Ergodic Actions of Semisimple Lie Groups on Compact Principal Bundles

Let G = SL(n, ?) (or, more generally, let G be a connected, noncompact, simple Lie group). For any compact Lie group K, it is easy to find a compact manifold M, such that there is a volume-preserving, connection-preserving, ergodic action of G on some smooth, principal K-bundle P over M. Can M can be chosen independent of K? We show that if M = H/Λ is a homogeneous space, and the action of G on M is by translations, then P must also be a homogeneous space H′Λ′. Consequently, there is a strong restriction on the groups K that can arise over this particular M.     

Free, Isometric Circle Actions on Compact Symmetric Spaces

Let S=G/K be a strongly irreducible, simply connected, compact symmetric space and let be its group of isometries. We classify the symmetric spaces among these that admit free, isometric circle actions. The existence of such actions is important in constructing examples of manifolds with positive sectional curvature.     

Constructive Axiomatic Approach to the Determination of the Orbit Spaces of Coregular Compact Linear Groups

We review the proposal of a constructive axiomatic approach to the determination of the orbit spaces of all the real compact linear groups, obtained through the computation of a metric matrix , which is defined only in terms of the scalar products between the gradients p₁(x),...,p_q(x) of the elements of a minimal integrity basis (MIB) for the ring [ⁿ]^G of G-invariant polynomials. The domain of semi-positivity of is known to realize the orbit space ⁿ/G of G as a semi-algebraic variety in the space ^q spanned by the variables p₁,...,p_q. The matrices can be obtained from the solutions of a universal differential equation (master equation), which satisfy convenient initial conditions. The master equation and the initial conditions involve as free parameters only the degrees d_a of the p_a(x)s. This approach tries to bypass the actual impossibility of explicitly determining a set of basic polynomial invariants for each group. Our results may be relevant in physical contexts where the study of covariant or invariant functions is important, like in the determination of patterns of spontaneous symmetry breaking in quantum field theory, in the analysis of phase spaces and structural phase transitions (Landaus theory), in covariant bifurcation theory, in crystal field theory and so on. Mathematics Subject Classifications (2000) 14L24, 13A50, 14L30.This paper is partially supported by INFN and MURST 40% and 60%.     

Group Actions on the Cubic Tree

It is known that every group which acts transitively on the ordered edges of the cubic tree ₃, with finite vertex stabilizer, is isomorphic to one of seven finitely presented subgroups of the full automorphism group of ₃–one of which is the modular group. In this paper a complete answer is given for the question (raised by Djokovi and Miller) as to whether two such subgroups which intersect in the modular group generate their free product with the modular group amalgamated.     

A Note on Local Rigidity

The aim of this note is to give a geometric proof for classical local rigidity of lattices in semisimple Lie groups. We are reproving well known results in a more geometric (and hopefully clearer) way.     

Entropy and Arithmetic Quotients for Simple Automorphism Groups of Geometric Manifolds

Let G be a connected simple Lie group, and suppose G acts on a compact real analytic manifold M. Assume that G preserves a unimodular rigid geometric structure, for example a connection and a volume form. The aim of this paper is to describe the measure theoretic structure of such actions, when -rank(G)2.     

Faithful Group Actions on Rooted Trees Induced by Actions of Quotients

We develop some new techniques of constructing (finite state) actions on rooted homogeneous trees and apply them to various groups. In particular we show that there is a faithful action of each amalgameted free product of the form ??_?? on a rooted homogeneous tree of finite degree, described by finite state automorphisms.     

Actions of Semisimple Lie Groups on Circle Bundles

Suppose G is a connected, simple, real Lie group with -rank(G) 2, M is an ergodic G-space with invariant probability measure , and : G × M Homeo( ) is a Borel cocycle. We use an argument of É. Ghys to show that there is a G-invariant probability measure on the skew product M × , such that the projection of to M is . Furthermore, if (G × M) Diff¹( ), then can be taken to be equivalent to × , where is Lebesgue measure on ; therefore, is cohomologous to a cocycle with values in the isometry group of .     

Group Actions, Double Cosets, and Homomorphisms: Unifying Concepts for the Constructive Theory of Discrete Structures

In the present paper we describe the use of group actions, double cosets and homomorphisms in the constructive theory of discrete structures, as we found it useful from both a theoretical and a practical point of view. By means of examples we should like to demonstrate that these methods are useful both as unifying principles and as efficient methods for applications.     

The first Cohomology of Affine $ \mathbb{Z}^{p} $ \mathbb{Z}^{p} -actions on Tori and applications to rigidity*

Let a minimal affine -action on the torus T ^q , p 2 and q 1. The cohomology of (see definition below) depends on both the algebraic properties of the induced action on H ¹(T ^q , ) and the arithmetical properties of the translation cocycle. We give a Diophantine condition that characterizes those affine actions whose first cohomology group is finite dimensional. In this case it is necessarily isomorphic to . Thus the -action F obtained by suspension of is parameter rigid, i.e., any other -action with the same orbit foliation is smoothly conjugate to a reparametrization of F by an automorphism of .*Partially supported by CNPq fellowship by Fondecyt Grant 1000047 and DGICT-UCN and fundación Andes, Chile.     

