A Product of Compact Normal Spaces Is Normal
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Nearly Metacompact Spaces
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Topology and its Applications 98 (1999) 191–201 Nearly metacompact spaces Elise Grabner a;∗, Gary Grabner a, Jerry E. Vaughan b a Department Mathematics, Slippery Rock University, Slippery Rock, PA 16057, USA b Department Mathematics, University of North Carolina at Greensboro, Greensboro, NC 27412, USA Received 28 May 1997; received in revised form 30 December 1997 Abstract A space X is called nearly metacompact (meta-Lindelöf) provided that if U is an open cover of X then there is a dense set D ⊆ X and an open refinement V of U that is point-finite (point-countable) on D: We show that countably compact, nearly meta-Lindelöf T3-spaces are compact. That nearly metacompact radial spaces are meta-Lindelöf. Every space can be embedded as a closed subspace of a nearly metacompact space. We give an example of a countably compact, nearly meta-Lindelöf T2-space that is not compact and a nearly metacompact T2-space which is not irreducible. 1999 Elsevier Science B.V. All rights reserved. Keywords: Metacompact; Nearly metacompact; Irreducible; Countably compact; Radial AMS classification: Primary 54D20, Secondary 54A20; 54D30 A space X is called nearly metacompact (meta-Lindelöf) provided that if U is an open cover of X then there is a dense set D ⊆ X and an open refinement V of U such that Vx DfV 2 V: x 2 V g is finite (countable) for all x 2 D. The class of nearly metacompact spaces was introduced in [8] as a device for constructing a variety of interesting examples of non orthocompact spaces. -
Or Dense in Itself)Ifx Has No Isolated Points
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 131, Number 11, Pages 3607{3616 S 0002-9939(03)06660-7 Article electronically published on February 24, 2003 TYCHONOFF EXPANSIONS BY INDEPENDENT FAMILIES WANJUN HU (Communicated by Alan Dow) Abstract. A method for Tychonoff expansions using independent families is introduced. Using this method we prove that every countable Tychonoff space which admits a partition into infinitely many open-hereditarily irre- solvable dense subspaces has a Tychonoff expansion that is !-resolvable but not strongly extraresolvable. We also show that, under Luzin's Hypothesis ! ! (2 1 =2 ), there exists an !-resolvable Tychonoff space of size !1 which is not maximally resolvable. Introduction A topological space X is called crowded (or dense in itself)ifX has no isolated points. E. Hewitt defined a resolvable space as a space which has two disjoint dense subsets (see [17]). A space is called κ-resolvable [3] if it admits κ-many pairwise disjoint dense sets. The cardinal ∆(X):=minfjUj : U is a non-empty open subset of Xg is called the dispersion character of the space X.ThespaceX is called maximally resolvable [2] if it is ∆(X)-resolvable; if every crowded subspace of X is not resolvable, then X is called hereditarily irresolvable (HI). Analogously, X is open-hereditarily irresolvable (or OHI) if every non-empty open subset of X is irresolvable. V.I. Malykhin introduced a similar concept of extraresolvable (see [20] + and [6]): a space is extraresolvable if there exists a family fDα : α<∆(X) g of dense susbets of X such that Dα \ Dβ is nowhere dense in X for distinct α and β. -
A Guide to Topology
