×

A categorical review of complete regularity. (English) Zbl 1523.54029

For a set \(X\), we denote by \(UX\) the set of ultrafilters on \(X\). For a map \(f:X \rightarrow Y\), the map \(Uf: UX \rightarrow UY\) sends \(x\in UX\) to its image under \(f\), defined by \[B\in Uf(x)\Leftrightarrow f^{-1}(B)\in x,\] for all \(B\subseteq Y\).
Definition 2.11. Denoting its reflection into \(U\)-ASpa by \(\beta_{X}:X\rightarrow BX\) (with \(B\) to be read as capital Beta), we say that a \(U\)-space \(X\) satisfies condition
(C)
if \(X\) carries the cartesian (= initial = weak) structure with respect to \(\beta_{X}\); that is, if for all \(x\in UX, y\in X\), one has \(x\rightsquigarrow y\) in \(X\) whenever \(U\beta_{X}(x)\rightsquigarrow \beta_{X}y\) in \(BX\);
(F)
if \(\beta_{X}\) is a monomorphism; that is: if \(\beta_{X}\) an injective map;
(CF)
if \(X\) satisfies (C) and (F).

In this paper the following results are proved:
Theorem 2.10. The category \(U\)-ASpa is reflective in \(U\)-RGph and, hence, also in \(U\)-Spa.
Theorem 2.12. Consider (a possible large or empty) family of monotone maps \(f_{i}:X \rightarrow Y_{i}(i\in I)\) of \(U\)-spaces with the given common domain \(X\).
(1)
Let \(X\) carry the weak (= initial) structure with respect to \((f_{i})_{i\in I}\). Then, if every \(Y_{i}\) satisfies (C), so does \(X\).
(2)
Let \((f_{i})_{i\in I}\) be point-separarating. Then, if every \(Y_{i}\) satisfies (F), so does \(X\).
(3)
\(U\)-CSpa is simultaneously epi- and mono-reflective in \(U\)-Spa, and \(U\)-FSpa is (regular epi)- reflective in \(U\)-Spa.

Theorem 4.7. \(T\)-\(U\)-ASpa(C) is reflective in \(T\)-\(U\)-Spa(C).
Theorem 4.11. Let \(f_{i} : X \rightarrow Y_{i} (i \in I)\) be (a possibly large) family of monotone morphisms of \(T\)-spaces with common domain \(X\). Then:
(1)
If \(X\) carries the initial structure respect to \((f_{i})_{i\in I}\) and the topological functor \(t\)-Spa(C)\(\rightarrow C\), and if every \(Y_{i}\) satisfies (C), then so does \(X\).
(2)
If \((f_{i})_{i\in I}\) is collectively monic, and if every \(Y_{i}\), satisfies (F), then so does \(X\).
(3)
\(T\)-CSpa(C) is simultancously epi-and mono-reflective in \(T\)-Spa(C).
(4)
\(T\)-FSpa(C) is (regular epi)-reflective in \(T\)-Spa(C).
(5)
\(T\)-CFSpa(C) is epi-reflective in \(T\)-Spa(C).

MSC:

54D15 Higher separation axioms (completely regular, normal, perfectly or collectionwise normal, etc.)
54D30 Compactness
54B30 Categorical methods in general topology
18A40 Adjoint functors (universal constructions, reflective subcategories, Kan extensions, etc.)
18C20 Eilenberg-Moore and Kleisli constructions for monads
18D30 Fibered categories
18F60 Categories of topological spaces and continuous mappings

References:

