Completely Regular Spaces
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Diagonal Cauchy Spaces D.C
BULL. AUSTRAL. MATH. SOC. 54E15, 54D10, 54D35, 54A20 VOL. 54 (1996) [255-265] DIAGONAL CAUCHY SPACES D.C. KENT AND G.D. RICHARDSON A diagonal condition is defined which internally characterises those Cauchy spaces which have topological completions. The T? diagonal Cauchy spaces allow both a finest and a coarsest T2 diagonal completion. The former is a completion functor, while the latter preserves uniformisability and has an extension property relative to ^-continuous maps. INTRODUCTION In 1954, Kowalsky [2] defined a diagonal axiom for convergence spaces subject to which every pretopological space is topological. In 1967, Cook and Fischer [1] gave a stronger version of the Kowalsky axiom relative to which a larger class of convergence spaces was shown to be topological. In [4], we showed that any convergence space satisfying the Cook-Fischer diagonal axiom is topological. In this paper, we introduce a diagonal axiom for Cauchy spaces which reduces to the Cook-Fischer axiom when the Cauchy space is complete. A diagonal Cauchy space is one which satisfies this axiom. We show that a Cauchy space is diagonal if and only if every Cauchy equivalence class contains a smallest Cauchy filter, and this smallest Cauchy filter has a base of open sets. Equivalently, a Cauchy space is diagonal if and only if it allows a diagonal (that is, topological) completion. The category of diagoaal Cauchy spaces and Cauchy continuous maps is shown to be a topological category. It is shown that a Ti diagonal Cauchy space has both a finest and a coarsest T2 diagonal completion. The "fine diagonal completion" determines a completion functor on the category of T2, diagonal Cauchy spaces. -
$ G $-Metrizable Spaces and the Images of Semi-Metric Spaces
Czechoslovak Mathematical Journal Ying Ge; Shou Lin g-metrizable spaces and the images of semi-metric spaces Czechoslovak Mathematical Journal, Vol. 57 (2007), No. 4, 1141–1149 Persistent URL: http://dml.cz/dmlcz/128231 Terms of use: © Institute of Mathematics AS CR, 2007 Institute of Mathematics of the Czech Academy of Sciences provides access to digitized documents strictly for personal use. Each copy of any part of this document must contain these Terms of use. This document has been digitized, optimized for electronic delivery and stamped with digital signature within the project DML-CZ: The Czech Digital Mathematics Library http://dml.cz Czechoslovak Mathematical Journal, 57 (132) (2007), 1141–1149 g-METRIZABLE SPACES AND THE IMAGES OF SEMI-METRIC SPACES Ying Ge, Jiangsu, Shou Lin, Fujian (Received November 8, 2005) Abstract. In this paper, we prove that a space X is a g-metrizable space if and only if X is a weak-open, π and σ-image of a semi-metric space, if and only if X is a strong sequence-covering, quotient, π and mssc-image of a semi-metric space, where “semi-metric” can not be replaced by “metric”. Keywords: g-metrizable spaces, sn-metrizable spaces, weak-open mappings, strong sequence-covering mappings, quotient mappings, π-mappings, σ-mappings, mssc-mappings MSC 2000 : 54C10, 54D55, 54E25, 54E35, 54E40 1. Introduction g-metrizable spaces as a generalization of metric spaces have many important properties [17]. To characterize g-metrizable spaces as certain images of metric spaces is an interesting question in the theory of generalized metric spaces, and many “nice” characterizations of g-metrizable spaces have been obtained ([6], [8], [7], [13], [18], [19]). -
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. -
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 . -
Generalized Solutions of Systems of Nonlinear Partial Differential Equations
Generalized solutions of systems of nonlinear partial differential equations by Jan Harm van der Walt Submitted in partial fulfilment of the requirements of the degree Philosophiae Doctor in the Department of Mathematics and Applied Mathematics in the Faculty of Natural and Agricultural Sciences University of Pretoria Pretoria February 2009 © University of Pretoria i DECLARATION I, the undersigned, hereby declare that the thesis submitted herewith for the degree Philosophiae Doctor to the University of Pretoria contains my own, independent work and has not been submitted for any degree at any other university. Name: Jan Harm van der Walt Date: February 2009 ii Title Generalized solutions of systems of nonlinear partial differential equations Name Jan Harm van der Walt Supervisor Prof E E Rosinger Co-supervisor Prof R Anguelov Department Mathematics and Applied Mathematics Degree Philosophiae Doctor Summary In this thesis, we establish a general and type independent theory for the existence and regularity of generalized solutions of large classes of systems of nonlinear partial differential equations (PDEs). In this regard, our point of departure is the Order Completion Method. The spaces of generalized functions to which the solutions of such systems of PDEs belong are constructed as the completions of suitable uniform convergence spaces of normal lower semi-continuous functions. It is shown that large classes of systems of nonlinear PDEs admit generalized solutions in the mentioned spaces of generalized functions. Furthermore, the gener- alized solutions that we construct satisfy a blanket regularity property, in the sense that such solutions may be assimilated with usual normal lower semi-continuous functions. These fundamental existence and regularity results are obtain as applica- tions of basic topological processes, namely, the completion of uniform convergence spaces, and elementary properties of real valued continuous functions. -
