二维流形上连续流的一个全局性质

<正> 对于在平面上由二阶微分方程组定义的动力系统,设C~+是包含在有界闭区域中的一条正半轨道.如果它的ω极限集Ω(C~+)不含奇点,那末或者C~+自身是一条周期轨道,或者Ω(C~+)是一条周期轨道.这就是著名的Poincare-Bendixson定理.在环面T~2上,由熟知的无理线性流的例子表明,每一条正半轨道的ω极限集都是整个T~2(在T~2上处处稠密).如M.M.Peixoto在[7]中指出,从T~2上的无理流出发可以在其它闭曲面M~2(除     

非极大偶阶真子群是2′-闭的有限群

作者在[1]中对满足下述条件的有限群进行了分类:(A)每个非极大偶阶真子群是2-闭的。特别,作者证明了:满足条件(A)的有限不可解群只有 A_5 和 SL(2,5)两个。本文将研究另一极端情形,即(B)每个非极大偶阶真子群是2′-闭的。     

群的形变收缩及Toeplitz代数

设G为一个离散群,(G,G_ )为一个拟偏序群使得G_ ~0=G_ ∩G_ ~(-1)为G的非平凡子群。令[G]为G关于G_ ~0的左倍集全体,|G_ |为[G]的正部。记T~(G_ )和T~([G_ ])为相应的Toeplitz代数。当存在一个从G到G_ ~0上的形变收缩映照时,我们证明了T~(G_ )酉同构于T~([G_ ])×C_r~*(G_ ~0)的一个C_-~*c子代数。若进一步,G_ ~0还为G的一个正规子群,则T~(G_ )与T~([G_ ])×C_r~*(G_ ~0)酉同构。     

具有非常余维数不动点集的可换对合

给定一组正整数l1相似文献   

格序群的l—自由积的表示

本文给出了两个非平凡可数格序群的ｌ－自由积在有理数集Ｑ上的高ｏ－可迁忠实表示。证明了格序群的ｌ－自由积的向量格复盖与格序群的向量格复盖的ｌ－自由积之间的同构关系；格序群与格序群的反向系统的ｌ－自由积的反向极限与反向极限的自由积之间的同构关系。作为应用，本文给出了两个非平凡可数格序群的ｌ－自由积的复盖在Ｑ上的高ｏ－可迁忠实表示和下有向偏序集上的非平凡可数格序群的反向系统的反向极限在Ｑ上的高ｏ－可迁忠     

三维闭流形上等周曲面的若干性质研究

本文包含两个主要定理,第一个定理考虑了数量曲率不小于相应空间形式的三维闭(即紧致无边)流形中小体积的等周曲面的面积上界估计,并指出等号成立当且仅当该流形截面曲率为常数;第二个定理假设三维闭流形的数量曲率R≥6,Ricci曲率非负且π_1(M~3)有限,在某个等周曲面面积满足一定条件时,原流形的体积必满足一个上界估计.     

(Z_2)~k作用下不动点集的一个必要条件

设(Z_2)~k作用于光滑闭流形M~n上,其不动点集具有常余维数(2~k-1),法丛分解为 (1,…,1). 2~k-1本文利用Kosniowski-Stong公式得出它的一个必要条件。(Z_2)~2作用于光滑闭流形M~n上,其不动点集具有常余维数3,法丛分解为P={(2,1,0),(2,0,1),(1,1,1)}.J_(n,2)~3(p)是具有上述性质的未定向的n维上协边类[M~n]构成的集合。本文通过构造上协边环MO_*的一组生成元决定了J_(n,2)~3(p)的群结构。     

超可解子群或为2-闭或为Schmidt群的有限群

本文研究超可解子群或为2-闭或为Schmidt群(即极小非幂零群)的有限群的构造,给出了这些群的一个精细的分类.     

三维梯度共轭系统的全周期性

本文中称三维系统x=f(x)(x∈R³)为梯度共轭系统,如果它满足条件1)divf=0,2)存在第一积分G(x)=C。得到的结果是:当其中,与G解析,G正规时,若每一曲面G=C上只有有限个平衡点,则在每一曲面G=C上除去中心,广义鞍点与鞍点的分界线之外全是闭轨.应用定理于几类捕食方程,得知它们除去平衡点与坐标面之外一切轨线全是闭的.所以可将它们看作是Volterra-Lotka捕食方程在三维的推广。     

环面上动力系统的唯一各态历经性

本文得到如下结果: 设f(p,t)是定义在环面T~2上的连续流。如果f不存在休止点和闭轨道,于是f是唯一各态历经的。     

Outer automorphism groups of certain 1-relator groups

Grossman first showed that outer automorphism groups of 1-relator groups given by orientable surface groups are residually finite, whence mapping class groups of orientable surfaces are residually finite. Allenby, Kim and Tang showed that outer automorphism groups of cyclically pinched 1-relator groups are residually finite, whence mapping class groups of orientable and non-orientable surfaces are residually finite. In this paper we show that outer automorphism groups of certain conjugacy separable 1-relator groups are residually finite.     