[1] J. Adámek, H. Herrlich and G.E. Strecker, Abstract and Concrete Categories: The Joy of Cats, John Wiley & Sons, New York, 1990. Republished in Reprints in Theory and Applications of Categories 17, 2006.
[2] J. Adámek and L. Sousa, D-ultrafilters and their monads, Advances in Mathematics 377:107486, 2020.
[3] M. Barr, Relational algebras, in: Lecture Notes in Mathematics 137, pp. 39-55, Springer-Verlag, Berlin-Heidelberg-New York, 1970. · Zbl 0204.33202
[4] R. Börger, Coproducts and ultrafilters, Journal of Pure and Applied Algebra 46 (1987), 35-47. · Zbl 0625.18001
[5] M Bukatin, R. Kopperman and S.G. Matthews, Partial metric spaces, American Mathematical Monthly 116:(8) (2009), 708-718. · Zbl 1229.54037
[6] A. Burroni, T -catégories (catégories dans un triple), Cahiers de Topologie et Géométrie Différentielle 12 (1971), 215-321. · Zbl 0246.18007
[7] G. Choquet, Convergences, Annales de l’Université de Grenoble. Nouvelle Série. Section Sciences Mathématiques et Physiques 23 (1948), 57-112. · Zbl 0031.28101
[8] M. M. Clementino and D. Hofmann, Topological features of lax algebras, Applied Categorical Structures 11 (2003), 267-286. · Zbl 1024.18003
[9] M. M. Clementino and W. Tholen, Metric, topology and multi category, Journal of Pure and Applied Algebra 179 (2003), 13-47. · Zbl 1015.18004
[10] M. M. Clementino, D. Hofmann, and W. Tholen, One setting for all: metric, topology, uniformity, approach structure, Applied Categorical Structures 12 (2004), 127-154. · Zbl 1051.18005
[11] R. C. Flagg and R. Kopperman, Continuity spaces: reconciling domains and metric spaces, Theoretical Computer Science 177 (1997), 111-138. · Zbl 0901.68109
[12] F. Hausdorff, Grundzüge der Mengenlehre, Veit, Leipzig, 1914. · JFM 45.0123.01
[13] H. Herrlich, Topologische Reflexionen und Coreflexionen, Lecture Notes in Math-ematics 78, Springer-Verlag, Berlin-Heidelberg-New York, 1968. · Zbl 0182.25302
[14] D. Hofmann, G. J. Seal and W. Tholen (editors), Monoidal Topology. A Cate-gorical Approach to Order, Metric, and Topology, Cambridge University Press, Cam-bridge, 2014. · Zbl 1297.18001
[15] S. H. Kamnitzer, Protoreflections, Relational Algebras and Topology, PhD thesis, University of Cape Town, Cape Town, 1974.
[16] R. Kopperman, All topologies come from generalized metrics, American Mathe-matical Monthly 95 (1988), 89-97. · Zbl 0653.54020
[17] H. Lai and W. Tholen, Quantale-valued topological spaces via closure and con-vergence, Topology and its Applications 230 (2017), 599-620. · Zbl 1376.54003
[18] F. W. Lawvere, Metric spaces, generalized logic, and closed categories, Rendiconti del Seminario Matematico e Fisico di Milano 43 (1973), 135-166. Republished in Reprints in Theory and Applications of Categories 1, 2002. · Zbl 0335.18006
[19] T. Leinster, Codensity and the ultrafilter monad, Theory and Applications of Categories 28:(13) (2013), 332-370. · Zbl 1273.18009
[20] R. Lowen, Approach Spaces: The Missing Link in the Topology-Uniformity-Metric Triad, Oxford University Press, Oxford, 1997. · Zbl 0891.54001
[21] S. Mac Lane, Categories for the Working Mathematician, Springer-Verlag, Berlin-Heidelbereg-New York, 1971; 2nd ed. 1994. · Zbl 0232.18001
[22] E. G. Manes, A Triple Miscellany, PhD thesis, Wesleyan University, Middletown CT, 1967.
[23] , A triple theoretic construction of compact algebras, in: Lecture Notes in Mathematics 80, pp. 91-118, Springer-Verlag, Berlin-Heidelberg-New York, 1969. [24] , Compact Hausdorff objects, General Topology and its Applications 4:(4) (1974), 341-360. · Zbl 0186.02901
[24] , Monads in Topology, Topology and its Applications 157 (2010), 961-989. · Zbl 1194.54016
[25] A. Möbus, Relational-Algebren, PhD thesis, Universität Düsseldorf, Düsseldorf, 1981. · Zbl 0516.18010
[26] R. J. Perry, Completely regular relational algebras, Cahiers de Topologie et Géométrie Différentielle Catégoriques 17:(2) (1976), 125-133. · Zbl 0384.18004
[27] G. J. Seal, Canonical and op-canonical lax algebras, Theory and Applications of Categories 14 (2005), 221-243. · Zbl 1088.18006
[28] W. Tholen, Semi-topological functors I, Journal of Pure and Applied Algebra 15 (1979), 53-73. · Zbl 0413.18001
[29] W. Tholen and L. Yeganeh, The comprehensive factorization of Burroni’s T -functors, Theory and Applications of Categories 36:(8) (2021), 206-249. · Zbl 1474.18006
[30] S. Willard, General Topology, Addison-Wesley, Reading MA, 1968.
[31] O. Wyler, Are there topoi in topology?, in: Lecture Notes in Mathematics 540, pp. 699-719, Springer-Verlag, Berlin-Heidelberg-New York, 1976. · Zbl 0354.54001
This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. In some cases that data have been complemented/enhanced by data from zbMATH Open. This attempts to reflect the references listed in the original paper as accurately as possible without claiming completeness or a perfect matching.