On a Generalization of Curvature Homogeneous Spaces

Sekigawa proved in 1977 that a 3-dimensional Riemannian manifold which is curvature homogeneous up to order 1 in the sense of I.M. Singer is always locally homogeneous. We deal here with the modification of the curvature homogeneity which is said to be “of type (1, 3)”. We give example of a 3-dimensional Riemannian manifold which is curvature homogeneous up to order 1 in the modified sense but still not locally homogeneous.     

The existence of light‐like homogeneous geodesics in homogeneous Lorentzian manifolds

In previous papers, a fundamental affine method for studying homogeneous geodesics was developed. Using this method and elementary differential topology it was proved that any homogeneous affine manifold and in particular any homogeneous pseudo‐Riemannian manifold admits a homogeneous geodesic through arbitrary point. In the present paper this affine method is refined and adapted to the pseudo‐Riemannian case. Using this method and elementary topology it is proved that any homogeneous Lorentzian manifold of even dimension admits a light‐like homogeneous geodesic. The method is illustrated in detail with an example of the Lie group of dimension 3 with an invariant metric, which does not admit any light‐like homogeneous geodesic.     

Locally homogeneous affine connections on compact surfaces

Global properties of locally homogeneous and curvature homogeneous affine connections are studied. It is proved that the only locally homogeneous connections on surfaces of genus different from 1 are metric connections of constant curvature. There exist nonmetrizable nonlocally symmetric locally homogeneous affine connections on the torus of genus 1. It is proved that there is no global affine immersion of the torus endowed with a nonflat locally homogeneous connection into .

     

Pólya Theory of Hypercubes

A homogeneous nilmanifold is a nilpotent Lie group endowed with a left-invariant metric. In the present work we classify up to isometry the homogeneous nilmanifolds of dimension 3 and 4 and we compute the corresponding isometry groups.     

The pseudo-hyperplanes and homogeneous pseudo-embeddings of AG(n, 4) and PG(n, 4)

We determine all homogeneous pseudo-embeddings of the affine space AG(n, 4) and the projective space PG(n, 4). We give a classification of all pseudo-hyperplanes of AG(n, 4). We also prove that the two homogeneous pseudo-embeddings of the generalized quadrangle Q(4, 3) are induced by the two homogeneous pseudo-embeddings of AG(4, 4) into which Q(4, 3) is fully embeddable.     

Generalized normal homogeneous spheres

We find new generalized normal homogeneous but not normal homogeneous Riemannian metrics on spheres of dimensions 4n+3, n ≥ 1, and all homogeneous space forms covered by them; all these spaces have zero Euler characteristic. Deriving consequences, alongside some other new results we obtain new proofs for analogous known results for all complex projective spaces of odd complex dimension starting from three.     

Affine homogeneity of indefinite real hypersurfaces in the space ℂ3

We develop a constructive approach to the problem of describing affinely homogeneous real hypersurfaces in 3-dimensional complex space having nondegenerate sign-indefinite Levi form. We construct the affine invariants of a nondegenerate indefinite hypersurface in terms of second-order jets of its defining function and introduce the notion of the affine canonical equation of this surface. Three main types of canonical equations are considered. For each of these types, we construct a family of Lie algebras related to affinely homogeneous surfaces of a particular type. As a result, a family (depending on two real parameters) of affinely different homogeneous submanifolds of 3-dimensional complex space is presented (as matrix algebras).     

3-齐次群与旗传递5-（v，k，3）设计

旗传递t-设计的分类是代数组合学的一个重要课题．本文主要讨论了旗传递5-（v,k,3）设计．由P．J．Cameron和C．E．Praeger的结论可知,此时设计的自同构群是3-齐次群．本文利用3-齐次群的分类,证明了设计的自同构群不能是仿射型群．     

FIVE LIMIT CYCLES TO A CLASS OF SYSTEMS IN R~3

In this paper, a bridge between near-homogeneous and homogeneous vector fields in R 3 is found. By the relationship between homogeneous vector fields and the induced tangent vector fields of two-dimensional manifold S 2 , we prove the existence of at least 5 isolated closed orbits for a class of n + 1 (n ≥ 2) systems in R 3 , which are located on the five invariant closed cones of the system.     

Classification of homogeneous CR-manifolds in dimension 4

Locally homogeneous CR-manifolds in dimension 3 were classified, up to local CR-equivalence, by E. Cartan. We classify, up to local CR-equivalence, all locally homogeneous CR-manifolds in dimension 4. The classification theorem enables us also to classify all symmetric CR-manifolds in dimension 4, up to local CR-equivalence.     

The Lyusternik‒Sobolev lemma and the specific asymptotic stability of solutions of linear homogeneous Volterra-type integro-differential equations of order 3

Sufficient conditions for the asymptotic stability on a half-line of the solutions of a linear homogeneous Volterra-type integro-differential equation of order 3 in the case where the solutions of the corresponding linear homogeneous differential equation are asymptotically unstable are determined. A new method is proposed, and an illustrative example is constructed.     