Finite group actions on bordered surfaces of small genus

This paper considers finite group actions on compact bordered surfaces — quotients of unbordered orientable surfaces under the action of a reflectional symmetry. Classification of such actions (up to topological equivalence) is carried out by means of the theory of non-euclidean crystallographic groups, and determination of normal subgroups of finite index in these groups, up to conjugation within their automorphism group. A result of this investigation is the determination, up to topological equivalence, of all actions of groups of finite order 6 or more on compact (orientable or non-orientable) bordered surfaces of algebraic genus p for 2≤p≤6. We also study actions of groups of order less than 6, or of prime order, on bordered surfaces of arbitrary algebraic genus p≥2.     

三维空间型中闭曲面和测地球面的比较

本文建立关于三维空间型中的可定向闭曲面的某些公式.引入积分绝对平均曲率来描述可定向闭曲面的平均弯曲程度.在此意义上,可定向闭曲面可以与作为包含该闭曲面的凸的测地球的边界测地球面进行比较,这种比较能用于说明空间型自身的某些属性     

On large orientation-reversing finite group-actions on 3-manifolds and equivariant Heegaard decompositions

We consider finite group-actions on closed, orientable and nonorientable 3-manifolds; such a finite group-action leaves invariant the two handlebodies of a Heegaard splitting of M of some genus g. The maximal possible order of a finite group-action of an orientable or nonorientable handlebody of genus $$g>1$$ is $$24(g-1)$$, and in the present paper we characterize the 3-manifolds M and groups G for which the maximal possible order $$|G| = 24(g-1)$$ is obtained, for some G-invariant Heegaard splitting of genus $$g>1$$. If M is reducible then it is obtained by doubling an action of maximal possible order $$24(g-1)$$ on a handlebody of genus g. If M is irreducible then it is a spherical, Euclidean or hyperbolic manifold obtained as a quotient of one of the three geometries by a normal subgroup of finite index of a Coxeter group associated to a Coxeter tetrahedron, or of a twisted version of such a Coxeter group.     

Generalized Embedding Schemes

Covering projections are used to provide a logical foundation for the generalized embedding schemes which describe embeddings of graphs on closed surfaces that are not necessarily orientable. Criteria are provided to resolve the question of when such schemes describe the same embedding, and when that embedding is orientable. The schemes are then applied to derive some interpolation-type theorems for embeddings on closed surfaces.     

Exotic Minimal Surfaces

We prove a general fusion theorem for complete orientable minimal surfaces in ?³ with finite total curvature. As a consequence, complete orientable minimal surfaces of weak finite total curvature with exotic geometry are produced. More specifically, universal surfaces (i.e., surfaces from which all minimal surfaces can be recovered) and space-filling surfaces with arbitrary genus and no symmetries.     

Finite maximal orientation reversing group actions on handlebodies and 3-manifolds

We study arbitrary (that is not necessarily orientation preserving) finite group actions on 3-dimensional orientable or nonorientable handlebodies of genus g. For g>1, the maximal possible order is 24(g−1); we characterize the corresponding groups of this order and also the occuring quotient orbifolds. Then we use this to study finite group actions of large order (with respect to the equivariant Heegaard genus g) on closed 3-manifolds, again concentrating on the maximal case of order 24(g−1). Our results extend corresponding results in the orientation preserving setting. Whereas for arbitrary finite group actions on handlebodies much more types of quotient orbifolds occur than in the orientation preserving case, it turns out that for closed 3-manifolds the situation is quite rigid, in contrast to the orientation preserving case where one has many possibilities to construct manifolds with large group actions.     

A Tutte polynomial for maps II: The non-orientable case

We construct a new polynomial invariant of maps (graphs embedded in a closed surface, orientable or non-orientable), which contains as specializations the Krushkal polynomial, the Bollobás—Riordan polynomial, the Las Vergnas polynomial, and their extensions to non-orientable surfaces, and hence in particular the Tutte polynomial. Other evaluations include the number of local flows and local tensions taking non-identity values in a given finite group.     

Rigidity for orientable functors

In the following paper we introduce the notion of orientable functor (orientable cohomology theory) on the category of projective smooth schemes and define a family of transfer maps. Applying this technique, we prove that with finite coefficients orientable cohomology of a projective variety is invariant with respect to the base-change given by an extension of algebraically closed fields. This statement generalizes the classical result of Suslin, concerning algebraic K-theory of algebraically closed fields. Besides K-theory, we treat such examples of orientable functors as etale cohomology, motivic cohomology, algebraic cobordism. We also demonstrate a method to endow algebraic cobordism with multiplicative structure and Chern classes.     

Each finite group is a symmetry group of some map (an “Atom”-bifurcation)

Maps are studied, i.e., cell decompositions of closed two-dimensional surfaces, or two-dimensional atoms which encode bifurcations of Liouville fibrations of non-degenerate integrable Hamiltonian systems. Any finite group G is proved to be a symmetry group of an orientable map (of an atom). Moreover, one such map X(G) is constructed algorithmically. Upper bounds are obtained for the minimal genus Mg(G) of an orientable map with the given symmetry group G and for the minimal number of vertices, edges, and sides of such maps.