Yang-Mills模空间上的有限群作用

设Ｘ是单连通光滑闭４－流形,有限群Ｇ光滑作用于Ｘ,令ｐ：Ｅ→Ｘ是Ｘ上具有光滑Ｇ作用的ＳＵ（２）从且ｐ是一个Ｇ映射．本文研究了丛Ｅ上等模空间Ｍ＾Ｇ和商丛Ｅ’→Ｘ’上的关键空间之间的关系,并讨论了不变模空间的紧致化。     

Nielsen Methods and Groups Acting on Hyperbolic Spaces

We show that for any n & in there exists a constant C(n) such that any n-generated group G which acts by isometries on a -hyperbolic space (with >0) is either free or has a nontrivial element with translation length at most C(n).     

Isometric Actions of Heisenberg Groups on Compact Lorentz Manifolds

We prove results toward classifying compact Lorentz manifolds on which Heisenberg groups act isometrically. We give a general construction, leading to a new example, of codimension-one actions – those for which the dimension of the Heisenberg group is one less than the dimension of the manifold. The main result is a classification of codimension-one actions, under the assumption they are real-analytic.     

局部紧的Vilenkin群上的加权局部Hardy空间

该文给出了Vilenkin群上加权局部Hardy空间的定义,它的原子分解及对偶性质,并且研究了一类卷积算子在其上的有界性．     

Isometric Group Actions on Hilbert Spaces: Growth of Cocycles

We study growth of 1-cocycles of locally compact groups, with values in unitary representations. Discussing the existence of 1-cocycles with linear growth, we obtain the following alternative for a class of amenable groups G containing polycyclic groups and connected amenable Lie groups: either G has no quasi-isometric embedding into a Hilbert space, or G admits a proper cocompact action on some Euclidean space. On the other hand, noting that almost coboundaries (i.e. 1-cocycles approximable by bounded 1-cocycles) have sublinear growth, we discuss the converse, which turns out to hold for amenable groups with “controlled” F?lner sequences; for general amenable groups we prove the weaker result that 1-cocycles with sufficiently small growth are almost coboundaries. Besides, we show that there exist, on a-T-menable groups, proper cocycles with arbitrary small growth. Received: August 2005 Revision: October 2006 Accepted: November 2006     

Actions of Compact Groups on Compact (4,m)-Quadrangles

We establish sharp upper bounds for the dimensions of compact groups which act effectively on finite-dimensional compact generalized quadrangles with four-dimensional point rows. These bounds are attained, or indeed approached, only for explicitly known actions of Lie groups on Moufang quadrangles.     

Vilenkin群上的加权Hardy空间中的乘子

本文主要研究了Vilenkin群上的加权Hardy空间的原子分解,证明了一个加权H^p乘子定理并给出其应用。     

Actions of SL $$(n,\mathbb{Z})$$ on Homology Spheres

Any continuous action of SL , where n > 2, on a r-dimensional mod 2 homology sphere factors through a finite group action if r < n −1. In particular, any continuous action of SL on the n-dimensional sphere factors through a finite group action.Mathematics Subject Classiffications (2000). Primary 57S25; Secondary 37C85, 57S17     

Certain Ergodic Properties of a Differential System on a Compact Differentiable Manifold

Let M ⁿ be an n-dimensional compact C ^∞-differentiable manifold, n ≥ 2, and let S be a C ¹-differential system on M ⁿ . The system induces a one-parameter C ¹ transformation group φ _t (−∞ < t < ∞) over M ⁿ and, thus, naturally induces a one-parameter transformation group of the tangent bundle of M ⁿ . The aim of this paper, in essence, is to study certain ergodic properties of this latter transformation group. Among various results established in the paper, we mention here only the following, which might describe quite well the nature of our study. (A) Let M be the set of regular points in M ⁿ of the differential system S. With respect to a given C ^∞ Riemannian metric of M ⁿ , we consider the bundle of all (n−2) spheres Q _x ⁿ⁻², x∈M, where Q _x ⁿ⁻² for each x consists of all unit tangent vectors of M ⁿ orthogonal to the trajectory through x. Then, the differential system S gives rise naturally to a one-parameter transformation group ψ _t ^# (−∞<t<∞) of . For an l-frame α = (u ₁, u ₂,⋯, u _l ) of M ⁿ at a point x in M, 1 ≥ l ≥ n−1, each u _i being in , we shall denote the volume of the parallelotope in the tangent space of M ⁿ at x with edges u ₁, u ₂,⋯, u _l by υ(α), and let . This is a continuous real function of t. Let

Some Groups Having Only Elementary Actions on Metric Spaces with Hyperbolic Boundaries

We study isometric actions of certain groups on metric spaces with hyperbolic-type bordifications. The class of groups considered includes SL _n (), Artin braid groups and mapping class groups of surfaces (except the lower rank ones). We prove that in various ways such actions must be elementary. Most of our results hold for non-locally compact spaces and extend what is known for actions on proper CAT(-1) and Gromov hyperbolic spaces. We also show that SL _n () for n 3 cannot act on a visibility space X without fixing a point in . Corollaries concern Floyd's group completion, linear actions on strictly convex cones, and metrics on the moduli spaces of compact Riemann surfaces. Some remarks on bounded generation are also included.