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1995, Proceedings of the American Mathematical Society
AI
This paper proves that a reflexive topological group is locally compact if and only if the evaluation mapping from the product of the group with itself into the torus is continuous. This result builds on Arens' theorem regarding admissible topologies in continuous functions and addresses a question previously posed by Megrelishvili. The work emphasizes the role of convergence spaces in establishing this characterization and highlights the distinction of locally compact Hausdorff abelian groups within the broader category of reflexive groups.
Topology and its Applications, 2010
Let G be a topological group with the identity element e. Given a space X, we denote by Cp(X, G) the group of all continuous functions from X to G endowed with the topology of pointwise convergence, and we say that X is: (a) G-regular if, for each closed set F ⊆ X and every point x ∈ X \ F , there exist f ∈ Cp(X, G) and g ∈ G \ {e} such that f (x) = g and f (F ) ⊆ {e}; (b) G ⋆ -regular provided that there exists g ∈ G \ {e} such that, for each closed set F ⊆ X and every point x ∈ X \ F , one can find f ∈ Cp(X, G) with f (x) = g and f (F ) ⊆ {e}. Spaces X and Y are G-equivalent provided that the topological groups Cp(X, G) and Cp(Y, G) are topologically isomorphic.
This paper discusses algebraic and topological conditions that are consequences of a convergence group action.
Computers & Mathematics With Applications, 2011
A function f on a topological space is sequentially continuous at a point u if, given a sequence (x n ), lim x n = u implies that lim f (x n ) = f (u). This definition was modified by Connor and Grosse-Erdmann for real functions by replacing lim with an arbitrary linear functional G defined on a linear subspace of the vector space of all real sequences. In this paper, we extend this definition to a topological group X by replacing G, a linear functional, with an arbitrary additive function defined on a subgroup of the group of all X -valued sequences and not only give new theorems in this generalized setting but also present theorems that have not been obtained for real functions so far.
Journal of Mathematical Analysis and Applications, 2005
The Pontryagin-van Kampen (P-vK) duality, defined for topological Abelian groups, is given in terms of the compact-open topology. Polar reflexive spaces, introduced by Köthe, are those locally convex spaces satisfying duality when the dual space is equipped with the precompact-open topology. It is known that the additive groups of polar reflexive spaces satisfy P-vK duality. In this note we consider the duality of topological Abelian groups when the topology of the dual is the precompactopen topology. We characterize the precompact reflexive groups, i.e., topological groups satisfying the group duality defined in terms of the precompact-open topology. As a consequence, we obtain a new characterization of polar reflexive spaces. We also present an example of a space which satisfies P-vK duality and is not polar reflexive. Some of our results respond to questions appearing in the literature.
2006
In Section 1 we prove several classical isomorphism theorems for topological groups. Furthermore, we state sufficient criteria for a topological group to be isomorphic to an inner direct product. In order to do so, we will need an open mapping theorem for topological groups which yields that every surjective morphism between topological groups is open if the groups satisfy certain compactness properties. We proceed in Section 2 by analyzing the structure of certain locally compact groups based on their subgroups. Weil's Lemma consists of two structure results for locally compact Hausdorff groups G. In particular, for each g ∈ G the cyclic group 〈g〉 is either discrete and infinite or has compact closure in G. We continue by classifying certain Abelian topological groups as direct products of a free Abelian group with an open subgroup. Additionally, we state an existence criterion for discrete subgroups of locally compact Abelian Hausdorff groups. Finally, we give some results of purely algebraic nature. This treatise was prepared for the seminar "Locally Compact Groups" held by PD. Dr. Ralf Gramlich in August 2010 at TU Darmstadt. The seminar was structured according to Markus Stroppel's book [3]. Further resources are provided under http://www3.mathematik.tu-darmstadt.de/index.php?id=84&evsid=23&&evsver=880. Notation The mappings will be denoted as actions from the right. The image of a point x under a map f will be written x f. Composition of mappings transforms likewisely, i.e. x (f •g) := x f g. 1 Topological Aspects of the Isomorphism Theorems Definition 1.1 (Quotient Map). Let f : X → Y be a surjective map between topological spaces. We call f a quotient map, if it induces the quotient topology on Y. This means every subset U ⊂ Y is open if and only U f −1 ⊂ X is open. Lemma 1.2 (Universal Mapping Property of Quotient Maps). We consider maps between topological spaces h: X → Y and g : Y → Z with f := h• g. If h is a quotient map and f is continuous, then g is also continuous. Hence, we have the following situation: X h , 2 f 8 8 Y g / / Z Proof. Let O ⊂ Z be open. As f is continuous, O f −1 is open. By definition of the mappings O g −1 h −1 = O f −1. As h induces the quotient topology on Y and O g −1 h −1 is open, also O g −1 is open.
