Countable Primitive Groups

We give a complete characterization of countable primitive groups in several settings including linear groups, subgroups of mapping class groups, groups acting minimally on trees and convergence groups. The latter category includes as a special case Kleinian groups as well as subgroups of word hyperbolic groups. As an application we calculate the Frattini subgroup in many of these settings, often generalizing results that were only known for finitely generated groups. In particular, we answer a question of G. Higman and B.H. Neumann on the Frattini group of an amalgamated product. Received: January 2006, Revision: May 2006, Accepted: May 2006     

Compactifications of Homeomorphism Groups of Linearly Ordered Compacta

Mathematical Notes - Topological groups of transformations are studied (their structure and equivariant compactifications) on which the pointwise convergence topology is an admissible group...     

Expansive Algebraic Actions of Countable Abelian Groups

This paper gives an algebraic characterization of expansive actions of countable abelian groups on compact abelian groups. This naturally extends the classification of expansive algebraic -actions given by Schmidt using complex varieties. Also included is an application to a natural class of examples arising from unit subgroups of integral domains. The author is very grateful for the helpful advice and comments of Tom Ward.     

可数sofic群的等距线性作用的维数

我们对复Banach空间上的可数sofic群的等距线性作用提出了一种新的维数,推广了复Banach空间上的可数顺从群的等距线性作用的Voiculescu维数,并且在可数sofic群的情形回答了Gromov的一个问题.     

Fundamental Groups of TriangleGeometries

利用Building理论获得一种计算某些三角几何的基本群的新的方法.这种方法能够容易地计算出无限多有限三角几何的基拓扑基本群.     

Homomorphisms between Stone-Cech Homomorphisms into Stone-Cech Remainders of Countable Groups

We study properties of continuous homomorphisms from β S into T^* and from S^* into T^*, where S denotes a countably infinite semigroup and T denotes a countably infinite group. We show that they have striking algebraic properties if they do not arise as continuous extensions of homomorphisms from S to T.     

三角几何的基本群(英)

利用Building理论获得一种计算某些三角几何的基本群的新的方法．这种方法能够容易地计算出无限多有限三角几何的基拓扑基本群．     

The Spectrum of Completely Positive Entropy Actions of Countable Amenable Groups

We prove that an ergodic free action of a countable discrete amenable group with completely positive entropy has a countable Lebesgue spectrum. Our approach is based on the Rudolph-Weiss result on the equality of conditional entropies for actions of countable amenable groups with the same orbits. Relative completely positive entropy actions are also considered. An application to the entropic properties of Gaussian actions of countable discrete abelian groups is given.     

Manifolds with Poly-Surface Fundamental Groups

We study closed topological 2n-dimensional manifolds M with poly-surface fundamental groups. We prove that if M is simple homotopy equivalent to the total space E of a Y-bundle over a closed aspherical surface, where Y is a closed aspherical n-manifold, then M is s-cobordant to E. This extends a well-known 4-dimensional result of Hillman in [14] to higher dimensions. Our proof is different from that of the quoted paper: we use Mayer-Vietoris techniques and the properties of the -theory assembly maps for such bundles.     

Four-Manifolds With Surface Fundamental Groups

We study the homotopy type of closed connected topological -manifolds whose fundamental group is that of an aspherical surface . Then we use surgery theory to show that these manifolds are -cobordant to connected sums of simply-connected manifolds with an -bundle over .

     

A Model-Theoretic Characterization of Countable Direct Sums of Finite Cyclic Groups #

We prove that an abelian group G is a countable direct sum of finite cyclic groups if and only if there exists a consistent existential theory Γ of abelian groups such that G is embeddable in every model of Γ.     

The Root-Class Residuality of the Fundamental Groups of Graphs of Groups

Siberian Mathematical Journal - Consider a class  $ \mathcal{C} $ of groups containing at least one nontrivial group and closed under subgroups, extensions, and Cartesian...     

Minimal Geodesics and Nilpotent Fundamental Groups

Hedlund [18] constructed Riemannian metrics on n-tori, n 3 for which minimal geodesics are very rare. In this paper we construct similar examples for every nilpotent fundamental group. These examples show that Bangert's existence results of minimal geodesics [4] are optimal for nilpotent fundamental groups.     

Fundamental Groups of Graphs of Cyclic Subgroup Separable and Weakly Potent Groups

We give a characterization of the cyclic subgroup separability and weak potency of the fundamental group of a graph of polycyclic-by-finite groups and free-by-finite groups amalgamating edge subgroups of the form × D,where h has infinite order and D is finite.     

First Fundamental Theorem for Covariants of Classical Groups

Let U(G) be a maximal unipotent subgroup of one of the classical groups G=GL(V), O(V), Sp(V). Let W be a direct sum of copies of V and its dual V*. For the natural action U(G) : W, we describe a minimal system of homogeneous generators for the algebra of U(G)-invariant regular functions on W. For G=O(V), Sp(V), this result is connected with a construction for the irreducible representations of G due to H. Weyl.     

高维Fuchsian群的基本域

应用Clifford矩阵表示,研究了高维Fuchsian群的基本域,对严格抛物元和严格双曲元给出了基本域的构造方法,将Beardon的关于平面Fuchsian群的两个定理推广到了高维情形.