totally quasi-umbilic timelike surfaces in $\mathbb{R}^{1,2}$.
Optimal transport and timelike lower Ricci curvature bounds on Finsler spacetimes
本文利用 q-Lorentz-Wasserstein 最优输运与概率测度 q-测地线，证明 Finsler 时空在加权 Ricci 曲率类时下界条件下满足 TCD_q(K,N)，并导出尖锐类时 Brunn-Minkowski 不等式。
Equivalence of the null energy condition to variable lower bounds on the timelike Ricci curvature for $C^2$-Lorentzian metrics
本文证明了麦卡恩对null能量条件的重新表述在$C^2$-洛伦兹度量中与经典null能量条件等价，并修正了原始证明。
In this article, we study spacelike and timelike rotational surfaces in a 3--dimensional de Sitter space $\mathbb{S}^3_1$ which are the orbit of a regular curve under the action of the orthogonal transformation of 4--dimensional Minkowski space $\mathbb{E}^4_1$ leaving a spacelike, a timelike or a degenerate plane pointwise fixed. We determine the profile curve of such Weingarten rotational surfaces parameterized by the principal curvature. Then, we classify spacelike and timelike Weingarten rotational surface in $\mathbb{S}^3_1$ with the principal curvatures $κ$ and $λ$ satisfying $κ=aλ+b$ or…
The Lamarle Formula, given by Kruppa in \cite{Kr}, is known as a relationship between the Gaussian curvature and the distribution parameter of a ruled surface in the surface theory. The ruled surfaces were investigated in 3 different classes with respect to the character of base curves and rulings, \cite{Tu1},\cite{Tu2}. In this paper on account of these studies, the relationships between the Gaussian curvatures and distribution parameters of spacelike ruled surface, timelike ruled surface with spacelike ruling and timelike ruled surface with timelike ruling are obtained, respectively. These r…
We prove that an isometric immersion of a timelike surface in four-dimensional Minkowski space is equivalent to a normalized spinor field which is a solution of a Dirac equation on the surface. Using the quaternions and the complex numbers, we obtain a spinor representation formula that relates the spinor field and the isometric immersion. Applying the representation formula, we deduce a new spinor representation of a timelike surface in three-dimensional De Sitter space; we give a formula for the Laplacian of the Gauss map of a minimal timelike surface in four-dimensional Minkowski space in t…
Singularities of generalized timelike minimal surfaces in Lorentz-Minkowski 3-space
本文研究Minkowski 3维空间中类时极小曲面的奇点，证明其存在性与不存在性定理，给出多类奇点的判别准则、对偶与不变性定理及例子。
It has been known for some time that there exist $5$ essentially different real forms of the complex affine Kac-Moody algebra of type $A_2^{(2)}$ and that one can associate $4$ of these real forms with certain classes of "integrable surfaces", such as minimal Lagrangian surfaces in $\mathbb {CP}^2$ and $\mathbb {CH}^2$, as well as definite and indefinite affine spheres in $\mathbb R^3$. In this paper we consider the class of timelike minimal Lagrangian surfaces in the indefinite complex hyperbolic two-space $\mathbb{CH}^2_1$. We show that this class of surfaces corresponds to the fifth real fo…
Causal convergence conditions through variable timelike Ricci curvature bounds
该研究提出非光滑整体双曲正则长度度量时空概念，引入带可变Ricci曲率界的熵类时曲率维数条件，统一广义相对论能量条件，推广相关几何估计与奇点定理，给出稳定性结果与类时几何不等式。
Let s be at least 2. We construct Ricci flat pseudo-Riemannian manifolds of signature (2s,s) which are not locally homogeneous but whose curvature tensors never the less exhibit a number of important symmetry properties. They are curvature homogeneous; their curvature tensor is modeled on that of a local symmetric space. They are spacelike Jordan Osserman with a Jacobi operator which is nilpotent of order 3; they are not timelike Jordan Osserman. They are k-spacelike higher order Jordan Osserman for $2\le k\le s$; they are k-timelike higher order Jordan Osserman for $s+2\le k\le 2s$, and they …