Transactions of the American Mathematical Society, 1967
The investigations leading to this paper were suggested by the papers of Hewitt [3] and Ross [5]. In [3] Hewitt was interested in proving that if an abelian group is locally compact in two topologies, one strictly stronger than the other, there is a character continuous in one topology and discontinuous in the other (actually a special case of a theorem of Kaplansky-see Theorem 1.1 of [2]). Actually Hewitt proved a stronger result. His arguments were based on the fact that both the additive group of reals, and the multiplicative group of complex numbers of absolute value 1 have the property that every stronger locally compact group topology is discrete. A natural question to ask is what other groups have this property. The answer is very simple (2.1 of this paper). We consider in the second section of this paper the obvious generalization. Namely which groups have the property that there are only finitely many stronger locally compact group topologies. The investigations in the first section of this paper were suggested by the paper of Ross [5]. Ross was considering the same question as Hewitt, and was led to consider the relationship between two locally compact group topologies on a group G such that G has the same closed subgroups in the two topologies. We investigate this further in the first section of this paper, and are able to say that many of the properties of O as a topological group can be recovered once we know the closed subgroups. 1. Closed subgroups of locally compact groups. We consider here what we can say about a locally compact group G once we know its closed subgroups. Throughout this section of the paper R will denote the additive group of real numbers, T will denote the multiplicative group of complex numbers of absolute value 1, and, except for 7? all our groups will be written multiplicatively. 1.1. Theorem. The closed subgroups of G determine the open subgroups ofG. Proof. In case G is abelian, the proof is very easy, for then a subgroup is open if and only if every subgroup containing it is closed. In the nonabelian case this does not characterize the open subgroups (every subgroup of 57(3, C) containing SU{3) is closed, and in fact is either SU{3) or SL{3, C), but SU{3) is not open). Thus for the general case we proceed by the following steps. (1) The closed subgroups of G determine the identity component C70 of G. In fact G0 is the intersection of the closed subgroups 77 with the property that for
Filomat, 2014
Let X be a first countable Hausdorff topological group. The limit of a sequence in X defines a function denoted by lim from the set of all convergent sequences to X. This notion has been modified by Connor and Grosse-Erdmann for real functions by replacing lim with an arbitrary linear functional G defined on a linear subspace of the vector space of all real sequences. Recently ?akall? has extended the concept to the topological group setting and introduced the concepts of G-sequential compactness, G-sequential continuity and sequential connectedness. In this paper we give a further investigation of G-sequential continuity in topological groups.
Glasgow Mathematical Journal, 2001
We introduce and study the type I-, II-, and III-L-complete continuity property of Banach spaces, where L is a subset of the dual group of a compact metrizable abelian group G.
Journal of Group Theory, 2000
We prove that every dense subgroup of a topological abelian group has the same 'convergence dual' as the whole group. By the 'convergence dual' we mean the character group endowed with the continuous convergence structure. We draw as a corollary that the continuous convergence structure on the character group of a precompact group is discrete and therefore a non-compact precompact group is never reflexive in the sense of convergence. We do not know if the same statement holds also for reflexivity in the sense of Pontryagin; at least in the category of metrizable abelian groups it does.
FOURTH INTERNATIONAL CONFERENCE OF MATHEMATICAL SCIENCES (ICMS 2020), 2021
It is well known that for a Hausdorff topological group X, the limits of convergent sequences in X define a function denoted by lim from the set of all convergent sequences in X to X. This notion has been modified by Connor and Grosse-Erdmann for real functions by replacing lim with an arbitrary linear functional G defined on a linear subspace of the vector space of all real sequences. Recently some authors have extended the concept to the topological group setting and introduced the concepts of Gcontinuity, G-compactness and G-connectedness. In this paper we prove some results about G-compactness for topological group with operations which include topological groups, topological rings without identity, R-modules, Lie algebras, Jordan algebras, and many others.
Motivated from , call a precompact group topology τ on an abelian group G ss-precompact (abbreviated from single sequence precompact) if there is a sequence u = (u n ) in G such that τ is the finest precompact group topology on G making u = (u n ) converge to zero. It is proved that a metrizable precompact abelian group (G, τ ) is ss-precompact iff it is countable. For every metrizable precompact group topology τ on a countably infinite abelian group G there exists a group topology η such that η is strictly finer than τ and the groups (G, τ ) and (G, η) have the same Pontryagin dual groups (in other words, (G, τ ) is not a Mackey group in the class of maximally almost periodic groups). We give a complete description of all ss-precompact abelian groups modulo countable ss-precompact groups from which we derive:
Axioms, 2015
For a locally quasi-convex topological abelian group (G, τ), we study the poset C (G, τ) of all locally quasi-convex topologies on G that are compatible with τ (i.e., have the same dual as (G, τ)) ordered by inclusion. Obviously, this poset has always a bottom element, namely the weak topology σ(G, G). Whether it has also a top element is an open question. We study both quantitative aspects of this poset (its size) and its qualitative aspects, e.g., its chains and anti-chains. Since we are mostly interested in estimates "from below", our strategy consists of finding appropriate subgroups H of G that are easier to handle and show that C (H) and C (G/H) are large and embed, as a poset, in C (G, τ). Important special results are: (i) if K is a compact subgroup of a locally quasi-convex group G, then C (G) and C (G/K) are quasi-isomorphic (3.15); (ii) if D is a discrete abelian group of infinite rank, then C (D) is quasi-isomorphic to the poset F D of filters on D (4.5). Combining both results, we prove that for an LCA (locally compact abelian) group G with an open subgroup of infinite co-rank (this class includes, among others, all non-σ-compact LCA groups), the poset C (G) is as big as the underlying topological structure of (G, τ) (and set theory) allows. For a metrizable connected compact group X, the group of null sequences G = c 0 (X) with the topology of uniform convergence is studied. We prove that C (G) is quasi-isomorphic to P(R) (6.9).
