### Browsing by Subject "Topology"

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- ItemOpen AccessAn approach to coincidence theory through universal covering spaces(1973) Harvey, Duncan Reginald Arthur; Schlagbauer, HThe close relationship between the theory of fixed points and the theory of coincidences of maps is well known. This presentation is aimed at recording one of the less well documented approaches to fixed point theory as extended to the more general situation of coincidences. The approach referred to is that by way of the Universal Covering Spaces. The existing theory of coincidences is geometrically well realised in this setting and after some consideration, the necessary extensions and generalizations of the techniques as utilized in fixed point theory lead to an appealing conceptual notion of "essentiality of coincidence classes". Many hints have been made in the literature (see [1] and "On the sharpness of the Δ₂ and Δ₁ Nielsen Numbers" by Robin Brooks, J.Reine Angew. Math. 259, (1973), 101-108.) that lifts of mappings and the theory of fibres and related topics lend themselves to coincidence theory. It is the intention of this presentation to follow some of the basic properties through this approach and to show, wherever it is thought desirable, the ties between this and two of the existing approaches - for example, in the definition of the Nielsen Number, which is fundamental to both fixed point theory and coincidence theory.
- ItemOpen AccessA categorial study of initiality in uniform topology(1971) Brümmer, Guillaume C L; Hardie, K AThis thesis consists of two chapters, of which the first presents a categorial study of the concept of initiality (also known as projective generation) and the second gives applications in the theory of uniform and quasi-uniform spaces.
- ItemOpen AccessCountable inductive limits(1972) Martens, Eric; Webb, John HInductive systems and inductive limits have by now become fairly well established in the general theory of topological vector spaces. It is a branch of Functional Analysis which is receiving a reasonable amount of attention by modern mathematicians. It is of course a very interesting subject of its own accord, but is also useful in solving problems and proving theorems which one does not suspect are intimately related to it. As an example we can consider the proof of the non-existence of a countably infinite dimensional metrisable barrelled space.
- ItemOpen AccessInterior algebras and topology(1990) Naturman, Colin Ashley; Rose, HenryIn this thesis connections between categories of interior algebras and categories of topological spaces, and generalizations of topological concepts to interior algebras, are investigated. The following are some of the most significant results we obtain: The establishment of a duality between topological spaces and complete atomic interior algebras formalized in terms of a category-theoretic co-equivalence between the category of topological spaces and continuous maps and the category of complete atomic interior algebras and maps known as complete topomorphisms (Theorem 2.1.7). Under this co-equivalence, continuous open maps correspond to complete homomorphisms (Theorem 2.1.8). We also establish a duality between arbitrary interior algebras and structures known as Stone fields in terms of a co-equivalence between the category of interior algebras and topomorphisms (see Definition 1.1.8) and the category of Stone fields and their morphisms the field maps (Theorem 2.2.14). Under this co-equivalence weakly open field maps (see Definition 2.2.17) correspond to homomorphisms (Theorem 2.2.18). The well-known connection between pre-ordered sets and interior algebras is shown to be a special case of topological duality (see section 4 of chapter 2). The topological concepts of neighbourhoods, convergence and accumulation are generalized to interior algebras (Chapter 3), and are used to generalize the topological separation and compactness properties to interior algebras (Chapter 4 and Chapter 5). What is particularly interesting with regard to the separation properties is that most of them are first order properties of interior algebras (see Theorem 4.5.11). This should be contrasted with the situation for frames/locales [12] and topological model theory [10]. By generalizing the concept of α-separation to interior algebras we obtain an ω chain of strictly elementary classes of interior algebras all of which have hereditarily undecidable first order theories (Theorem 4.3.14). Characterizations of irreducibility properties for interior algebras are also found. These properties (subdirect irreducibility, finite subdirect irreducibility, direct indecomposability, simplicity and semi-simplicity) can be characterized in many different ways. Characterizations in terms of open elements (fixed points of the interior operator) are found (Theorem 1.3.18 and Theorem 1.3.21) and these are used to obtain further characterizations. In particular a characterization in terms of topological properties of Stone spaces of interior algebras is obtained (Theorem 2.3.9). We also find characterizations of the irreducibility properties in the power set interior algebras of topological spaces (Theorem 2.1.15) and in interior algebras obtained from pre-ordered sets (Theorem 2.4.16). What is particularly striking is that the irreducibility properties correspond to very natural topological properties. (Other results characterizing or related to the irreducibility properties are 2.4.11, 2.4.17, 5.1.13, and 5.1.15). Bibliography: pages 134-135.
- ItemOpen AccessMetrization of ordered topological spaces(1974) Colquhoun, Alan; Salbany, Sergio de OrnelasIn 1969, Lutzer proved that a linearly ordered topological space with a Gδ-diagonal is metrizable. This appears to be the first work in the field of metrization of ordered topological spaces. Very little seems to have been done in this direction. This thesis is a study of the various conditions necessary for metrizability of such spaces. One of the earliest papers concerned with ordered topological spaces is that of Eilenberg. Since then, ordered spaces have been considered by various authors, but few considered the conditions under which such spaces would be metrizable. Bennet gave a characterization of metrizability for a linearly ordered topological space with a σ-point finite base. A linearly ordered topological space is a space for which the interval topology coincides with the original topology for the space. We investigate the metrizability of linearly ordered topological space satisfying certain covering properties, countability conditions on the base, certain conditions on the diagonal and spaces which admit a symmetric. We obtain four characterizations of metrizability for linearly ordered topological space in terms of some of the above notions.