Bcklund变换与类时极值平移曲面

首先通过选取适当的等温参数将三维Ｍｉｎｋｏｗｓｋｉ空间Ｒ２．１中的全脐点类时曲面与Ｌｉｏｕｖｉｌｅ方程相联系．其次，通过类时曲面上的类光曲线坐标将Ｒ２．１中的类时极值曲面与齐次波动方程相联系．进一步，利用Ｌｉｏｕｖｉｌｅ方程与齐次波动方程之间的Ｂａｃｋｌｕｎｄ变换，我们可以从三维Ｍｉｎｋｏｗｓｋｉ空间中一个全脐点的类时曲面得到该空间中一个类时极值平移曲面．     

方型锥上第一类Siegel域

<正> 记V为R~n中不包含直线的仿射齐性开凸锥(简称齐性锥),则C~n中点集■(V)={z∈C~n|Im(z)∈V}称为齐性锥V上第一类Siegel域,它仿射齐性.熟知齐性锥V上第一类Siegel域在解析等价下的分类即齐性锥在仿射等价下的分类.这方面已有结果为Vinberg关于仿射齐性自共轭锥的分类. 本文考虑方型锥,即这种齐性锥,它仿射等价于适合条件     

Maximal surfaces in a certain 3-dimensional homogeneous spacetime

A generalized integral representation formula for spacelike maximal surfaces in a certain 3-dimensional homogeneous spacetime is obtained. This spacetime has a solvable Lie group structure with left invariant metric. The normal Gauß map of maximal surfaces in the homogeneous spacetime is discussed and the harmonicity of the normal Gauß map is studied.     

Compact projective planes with homogeneous ovals

Closed ovals exist only in 2-or 4-dimensional compact projective planes. We show that a plane of dimension 2 or 4 contains ahomogeneous closed oval iff the automorphism group contains SO₂ or SO₃, respectively. In 4-dimensional planes, existence of a homogeneous oval and existence of a homogeneous Baer subplane are equivalent. We determine the possible full automorphism groups of planes containing homogeneous ovals except for two possibilities in dimension 4 whose existence remains uncertain.     

Characterization of isolated homogeneous hypersurface singularities in C4

Let V be a hypersurface with an isolated singularity at the origin in Cn+1. It is a natural question to ask when V is defined by weighted homogeneous polynomial or homogeneous polynomial up to biholomorphic change of coordinates. In 1971, a beautiful theorem of Saito gives a necessary and sufficient condition for V to be defined by a weighted homogeneous polynomial.For a two-dimensional isolated hypersurface singularity V, Xu and Yau found a coordinate free characterization for V to be defined by a homogeneous polynomial. Recently Lin and Yau gave necessary and sufficient conditions for a 3-dimensional isolated hypersurface singularity with geometric genus bi.er than zero to be defined by a homogeneous polynomial. The purpose of this paper is to prove that Lin-Yau's theorem remains true for singularities with geometric genus equal to zero.     

Characterization of isolated homogeneous hypersurface singularities in C~4 Dedicated to Professor Sheng GONG on the occasion of his 75th birthday

Let V be a hypersurface with an isolated singularity at the origin in Cn 1. It is a natural question to ask when V is defined by weighted homogeneous polynomial or homogeneous polynomial up to biholomorphic change of coordinates. In 1971, a beautiful theorem of Saito gives a necessary and sufficient condition for V to be defined by a weighted homogeneous polynomial. For a two-dimensional isolated hypersurface singularity V, Xu and Yau found a coordinate free characterization for V to be defined by a homogeneous polynomial. Recently Lin and Yau gave necessary and sufficient conditions for a 3-dimensional isolated hypersurface singularity with geometric genus bigger than zero to be defined by a homogeneous polynomial. The purpose of this paper is to prove that Lin-Yau's theorem remains true for singularities with geometric genus equal to zero.     

Characterization of isolated homogeneous hypersurface singularities in ℂ4

Let V be a hypersurface with an isolated singularity at the origin in ? ⁿ⁺¹. It is a natural question to ask when V is defined by weighted homogeneous polynomial or homogeneous polynomial up to biholomorphic change of coordinates. In 1971, a beautiful theorem of Saito gives a necessary and sufficient condition for V to be defined by a weighted homogeneous polynomial. For a two-dimensional isolated hypersurface signularity V, Xu and Yau found a coordinate free characterization for V to be defined by a homogeneous polynomial. Recently Lin and Yau gave necessary and sufficient conditions for a 3-dimensional isolated hypersurface singularity with geometric genus bigger than zero to be defined by a homogeneous polynomial. The purpose of this paper is to prove that Lin-Yau’s theorem remains true for singularities with geometric genus equal to zero.