Locally compact subgroups of the spectrum of the measure algebra II

The maximal ideal space Δ_G of the measure algebra M(G) of a locally compact abelian group G is a compact commutative semitopological semigroup. In this paper we show that cℓ Ĝ the closure of Ĝ, the dual of G, in Δ_G can contain maximal subgroups which are not locally compact. We have previously characterized the locally compact maximal subgroups of cℓ Ĝ as arising from locally compact topologies on G which are finer than the original topology. This research was supported in part by NSF contract number GP-19852.     

Hyperbolic groups have flat-rank at most 1

The flat-rank of a totally disconnected, locally compact group G is an integer, which is an invariant of G as a topological group. We generalize the concept of hyperbolic groups to the topological context and show that a totally disconnected, locally compact, hyperbolic group has flat-rank at most 1. It follows that the simple totally disconnected locally compact groups constructed by Paulin and Haglund have flat-rank at most 1.     

Low discrepancy sequences failing Poissonian pair correlations

Let G be a locally compact totally disconnected topological group. Under a necessary mild assumption, we show that the irreducible unitary representations of G are uniformly admissible if and only if the irreducible smooth representations of G are uniformly admissible. An analogous result for *-algebras is also established. We further show that the property of having uniformly admissible irreducible smooth representations is inherited by finite-index subgroups and overgroups of G.     

Algebraic endomorphisms of LCA-groups that preserve closed subgroups

The object of this paper is the classification of those algebraic (i.e. not necessarily continuous) endomorphisms of a locally compact abelian group leaving invariant all closed subgroups. In a canonical way they turn out to form again a locally compact abelian group which can be determined up to isomorphism. If the group is totally disconnected or not periodic all endomorphisms with this property are continuous and form a topological ring.     

Dense orbits of automorphisms and compactness of groups

It is proved, by using topological properties, that when a group automorphism of a locally compact totally disconnected group is ergodic under the Haar measure, the group is compact. The result is an answer for Halmos's question that has remained open for the totally disconnected case.     

Limits of commutative triangular systems on locally compact groups

On a locally compact group G, if , for some probability measuresv _n and μ onG, then a sufficient condition is obtained for the set to be relatively compact; this in turn implies the embeddability of a shift of μ. The condition turns out to be also necessary when G is totally disconnected. In particular, it is shown that ifG is a discrete linear group over R then a shift of the limit μ is embeddable. It is also shown that any infinitesimally divisible measure on a connected nilpotent real algebraic group is embeddable.     

Actions of totally disconnected groups and equivariant singular homology

We study equivariant singular homology in the case of actions of totally disconnected locally compact groups on topological spaces. Theorem A says that if G is a totally disconnected locally compact group and X is a G-space, then any short exact sequence of covariant coefficient systems for G induces a long exact sequence of corresponding equivariant singular homology groups of the G-space X. In particular we consider the case where G is a totally disconnected compact group, i.e., a profinite group, and G acts freely on X. Of special interest is the case where G is a p-adic group, p a prime. The conjecture that no p-adic group, p a prime, can act effectively on a connected topological manifold, is namely known to be equivalent to the famous Hilbert-Smith conjecture. The Hilbert-Smith conjecture is the statement that, if a locally compact group G acts effectively on a connected topological manifold M, then G is a Lie group.     

一类全不连通群上的BMO和奇异积分算子

本文主要讨论了定义在局部紧的全不连通群Ｇ上的一类卷积算子在加权Ｌ^∞（Ｇ）和ＢＭＯ（ａ）空间的性态．证明了如果卷积算子的核满足适当的条件,则算子是Ｌ^∞（Ｇ）到ＢＭＯ（ａ）有界的或是ＢＭＯ（ａ）到ＢＭＯ（ａ）有界的．     

Compactness conditions for groups of automorphisms of topological groups

It is proved that if G is a compact, totally disconnected Abelian group and Aut G is its group of topological automorphisms (with the natural topology), then the following conditions are equivalent: (a) Aut G is compact; (b) Aut G is locally compact; (c) Aut G has small invariant neighborhoods of the identity; (d) Aut G is an -group; (e) the factor group of Aut G by its center is compact; (f) the closure of the commutator subgroup of Aut G is compact; (g) , where F_p is a finite p-group, Z_p is the additive group of p-adic integers, and n_p < .Translated from Matematicheskie Zametki, Vol. 19, No. 5, pp. 735–743, May, 1976.In conclusion, the author thanks V. P. Platonov for his constant attention to this paper.     

Equivariant sheaf cohomdlogy

In this paper we study Grothendieck's equivariant sheaf cohomology H(X,G;G) for non-discrete topological groups G and G-sheavesG on a G-Space X. For compact groups and locally compact, totally disconnected groups we obtain detailed results relating H(X,G;-) to H(X;-)^G and H(X/G;-). Furthermore we point out the connection between H(X,G;-) and Borel's equivariant cohomology H_G(X;-).     

Contraction groups and passage to subgroups and quotients for endomorphisms of totally disconnected locally compact groups

The concepts of the scale and tidy subgroups for an automorphism of a totally disconnected locally compact group were defined in seminal work by George A. Willis in the 1990s, and recently generalized to the case of endomorphisms (G. A.Willis, Math. Ann. 361 (2015), 403–442). We show that central facts concerning the scale, tidy subgroups, quotients, and contraction groups of automorphisms extend to the case of endomorphisms. In particular, we obtain results concerning the domain of attraction around an invariant closed subgroup.     