We prove that if $ϕ\colon \mathbb{R}^2 \to \mathbb{R}^{1+2}$ is a smooth proper timelike immersion with vanishing mean curvature, then necessarily $ϕ$ is an embedding, and every compact subset of $ϕ(\mathbb{R}^2)$ is a smooth graph. It follows that if one evolves any smooth self-intersecting spacelike curve (or any planar spacelike curve whose unit tangent vector spans a closed semi-circle) so as to trace a timelike surface of vanishing mean curvature in $\mathbb{R}^{1+2}$, then the evolving surface will either fail to remain timelike, or it will fail to remain smooth. We show that, even allow…
A spacelike surface in Minkowski space $\mathbb{R}_1^3$ is called a $K^α$-translator of the flow by the powers of Gauss curvature if satisfies $K^α= \langle N,\vec{v}\rangle$, $α\neq 0$, where $K$ is the Gauss curvature, $N$ is the unit normal vector field and $\vec{v}$ is a direction of $\mathbb{R}_1^3$. In this paper, we classify all rotational $K^α$-translators. This classification will depend on the causal character of the rotation axis. Although the theory of the $K^α$-flow holds for spacelike surfaces, the equation describing $K^α$-translators is still valid for timelike surfaces. So we …
In this paper, we prove that the static triple with half harmonic Weyl curvature and positive scalar curvature must be the standard hemisphere.
In this note we classify compact 4-manifolds with harmonic Weyl tensor and nonnegative biorthogonal curvature
We prove that if the Ricci curvature is uniformly bounded under the Ricci-Harmonic flow for all times $t$ \in[0, T), then the curvature tensor has to be uniformly bounded as well.
Convergence of Lorentzian spaces and curvature bounds for generalized cones
本文引入了洛伦兹前长度空间的收敛概念，证明了时状曲率和时状曲率-维度界在该收敛下稳定，并展示了广义洛伦兹锥在该收敛下的适应性，最终获得预紧致定理。
Conformal geometry of quasi-umbilical timelike surfaces
本文研究拟脐类时曲面的共形几何，证明其等温性、共形形变依赖单变量函数及共形高斯映射调和，并从零曲线构造所有此类曲面。
Almost-Kähler four manifolds with harmonic self-dual Weyl curvature
本文研究具有调和自对偶Weyl曲率的紧致近Kähler四维流形，证明了在常数量曲率且$c_{1}\cdotω\geq 0$条件下该流形为Kähler流形，并建立了一个积分曲率不等式。
It was proved that the fundamental group of the space of harmonic polynomials of degree $n(n \geq 2)$, with the same Gaussian curvature is not trivial. Furthermore, we give an example of topologically nonequivalent conjugate harmonic functions having the same Gaussian curvature.
We prove local existence of complex-valued harmonic morphisms from any Riemannian homogeneous spaces of positive curvature, except the Berger space Sp(2)/SU(2).
On a compact n-dimensional manifold, it has been conjectured that a critical point metric of the total scalar curvature, restricted to the space of metrics with constant scalar curvature of unit volume, will be Einstein. This conjecture was proposed in 1984 by Besse, but has yet to be proved. In this paper, we prove that if the manifold with the critical point metric has harmonic curvature, then it is isometric to a standard sphere.
On intrinsic rotational surfaces in the Lorentz-Minkowski space
研究Lorentz-Minkowski空间中的内蕴旋转曲面，通过替换平均曲率常值性为Weingarten自同态的特定表达式，证明平均曲率为常数且旋转角是线性函数，结果涵盖类空/类时情况，还确定了零平均曲率下的相关曲面族。
J.Eells and L. Lemaire introduced k-harmonic maps, and T. Ichiyama, J. Inoguchi and H.Urakawa showed the first variation formula. In this paper, we describe the ordinary differential equations of $3$-harmonic curves into a Riemannian manifold with constant sectional curvature, and show biharmonic curve is k-harmonic curve $(k\geq 2)$.
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