i i “topguide” — 2010/12/8 — 17:36 — page i — #1 i i A Guide to Topology i i i i i i “topguide” — 2011/2/15 — 16:42 — page ii — #2 i i c 2009 by The Mathematical Association of America (Incorporated) Library of Congress Catalog Card Number 2009929077 Print Edition ISBN 978-0-88385-346-7 Electronic Edition ISBN 978-0-88385-917-9 Printed in the United States of America Current Printing (last digit): 10987654321 i i i i i i “topguide” — 2010/12/8 — 17:36 — page iii — #3 i i The Dolciani Mathematical Expositions NUMBER FORTY MAA Guides # 4 A Guide to Topology Steven G. Krantz Washington University, St. Louis ® Published and Distributed by The Mathematical Association of America i i i i i i “topguide” — 2010/12/8 — 17:36 — page iv — #4 i i DOLCIANI MATHEMATICAL EXPOSITIONS Committee on Books Paul Zorn, Chair Dolciani Mathematical Expositions Editorial Board Underwood Dudley, Editor Jeremy S. Case Rosalie A. Dance Tevian Dray Patricia B. Humphrey Virginia E. Knight Mark A. Peterson Jonathan Rogness Thomas Q. Sibley Joe Alyn Stickles i i i i i i “topguide” — 2010/12/8 — 17:36 — page v — #5 i i The DOLCIANI MATHEMATICAL EXPOSITIONS series of the Mathematical Association of America was established through a generous gift to the Association from Mary P. Dolciani, Professor of Mathematics at Hunter College of the City Uni- versity of New York. In making the gift, Professor Dolciani, herself an exceptionally talented and successfulexpositor of mathematics, had the purpose of furthering the ideal of excellence in mathematical exposition. -
Several Results on Compact Metrizable Spaces in ZF
Monatshefte für Mathematik (2021) 196:67–102 https://doi.org/10.1007/s00605-021-01582-0 Several results on compact metrizable spaces in ZF Kyriakos Keremedis1 · Eleftherios Tachtsis2 · Eliza Wajch3 Received: 1 October 2020 / Accepted: 10 June 2021 / Published online: 29 June 2021 © The Author(s) 2021 Abstract In the absence of the axiom of choice, the set-theoretic status of many natural state- ments about metrizable compact spaces is investigated. Some of the statements are provable in ZF, some are shown to be independent of ZF. For independence results, distinct models of ZF and permutation models of ZFA with transfer theorems of Pin- cus are applied. New symmetric models of ZF are constructed in each of which the power set of R is well-orderable, the Continuum Hypothesis is satisfied but a denumer- able family of non-empty finite sets can fail to have a choice function, and a compact metrizable space need not be embeddable into the Tychonoff cube [0, 1]R. Keywords Weak forms of the Axiom of Choice · Metrizable space · Totally bounded metric · Compact space · Permutation model · Symmetric model Mathematics Subject Classification 03E25 · 03E35 · 54A35 · 54E35 · 54D30 Communicated by S.-D. Friedman. B Eliza Wajch [email protected] Kyriakos Keremedis [email protected] Eleftherios Tachtsis [email protected] 1 Department of Mathematics, University of the Aegean, Karlovassi, Samos 83200, Greece 2 Department of Statistics and Actuarial-Financial Mathematics, University of the Aegean, Karlovassi, Samos 83200, Greece 3 Institute of Mathematics, Faculty of Exact and Natural Sciences, Siedlce University of Natural Sciences and Humanities, ul. 3 Maja 54, 08-110 Siedlce, Poland 123 68 K. -
On Dimension and Weight of a Local Contact Algebra
Filomat 32:15 (2018), 5481–5500 Published by Faculty of Sciences and Mathematics, https://doi.org/10.2298/FIL1815481D University of Nis,ˇ Serbia Available at: http://www.pmf.ni.ac.rs/filomat On Dimension and Weight of a Local Contact Algebra G. Dimova, E. Ivanova-Dimovaa, I. D ¨untschb aFaculty of Math. and Informatics, Sofia University, 5 J. Bourchier Blvd., 1164 Sofia, Bulgaria bCollege of Maths. and Informatics, Fujian Normal University, Fuzhou, China. Permanent address: Dept. of Computer Science, Brock University, St. Catharines, Canada Abstract. As proved in [16], there exists a duality Λt between the category HLC of locally compact Hausdorff spaces and continuous maps, and the category DHLC of complete local contact algebras and appropriate morphisms between them. In this paper, we introduce the notions of weight wa and of dimension dima of a local contact algebra, and we prove that if X is a locally compact Hausdorff space then t t w(X) = wa(Λ (X)), and if, in addition, X is normal, then dim(X) = dima(Λ (X)). 1. Introduction According to Stone’s famous duality theorem [43], the Boolean algebra CO(X) of all clopen (= closed and open) subsets of a zero-dimensional compact Hausdorff space X carries the whole information about the space X, i.e. the space X can be reconstructed from CO(X), up to homeomorphism. It is natural to ask whether the Boolean algebra RC(X) of all regular closed subsets of a compact Hausdorff space X carries the full information about the space X (see Example 2.5 below for RC(X)). -