Countably Compact Paratopological Groups

By means of the theory of bispaces we show that a countably compact T₀ paratopological group (G, τ) is a topological group if and only if (G, τ ∨ τ^-1) is ω-bounded (here τ^-1 is the conjugate topology of τ). Our approach is premised on the fact that every paratopological countably compact paratopological group is a Baire space and on the notion of a 2-pseudocompact space. We also prove that every ω-bounded (respectively, topologically periodic) Baire paratopological group admits a weaker Hausdorff group topology. In particular, ω-bounded (respectively, topologically periodic) 2-pseudocompact (so, also countably compact) paratopological groups enjoy this property. Some topological properties turning countably compact topological semigroups into topological groups are presented and some open questions are posed.     

Über die Wiener-Eigenschaft bei projektiven Limites lokalkompakter Gruppen

Leptin posed in [1] the problem to determine the class [W] of locally compact groups G characterized by the following property: Every proper closed two-sided idealJ in the Banach-*-algebraL ¹(G) is annihilated by some nondegenerate continuous *-representation ofL ¹(G) in a Hilbert space. Our main result: A locally compact group G, which is representable as a projective limit of a system of factor groups G/k, k compact normal subgroups, lies in [W] if and only if all the G/k are in [W].     

On the Theorem of Maschke

It will be shown that the Theorem of Maschke can be carried over to certain classes of topological groups in the following way. Let A be a locally compact abelian strictly -divisible group, and let G be a compact totally disconnected -group of automorphisms of A, where is a set of prime numbers. If a G-admissible subgroup B of A is isolated as a direct summand, then it has a G-admissible complement in A.Translated from Matematicheskie Zametki, Vol. 4, No. 2, pp. 151–154, August, 1968.     

Locally minimal topological groups 2

We continue in this paper the study of locally minimal groups started in Außenhofer et al. (2010) [4]. The minimality criterion for dense subgroups of compact groups is extended to local minimality. Using this criterion we characterize the compact abelian groups containing dense countable locally minimal subgroups, as well as those containing dense locally minimal subgroups of countable free-rank. We also characterize the compact abelian groups whose torsion part is dense and locally minimal. We call a topological group G almost minimal if it has a closed, minimal normal subgroup N such that the quotient group G/N is uniformly free from small subgroups. The class of almost minimal groups includes all locally compact groups, and is contained in the class of locally minimal groups. On the other hand, we provide examples of countable precompact metrizable locally minimal groups which are not almost minimal. Some other significant properties of this new class are obtained.     

Final group topologies,Kac-Moody groups and Pontryagin duality

We study final group topologies and their relations to compactness properties. In particular, we are interested in situations where a colimit or direct limit is locally compact, a k _ω-space, or locally k _ω. As a first application, we show that unitary forms of complex Kac-Moody groups can be described as the colimit of an amalgam of subgroups (in the category of Hausdorff topological groups, and the category of k _ω-groups). Our second application concerns Pontryagin duality theory for the classes of almost metrizable topological abelian groups, resp., locally k _ω topological abelian groups, which are dual to each other. In particular, we explore the relations between countable projective limits of almost metrizable abelian groups and countable direct limits of locally k _ω abelian groups.     

Locally minimal topological groups 1

The aim of this paper is to go deeper into the study of local minimality and its connection to some naturally related properties. A Hausdorff topological group (G,τ) is called locally minimal if there exists a neighborhood U of 0 in τ such that U fails to be a neighborhood of zero in any Hausdorff group topology on G which is strictly coarser than τ. Examples of locally minimal groups are all subgroups of Banach-Lie groups, all locally compact groups and all minimal groups. Motivated by the fact that locally compact NSS groups are Lie groups, we study the connection between local minimality and the NSS property, establishing that under certain conditions, locally minimal NSS groups are metrizable. A symmetric subset of an abelian group containing zero is said to be a GTG set if it generates a group topology in an analogous way as convex and symmetric subsets are unit balls for pseudonorms on a vector space. We consider topological groups which have a neighborhood basis at zero consisting of GTG sets. Examples of these locally GTG groups are: locally pseudoconvex spaces, groups uniformly free from small subgroups (UFSS groups) and locally compact abelian groups. The precise relation between these classes of groups is obtained: a topological abelian group is UFSS if and only if it is locally minimal, locally GTG and NSS. We develop a universal construction of GTG sets in arbitrary non-discrete metric abelian groups, that generates a strictly finer non-discrete UFSS topology and we characterize the metrizable abelian groups admitting a strictly finer non-discrete UFSS group topology. Unlike the minimal topologies, the locally minimal ones are always available on “large” groups. To support this line, we prove that a bounded abelian group G admits a non-discrete locally minimal and locally GTG group topology iff |G|?c.     

Non-commutative Clarkson Inequalities for n-Tuples of Operators

Let A ₀, ... , A _n−1 be operators on a separable complex Hilbert space , and let α₀,..., α _n−1 be positive real numbers such that 1. We prove that for every unitarily invariant norm,

三个或多个素变数的线性方程

设G是有限群,p是|G|的一个素因子,P是G的一个Sylow p-子群.若下列条件之一满足,则G是p-幂零:(1)P的极大子群均在G中S-半正规且(|G|,p-1)=1;(2)P的二次极大子群均在G中S-半正规且(|G|,p2-1)=1.