On Α-Normal and Β-Normal Spaces
Comment.Math.Univ.Carolin. 42,3 (2001)507–519 507 On α-normal and β-normal spaces A.V. Arhangel’skii, L. Ludwig Abstract. We define two natural normality type properties, α-normality and β-normality, and compare these notions to normality. A natural weakening of Jones Lemma immedi- ately leads to generalizations of some important results on normal spaces. We observe that every β-normal, pseudocompact space is countably compact, and show that if X is a dense subspace of a product of metrizable spaces, then X is normal if and only if X is β-normal. All hereditarily separable spaces are α-normal. A space is normal if and only if it is κ-normal and β-normal. Central results of the paper are contained in Sections 3 and 4. Several examples are given, including an example (identified by R.Z. Buzyakova) of an α-normal, κ-normal, and not β-normal space, which is, in fact, a pseudocompact topological group. We observe that under CH there exists a locally compact Hausdorff hereditarily α-normal non-normal space (Theorem 3.3). This example is related to the main result of Section 4, which is a version of the famous Katˇetov’s theorem on metrizability of a compactum the third power of which is hereditarily normal (Corollary 4.3). We also present a Tychonoff space X such that no dense subspace of X is α-normal (Section 3). Keywords: normal, α-normal, β-normal, κ-normal, weakly normal, extremally discon- nected, Cp(X), Lindel¨of, compact, pseudocompact, countably compact, hereditarily sep- arable, hereditarily α-normal, property wD, weakly perfect, first countable Classification: 54D15, 54D65, 54G20 §0. -
On Finest Unitary Extensions of Topological Monoids
Topol. Algebra Appl. 2015; 3:1–10 Research Article Open Access Boris G. Averbukh* On finest unitary extensions of topological monoids Abstract: We prove that the Wyler completion of the unitary Cauchy space on a given Hausdorff topological monoid consisting of the underlying set of this monoid and of the family of unitary Cauchy filters on it, is a T2-topological space and, in the commutative case, an abstract monoid containing the initial one. Keywords: topological monoid, Cauchy space, completion MSC: 22A15, 54D35, 54E15 DOI 10.1515/taa-2015-0001 Received September 11, 2014; accepted November 11, 2014. Introduction For an arbitrary Hausdorff topological monoid, the concept of a unitary Cauchy filter (for brevity, wecallthem C-filters) which was defined in [1], generalizes the notion of a fundamental sequence of reals. Itwasproved in [1] that the underlying set X of this monoid X endowed with the family of such filters forms a Cauchy space called a unitary Cauchy space over X. Its convergence structure defines a T 1 -topology on X which is also 3 2 said to be unitary. In this paper, we begin the consideration of unitary completions of this monoid, i.e. such its extensions where all its C-filters converge. We use some principal ideas of the theory of Cauchy spaces but almost donot use its results since stronger statements are true in our case. However, we adduce all necessary definitions to make the paper as self-contained as it is possible. Each Cauchy space has many completions which have been studied in papers of many authors (see [2– 11]). -
Locally Compact Spaces of Measures
LOCALLY COMPACT SPACES OF MEASURES NORMAN Y. LUTHER Abstract. Under varying conditions on the topological space X, the spaces of <r-smooth, T-smooth, and tight measures on X, respectively, are each shown to be locally compact in the weak topology if, and only if, X is compact. 1. Introduction. Using the methods and results of Varadarajan [5], we give necessary and sufficient conditions on the topological space X for the local compactness of the spaces of ff-smooth, r- smooth, and tight measures [resp., signed measures] on X, respec- tively, when endowed with the weak topology. For such spaces of signed measures, the finiteness of X is easily seen to be necessary and sufficient (Remark 4). For such spaces of (nonnegative) measures, we find that, under various conditions on X, the compactness of X is necessary and sufficient (Corollaries 3 and 4); in fact, it is the dividing line between when the space is locally compact and when it has no compact neighborhoods whatsoever (Theorems 1, 2, and 5). 2. Preliminaries and main results. Our definitions, notation, and terminology follow quite closely those in the paper of Varadarajan [5]. In particular, our topological definitions and terminology co- incide with those of Kelley [3]. X will denote a topological space and C(X) the space of all real-valued, bounded, continuous functions on X. We use £ to denote the class of all zero-sets on X; i.e., $ = {/-1(0); fEC(X)}. The class of all co-zero sets (i.e., complements of zero-sets) in X shall be denoted by U. -