MTH 304: General Topology Semester 2, 2017-2018
MTH 304: General Topology Semester 2, 2017-2018 Dr. Prahlad Vaidyanathan Contents I. Continuous Functions3 1. First Definitions................................3 2. Open Sets...................................4 3. Continuity by Open Sets...........................6 II. Topological Spaces8 1. Definition and Examples...........................8 2. Metric Spaces................................. 11 3. Basis for a topology.............................. 16 4. The Product Topology on X × Y ...................... 18 Q 5. The Product Topology on Xα ....................... 20 6. Closed Sets.................................. 22 7. Continuous Functions............................. 27 8. The Quotient Topology............................ 30 III.Properties of Topological Spaces 36 1. The Hausdorff property............................ 36 2. Connectedness................................. 37 3. Path Connectedness............................. 41 4. Local Connectedness............................. 44 5. Compactness................................. 46 6. Compact Subsets of Rn ............................ 50 7. Continuous Functions on Compact Sets................... 52 8. Compactness in Metric Spaces........................ 56 9. Local Compactness.............................. 59 IV.Separation Axioms 62 1. Regular Spaces................................ 62 2. Normal Spaces................................ 64 3. Tietze's extension Theorem......................... 67 4. Urysohn Metrization Theorem........................ 71 5. Imbedding of Manifolds.......................... -
Chapter 7 Separation Properties
Chapter VII Separation Axioms 1. Introduction “Separation” refers here to whether or not objects like points or disjoint closed sets can be enclosed in disjoint open sets; “separation properties” have nothing to do with the idea of “separated sets” that appeared in our discussion of connectedness in Chapter 5 in spite of the similarity of terminology.. We have already met some simple separation properties of spaces: the XßX!"and X # (Hausdorff) properties. In this chapter, we look at these and others in more depth. As “more separation” is added to spaces, they generally become nicer and nicer especially when “separation” is combined with other properties. For example, we will see that “enough separation” and “a nice base” guarantees that a space is metrizable. “Separation axioms” translates the German term Trennungsaxiome used in the older literature. Therefore the standard separation axioms were historically named XXXX!"#$, , , , and X %, each stronger than its predecessors in the list. Once these were common terminology, another separation axiom was discovered to be useful and “interpolated” into the list: XÞ"" It turns out that the X spaces (also called $$## Tychonoff spaces) are an extremely well-behaved class of spaces with some very nice properties. 2. The Basics Definition 2.1 A topological space \ is called a 1) X! space if, whenever BÁC−\, there either exists an open set Y with B−Y, CÂY or there exists an open set ZC−ZBÂZwith , 2) X" space if, whenever BÁC−\, there exists an open set Ywith B−YßCÂZ and there exists an open set ZBÂYßC−Zwith 3) XBÁC−\Y# space (or, Hausdorff space) if, whenever , there exist disjoint open sets and Z\ in such that B−YC−Z and . -
Some Results on Separate and Joint Continuity
