矩形拟正则半群及矩形群的半直积和圈积

给出了两个半群的半直积和圈积为矩形拟正则半群和矩形群的充要条件。     

On the Decidability of Iterated Semidirect Products with Applications to Complexity

The notion of hyperdecidability has been introduced by the firstauthor as a tool to prove decidability of semidirect productsof pseudovarieties of semigroups. In this paper we considersome stronger notions which lead to improved decidability resultsallowing us in turn to establish the decidability of some iteratedsemidirect products. Roughly speaking, the decidability of asemidirect product follows from a mild, commonly verified propertyof the first factor plus the stronger property for all the otherfactors. A key role in this study is played by intermediatefree semigroups (relatively free objects of expanded type lyingbetween relatively free and relatively free profinite objects).As an application of the main results, the decidability of theKrohn–Rhodes (group) complexity is shown to follow fromnon-algorithmic abstract properties likely to be satisfied bythe pseudovariety of all finite aperiodic semigroups, therebysuggesting a new approach to answer (affirmatively) the questionas to whether complexity is decidable. 1991 Mathematics SubjectClassification: primary 20M05, 20M07, 20M35; secondary 08B20.     

群伪簇与幂零半群伪簇的两种积

本文确定了群伪簇Ｇ与幂零半群伪簇的Ｎ的半直积Ｇ＊Ｎ,Ｎ＊Ｇ和Ｍａｌｃｅｖ积Ｇ＾－１Ｎ,Ｎ＾－１Ｇ。     

Implicit operations on the categories of universal algebras

We present a series of concepts and prove some results on implicit operations on the various categories of universal algebras. This generalizes the previous results for pseudovarieties, pseudouniversal classes of algebras, positively conditional pseudovarieties, and so on.     

Rank Gradient and p-gradient of Amalgamated Free Products and HNN Extensions

We calculate the rank gradient and p-gradient of free products with amalgamation over an amenable subgroup and HNN extensions with an amenable associated subgroup. The notion of cost is used to compute the rank gradient of amalgamated free products and HNN extensions. For the p-gradient the Kurosh subgroup theorems for amalgamated free products and HNN extensions will be used.     

Grothendieck''s problem on profinite completions and representations of groups

The paper is concerned with Grothendieck's problem on profinite completions of groups. The relationship of this problem to the representation theory of finitely generated groups and to the problem of arithmeticity of Platonov are treated.To Professor A. Grothendieck on the Occasion of his 60th Birthday     

Fréchet-Urysohn spaces in free topological groups

Let and be respectively the free topological group and the free Abelian topological group on a Tychonoff space . For every natural number we denote by () the subset of () consisting of all words of reduced length . It is well known that if a space is not discrete, then neither nor is Fréchet-Urysohn, and hence first countable. On the other hand, it is seen that both and are Fréchet-Urysohn for a paracompact Fréchet-Urysohn space . In this paper, we prove first that for a metrizable space , () is Fréchet-Urysohn if and only if the set of all non-isolated points of is compact and is Fréchet-Urysohn if and only if is compact or discrete. As applications, we characterize the metrizable space such that is Fréchet-Urysohn for each and is Fréchet-Urysohn for each except for . In addition, however, there is a first countable, and hence Fréchet-Urysohn subspace of () which is not contained in any (). We shall show that if such a space is first countable, then it has a special form in (). On the other hand, we give an example showing that if the space is Fréchet-Urysohn, then it need not have the form.

     

左Clifford拟正则半群的半直积和圈积

在半群不含单位元的情况下,给出了两个半群的半直积和圈积是左Clifford拟正则半群的充分必要条件.     

4p阶群及2p2阶群的自同构群

给出了4p阶群和2p^2阶群的自同构群的结构,这里P是奇素数。     

Almost periodic compactifications of group extensions

Let and be groups and let be an extension of by . Given a property of group compactifications, one can ask whether there exist compactifications and of N and K such that the universal -compactification of G is canonically isomorphic to an extension of by . We prove a theorem which gives necessary and sufficient conditions for this to occur for general properties and then apply this result to the almost periodic and weakly almost periodic compactifications of G.     