M. KULA, T3 and T4-Objects in the Topological Category of Cauchy
Commun. Fac. Sci. Univ. Ank. Sér. A1 Math. Stat. Volume 66, Number 1, Pages 29—42 (2017) DOI: 10.1501/Commua1_0000000772 ISSN 1303—5991 T3 AND T4-OBJECTS IN THE TOPOLOGICAL CATEGORY OF CAUCHY SPACES MUAMMER KULA Abstract. There are various generalization of the usual topological T3 and T4 axioms to topological categories defined in [2] and [7]. [7] is shown that they lead to different T3 and T4 concepts, in general. In this paper, an explicit characterization of each of the separation properties T3 and T4 is given in the topological category of Cauchy spaces. Moreover, specific relationships that arise among the various Ti, i = 0, 1, 2, 3, 4, P reT2, and T2 structures are examined in this category. 1. Introduction In general topology and analysis, a Cauchy space is a generalization of metric spaces and uniform spaces for which the notion of Cauchy convergence still makes sense. When filters came into existence and uniform spaces were introduced, Cauchy filters appeared in topological theory as a generalization of Cauchy sequences. The theory of Cauchy spaces was initiated by H. J. Kowalsky [26]. Cauchy spaces were introduced by H. Keller [22] in 1968, as an axiomatic tool derived from the idea of a Cauchy filter in order to study completeness in topological spaces. In that paper the relation between Cauchy spaces, uniform convergence spaces, and convergence spaces was developed. In the completion theory of uniform convergence spaces and convergence vector spaces, Cauchy spaces play an essential role ([19], [25], [39]). This fact explain why most work on Cauchy spaces deals mainly with completions ([17], [18], [29]). -
18 | Compactification
18 | Compactification We have seen that compact Hausdorff spaces have several interesting properties that make this class of spaces especially important in topology. If we are working with a space X which is not compact we can ask if X can be embedded into some compact Hausdorff space Y . If such embedding exists we can identify X with a subspace of Y , and some arguments that work for compact Hausdorff spaces will still apply to X. This approach leads to the notion of a compactification of a space. Our goal in this chapter is to determine which spaces have compactifications. We will also show that compactifications of a given space X can be ordered, and we will look for the largest and smallest compactifications of X. 18.1 Proposition. Let X be a topological space. If there exists an embedding j : X → Y such that Y is a compact Hausdorff space then there exists an embedding j1 : X → Z such that Z is compact Hausdorff and j1(X) = Z. Proof. Assume that we have an embedding j : X → Y where Y is a compact Hausdorff space. Let j(X) be the closure of j(X) in Y . The space j(X) is compact (by Proposition 14.13) and Hausdorff, so we can take Z = j(X) and define j1 : X → Z by j1(x) = j(x) for all x ∈ X. 18.2 Definition. A space Z is a compactification of X if Z is compact Hausdorff and there exists an embedding j : X → Z such that j(X) = Z. 18.3 Corollary. -
Math 131: Introduction to Topology 1
Math 131: Introduction to Topology 1 Professor Denis Auroux Fall, 2019 Contents 9/4/2019 - Introduction, Metric Spaces, Basic Notions3 9/9/2019 - Topological Spaces, Bases9 9/11/2019 - Subspaces, Products, Continuity 15 9/16/2019 - Continuity, Homeomorphisms, Limit Points 21 9/18/2019 - Sequences, Limits, Products 26 9/23/2019 - More Product Topologies, Connectedness 32 9/25/2019 - Connectedness, Path Connectedness 37 9/30/2019 - Compactness 42 10/2/2019 - Compactness, Uncountability, Metric Spaces 45 10/7/2019 - Compactness, Limit Points, Sequences 49 10/9/2019 - Compactifications and Local Compactness 53 10/16/2019 - Countability, Separability, and Normal Spaces 57 10/21/2019 - Urysohn's Lemma and the Metrization Theorem 61 1 Please email Beckham Myers at [email protected] with any corrections, questions, or comments. Any mistakes or errors are mine. 10/23/2019 - Category Theory, Paths, Homotopy 64 10/28/2019 - The Fundamental Group(oid) 70 10/30/2019 - Covering Spaces, Path Lifting 75 11/4/2019 - Fundamental Group of the Circle, Quotients and Gluing 80 11/6/2019 - The Brouwer Fixed Point Theorem 85 11/11/2019 - Antipodes and the Borsuk-Ulam Theorem 88 11/13/2019 - Deformation Retracts and Homotopy Equivalence 91 11/18/2019 - Computing the Fundamental Group 95 11/20/2019 - Equivalence of Covering Spaces and the Universal Cover 99 11/25/2019 - Universal Covering Spaces, Free Groups 104 12/2/2019 - Seifert-Van Kampen Theorem, Final Examples 109 2 9/4/2019 - Introduction, Metric Spaces, Basic Notions The instructor for this course is Professor Denis Auroux. His email is [email protected] and his office is SC539.