SOME RESULTS ON SEPARATE AND JOINT CONTINUITY A. BARECHE AND A. BOUZIAD Abstract. Let f : X × K → R be a separately continuous function and C a countable collection of subsets of K. Following a result of Calbrix and Troallic, there is a residual set of points x ∈ X such that f is jointly continuous at each point of {x}×Q, where Q is the set of y ∈ K for which the collection C includes a basis of neighborhoods in K. The particular case when the factor K is second countable was recently extended by Moors and Kenderov to any Cech-completeˇ Lindel¨of space K and Lindel¨of α-favorable X, improving a generalization of Namioka’s theorem obtained by Talagrand. Moors proved the same result when K is a Lindel¨of p-space and X is conditionally σ-α-favorable space. Here we add new results of this sort when the factor X is σC(X)-β-defavorable and when the assumption “base of neighborhoods” in Calbrix-Troallic’s result is replaced by a type of countable completeness. The paper also provides further information about the class of Namioka spaces. 1. Introduction If K, X are topological spaces, a mapping f : X ×K → R is said to be separately continuous if for every x ∈ X and y ∈ K, the mappings f(x, .): K → R and f(., y): X → R are continuous, the reals being equipped with the usual topology. The spaces K and X satisfy the Namioka property N (X,K) if every separately continuous map f : X × K → R is (jointly) continuous at each point of a subset of X×K of the form R×K, where R is a dense subset of X [20]. -
On Pseudo-K-Spaces
Applied General Topology c Universidad Polit´ecnica de Valencia @ Volume 9, No. 2, 2008 pp. 177-184 On pseudo-k-spaces Annamaria Miranda Abstract. In this note a new class of topological spaces generalizing k-spaces, the pseudo-k-spaces, is introduced and investigated. Particu- lar attention is given to the study of products of such spaces, in analogy to what is already known about k-spaces and quasi-k-spaces. 2000 AMS Classification: 54D50, 54D99, 54B10, 54B15 Keywords: Quotient map, product space, locally compact space, (locally) pseudocompact space, pseudo-k-space. 1. Introduction The first example of two k-spaces whose cartesian product is not a k-space was given by Dowker (see [2]). So a natural question is when a k-space satisfies that its product with every k-spaces is also a k-space. In 1948 J.H.C. Whitehead proved that if X is a locally compact Hausdorff space then the cartesian product iX × g, where iX stands for the identity map on X, is a quotient map for every quotient map g. Using this result D.E. Cohen proved that if X is locally compact Hausdorff then X × Y is a k-space for every k-space Y (see Theorem 3.2 in [1]). Later the question was solved by Michael who showed that a k-space has this property iff it is a locally compact space (see [5]). A similar question, related to quasi-k-spaces, was answered by Sanchis (see [8]). Quasi-k-spaces were investigated by Nagata (see [7]) who showed that “a space X is a quasi-k-space (resp. -
Fixed Point Theory on Extension-Type Spaces and Essential Maps on Topological Spaces
FIXED POINT THEORY ON EXTENSION-TYPE SPACES AND ESSENTIAL MAPS ON TOPOLOGICAL SPACES DONAL O’REGAN Received 19 November 2003 We present several new fixed point results for admissible self-maps in extension-type spaces. We also discuss a continuation-type theorem for maps between topological spaces. 1. Introduction In Section 2, we begin by presenting most of the up-to-date results in the literature [3, 5, 6, 7, 8, 12] concerning fixed point theory in extension-type spaces. These results are then used to obtain a number of new fixed point theorems, one concerning approximate neighborhood extension spaces and another concerning inward-type maps in extension- type spaces. Our first result was motivated by ideas in [12] whereas the second result is based on an argument of Ben-El-Mechaiekh and Kryszewski [9]. Also in Section 2 we present a new continuation theorem for maps defined between Hausdorff topological spaces, and our theorem improves results in [3]. For the remainder of this section we present some definitions and known results which will be needed throughout this paper. Suppose X and Y are topological spaces. Given a class ᐄ of maps, ᐄ(X,Y) denotes the set of maps F : X → 2Y (nonempty subsets of Y) belonging to ᐄ,andᐄc the set of finite compositions of maps in ᐄ.Welet Ᏺ(ᐄ) = Z :FixF =∅∀F ∈ ᐄ(Z,Z) , (1.1) where FixF denotes the set of fixed points of F. The class Ꮽ of maps is defined by the following properties: (i) Ꮽ contains the class Ꮿ of single-valued continuous functions; (ii) each F ∈ Ꮽc is upper semicontinuous and closed valued; n n n (iii) B ∈ Ᏺ(Ꮽc)foralln ∈{1,2,...