A Sufficient Condition for a Smash Product as a Transfinite Left Free Normalizing Extension and Its Application

In this paper, a sufficient condition is given under which the smash product A#H is a transfinite left free normalizing extension of an algebra A. Moreover, the result is applied to a skew semigroup ring, a skew group ring and the quantum group U _q (sl(2)) such that some properties are shown. Received April 14, 1997, Accepted August 10, 1998     

Functorial topologies and finite-index subgroups of abelian groups

In the general context of functorial topologies, we prove that in the lattice of all group topologies on an abelian group, the infimum between the Bohr topology and the natural topology is the profinite topology. The profinite topology and its connection to other functorial topologies is the main objective of the paper. We are particularly interested in the poset C(G) of all finite-index subgroups of an abelian group G, since it is a local base for the profinite topology of G. We describe various features of the poset C(G) (its cardinality, its cofinality, etc.) and we characterize the abelian groups G for which C(G)?{G} is cofinal in the poset of all subgroups of G ordered by inclusion. Finally, for pairs of functorial topologies T, S we define the equalizer E(T,S), which permits to describe relevant classes of abelian groups in terms of functorial topologies.     

Extending partial automorphisms and the profinite topology on free groups

A class of structures is said to have the extension property for partial automorphisms (EPPA) if, whenever and are structures in , finite, , and are partial automorphisms of extending to automorphisms of , then there exist a finite structure in and automorphisms of extending the . We will prove that some classes of structures have the EPPA and show the equivalence of these kinds of results with problems related with the profinite topology on free groups. In particular, we will give a generalisation of the theorem, due to Ribes and Zalesskii stating that a finite product of finitely generated subgroups is closed for this topology.

     

Classification and discrete cocompact subgroups of some 7-dimensional connected nilpotent groups

Summary For real connected nilpotent groups, 7 is the lowest dimension where there are infinitely many non-isomorphic groups, and also where some groups (indeed, uncountably many) have no discrete cocompact subgroups. In [21] one infinite family <InlineEquation ID=IE"1"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"2"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"3"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"4"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"5"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"6"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"7"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"8"><EquationSource Format="TEX"><![CDATA[$]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>\mathcal{G}$ of 7-dimensional groups was identified and classified. Discrete cocompact subgroups H were identified for some groups in $\mathcal{G}$ in [10], along with simple quotients of $C^{*}(\mathrm{H})$ and relevant flows $(\mathrm{H}_3,\mathbf{T}^3)$. In this paper, such H and attributes are determined for more groups in $\mathcal{G}$; in particular, the members of $\mathcal{G}$ that admit discrete cocompact subgroups are identified precisely. In achieving some of these results, we consider other known ways of classifying the groups in $\mathcal{G}$, and also the classification of the analogous family of complex groups.     

Self-similar groups in the sense of an iterated function system and their properties

The notion of self-similarity in the sense of iterated function system (IFS) for compact topological groups is given by ?. Koçak in Definition 3. In this work, first we give the definition of strong self-similar group in the sense of IFS. Then, we investigate the main properties of these groups. We also obtain the relations between profinite groups and strong self-similar groups in the sense of IFS. Finally, we construct some examples of these groups.     

Covering groups of nonconnected topological groups and 2-groups

We investigate the universal cover of a topological group that is not necessarily connected. Its existence as a topological group is governed by a Taylor cocycle, an obstruction in 3-cohomology. Alternatively, it always exists as a topological 2-group. The splitness of this 2-group is also governed by an obstruction in 3-cohomology, a Sinh cocycle. We give explicit formulas for both obstructions and show that they are equal.     

A problem in the theory of formations of finite groups

In 1968 Carter, Fisher, and Hawkes developed the method of extremal classes to study local formations and their critical groups in the class of finite solvable groups. In the present paper the problem of Shemetkov on the extension of this method to arbitrary finite groups is solved. Translated fromMatematicheskie Zametki, Vol. 62, No. 1, pp. 52–58, July, 1997. Translated by A. I. Shtern