For an abelian locally compact group X let X ∧ p be the group of continuous homomorphisms from X into the unit circle T of the complex plane endowed with the pointwise convergence topology. It is proved that X is metrizable iff X ∧ p is K-analytic iff X endowed with its Bohr topology σ(X, X ∧) has countable tightness. Using this result, we establish a large class of topological groups with countable tightness which are not sequential, so neither Fréchet-Urysohn.
Journal of Pure and Applied Algebra, 2005
We prove that direct and inverse limits of sequences of reflexive Abelian groups that are metrizable or k -spaces, but not necessarily locally compact, are reflexive and dual of each other provided some extra conditions are satisfied by the sequences.
Open Problems in Topology II, 2007
2017
This article fits in the context of the approach to topological problems in terms of the underlying convergence space structures, and serves as yet another illustration of the power of the method. More specifically, we spell out convergence-theoretic characterizations of the notions of weak base, weakly first-countable space, semi-metrizable space, and symmetrizable spaces. With the help of the already established similar characterizations of the notions of Fr\'echet-Ursyohn, sequential, and accessibility spaces, we give a simple algebraic proof of a classical result regarding when a symmetrizable (respectively, weakly first-countable, respectively sequential) space is semi-metrizable (respectively first-countable, respectively Fr\'echet) that clarifies the situation for non-Hausdorff spaces. Using additionally known results on the commutation of the topologizer with product, we obtain simple algebraic proofs of various results of Y. Tanaka on the stability under product of ...
Bulletin of The Australian Mathematical Society, 1988
Using the Iwasawa structure theorem for connected locally compact Hausdorff groups we show that every locally compact Hausdorff group G is homeomorphic to R n x K x D, where n is a non-negative integer, if is a compact group and D is a discrete group. This makes recent results on cardinal numbers associated with the topology of locally compact groups more transparent. For abelian G, we note that the dual group, G, is homeomorphic to R n x K X D. This leads us to the relationship card G = U» 0 (G) + 2"o(<3) > w u e r e u> (respectively, wo ) denotes the weight (respectively local weight) of the topological group. From this classical results such as card G -1 c a a for compact Hausdorff abelian groups, and u>(G) = <*(G\ for general locally compact Hausdorff abelian groups are easily derived.
Revista de la Real Academia de Ciencias Exactas, Físicas y Naturales. Serie A. Matemáticas
We provide some characterizations of precompact abelian groups G whose dual group G ∧ p endowed with the pointwise convergence topology on elements of G contains a nontrivial convergent sequence. In the special case of precompact abelian torsion groups G, we characterize the existence of a nontrivial convergent sequence in G ∧ p by the following property of G: No infinite quotient group of G is countable. Finally, we present an example of a dense subgroup G of the compact metrizable group Z(2) ω such that G is of the first category in itself, has measure zero, but the dual group G ∧ p does not contain infinite compact subsets. This complements Theorem 1.6 in [J.E. Hart and K. Kunen, Limits in function spaces and compact groups, Topol. Appl. 151 (2005), 157-168]. As a consequence, we obtain an example of a precompact reflexive abelian group which is of the first Baire category.
Topology and its Applications, 2009
Topological group Cech-Stone compactification Epimorphism Monomorphism Epi-topology Compact-open topology Compact-zero topology Space with filter Frame Lattice-ordered group Pressing down Aronszajn tree We address questions of when (C(X), +) is a topological group in some topologies which are meets of systems of compact-open topologies from certain dense subsets of X. These topologies have arisen from the theory of epimorphisms in lattice-ordered groups (in this context called "epi-topology"). A basic necessary and sufficient condition is developed, which at least yields enough insight to provide the general answer "sometimes Yes and sometimes No". After some reduction the situation seems to become Set Theory (which view will be reinforced by a sequel to this paper "Topological group criterion for C (X) in compact-open-like topologies, II").
Applied general topology, 2005
We study the sequences of integers (un) that converge to 0 in some precompact group topology on Z and the properties of the finest topology with this property when (un) satisfies a linear recurrence relation with bounded coefficients. Some of the results are extended to the case of sequences in arbitrary Abelian groups.
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