};hereB ={x ∈ R : x≤1}. -
A Absolutely Countably Compact Space, 286 Accumulation Point, 81 of a Sequence of Sets, 80 Action, 218, 220, 222–228, 231
Index A B Absolutely countably compact space, 286 Baire property, 9, 69 Accumulation point, 81 Baire space, 9–11 of a sequence of sets, 80 Banach space, 80 Action, 218, 220, 222–228, 231, 234, 241, Base for the closed sets, 133 242 Base Tree Theorem, 274 Bernstein set, 158 algebraic, 220, 223, 234, 236 b f -group, 221, 224, 226–228 b f -action, 226 b f -space, 108, 122, 124, 125, 221, 222, 224, diagonal, 222 226–228, 231, 237 effective, 232 Birkhoff–Kakutani theorem, 39 faithful, 232 b-lattice, 164–167 free, 231 Bounded linearization of, 241 lattice, 164 trivial, 218, 229, 230, 240 neighborhood, 123 AD, see almost disjoint family open subset, 110 subset, 107–113, 115–119, 121–126, Additive group of the reals, 242 128–133, 136, 137, 139–142, 144–146, Admissible topology, 234 158, 221, 222 Alexandroff one-point compactification, 233 Boundedness, 111 Algebra of continuous functions, 221 Bounding number, 268 Almost compact space, 16, 17, 266 br -space, 108, 122, 124, 128, 129, 137 Almost disjoint family, 7, 8, 17, 28, 36, 195, 253–261, 266, 269, 271, 272, 280– 282, 284, 286 C Almost dominance with respect to a weak Canonical η-layered family, 263 selection, 280 Canonical open set in a GO-space, 170, 173 Almost metrizable topological group, 60 C-compact subset, 34, 36, 108, 115–117, Almost periodic point, 242 126–129, 137, 140, 184 ˇ Almost periodic subset, 242 Cech-complete space, 174–176, 181 Cech-completeˇ topological group, 181 Almost pseudo-ω-bounded space, 19, 20 Cellular family, 19 Almost P-space, 176 Cellularity, 49 (α, ) D -pseudocompact space, 77, 78, 85 C-embedded subset, 2–4, 15, 16, 18, 26, 110, Arcwise connected space, 184 122, 225 Arzelà-Ascoli theorem, 108, 239, 240 C∗-embedded subset, 2, 13, 17, 24, 25 Ascoli theorem, 80 Centered family, 275 © Springer International Publishing AG, part of Springer Nature 2018 291 M. -
D-18 Theˇcech–Stone Compactifications of N and R
d-18 The Cech–Stoneˇ compactifications of N and R 213 d-18 The Cech–Stoneˇ Compactifications of N and R c It is safe to say that among the Cech–Stoneˇ compactifica- The proof that βN has cardinality 2 employs an in- tions of individual spaces, that of the space N of natural dependent family, which we define on the countable set numbers is the most widely studied. A good candidate for {hn, si: n ∈ N, s ⊆ P(n)}. For every subset x of N put second place is βR. This article highlights some of the most Ix = {hn, si: x ∩ n ∈ s}. Now if F and G are finite dis- striking properties of both compactifications. T S joint subsets of P(N) then x∈F Ix \ y∈G Iy is non-empty – this is what independent means. Sending u ∈ βN to the P( ) ∗ point pu ∈ N 2, defined by pu(x) = 1 iff Ix ∈ u, gives us a 1. Description of βN and N c continuous map from βN onto 2. Thus the existence of an independent family of size c easily implies the well-known The space β (aka βω) appeared anonymously in [10] as an N fact that the Cantor cube c2 is separable; conversely, if D is example of a compact Hausdorff space without non-trivial c dense in 2 then setting Iα = {d ∈ D: dα = 0} defines an converging sequences. The construction went as follows: for ∗ ∈ independent family on D. Any closed subset F of N such every x (0, 1) let 0.a1(x)a2(x)..