- ItemOpen AccessThe pullback closure, perfect morphisms and completions(1995) Holgate, David; Brümmer, Guillaume C LClosure operations within objects of various categories have played an important role in the development of Categorical Topology. Notably they have been used to characterise epimorphisms and investigate cowellpoweredness in specific categories, to generalise Hausdorff separation through diagonal theorems, and to extend topological notions such as compactness of objects and perfectness of morphisms to abstract categories. The categorical theory of factorisation structures for families of morphisms which developed in the 1970's laid the foundation for an axiomatic theory of categorical closure operators. This theory drew together many endeavours involving closure operations, and was coalesced in [Dikranjan, Giuli 1987]. The literature on categorical closure operators continues to extend the theory as well as apply it to problems in Category Theory. Central to our thesis is a particular closure operator (in the sense of [Dikranjan, Giuli 1987]) which we name the "pullback closure operator". Its construction is not entirely new, but no author has studied this operator in its own right. We investigate some of the operator's properties, present several examples and then apply it in two areas of Categorical Topology. First we use the pullback closure operator to establish links between two previously disjoint theories of perfect morphisms. One theory, which developed in the 1970's, exploits the orthogonality properties and functor related properties of perfect continuous maps. Another theory, which has developed more recently, generalises the closure and compactness properties of perfect continuous maps. (We should note that this does not include the recent work in [Clementino, Giuli, Tholen 1995] which takes another approach to perfect morphisms via closure operators.) Our investigations centre around finding conditions that are sufficient to ensure that the links between these two theories can be utilised. Our second use of the pullback closure operator is in pursuing the precategorical ideas expressed in [Birkhoff 1937], and some developments of these ideas in [Brummer, Giuli, Herrlich 1992] and [Brummer, Giuli 1992], to build a theory of completion of objects in an abstract category. In this context the pullback closure operator is shown to be appropriate in characterising complete objects, illuminating links with previously studied completion notions and describing epimorphisms in the category in which we are working. (In fact the pullback closure operator can be used to describe epimorphisms in even wider contexts.) Our methodology is what has been termed colloquially as "doing topology in categories". Topological notions and results are expressed in the language of category theory. Using these reformulations, new results are pursued at the level of categories, and are then applied in specific topological or algebraic contexts. Within this, our approach has been to make as few global assumptions as possible. The pullback closure operator is strictly a tool, in the sense that when assumptions are made, they concern the underlying categories, functors and classes of morphisms and objects and not the operator itself.
- ItemOpen AccessQuasireflections and quasifactorizations(1996) Henning, Peter; Brümmer, Guillaume C L; Bargenda, Hubertus WThe study of reflections in abstract category theory is widespread, and has often been used to study the concrete notion of "completion of an object" that occurs in. many fields of Mathematics, such as the Cech-Stone compactification of a Tychonoff space ([Cech 37]) or the completion of a uniform space ([Weil 38]). More recent work relating reflections to completions was published by Brummer and Giuli [Brummer Giuli 92], and in this thesis many of their ideas are extended to the more general setting of quasireflections (Bargenda 94]. In particular, one would like to view the well-known concept of an injective hull as a "completion", and this can be accomplished via a Galois correspondence between such hulls on one hand, and quasireflections on the other. Thus the theory of completion of objects can be extended to include many widely studied and significant examples, the most paradigmatic of which is the Mac Neille completion of a partially ordered set [Mac Neille 37]. These ideas are presented in chapters 1 and 2 of the present thesis. Further, the widely accepted characterization of factorization structures for sources in terms of certain colimits (pushouts and cointersections) was successfully extended to a characterization of factorization structures relative to a subcategory in the PhD thesis of Vaclav Vajner ([Vajner 94]). In chapter 3 of this thesis, the characterization is further generalized to include quasifactorization structures relative to a subcategory. This result relates to the results of chapters 1 and 2 via an important result of Bargenda's, which proves a Galois correspondence between quasireflective subcategories and relative quasifactorization structures (proposition 3.7).
- ItemOpen AccessThe strict typology : theory, generalizations and applications(1972) Diss, Gordon Fletcher; Webb, J HThe strict topology β was first defined on the space of bounded complex-valued continuous functions Cb(X), on a locally compact Hausdorff space X, by Buck. It was found to have many applications in Approximation Theory, spectral synthesis, spaces of bounded holomorphic functions and multipliers of Banach Algebras.
- ItemOpen Access(Strongly) zero-dimensional ordered spaces(1993) Nailana, Kwena Rufus; Brümmer, Guillaume C LThe relationship between transitive uniform spaces and zero-dimensional topological spaces was first established by Banaschewski [1957], and was later investigated by Levine [1969]. The theory of transitive quasi-uniform spaces is treated in [Fletcher and Lindgren 1972], [Brummer 1984] and [Kiinzi 1990, 1992a, 1992b,1993]; a convenient presentation for our purpose is to be found in [Fletcher and Lindgren 1982]. After Reilly [1972] introduced the notion of zero-dimensionality in bitopological spaces, Birsan [1974] and Halpin [1974] studied the relationship between transitive quasi-uniform spaces and zero-dimensional bitopological spaces. In this thesis we define a notion of zero-dimensionality in ordered topological spaces and examine the relationship between transitive quasi-uniform spaces and zero-dimensional ordered topological spaces. To a large extent, our presentation is influenced by the situation in bitopological spaces (cf. [Halpin 1974] and [Birsan 1974]), and uses the commutative diagrams which occur in [Schauerte 1988] and [Brummer 1977, 1982]. We also study strongly zero-dimensional ordered topological spaces and their relation with functorial quasi-uniformities. In this respect, our results are influenced by those of [Fora 1984], [Banaschewski and Brummer 1990] and [Kiinzi 1990] for strongly zero-dimensional bitopological spaces.
- ItemOpen AccessStructured frames(1986) Frith, John L; Hardie, K AEhresmann in 1959 first articulated the view that a complete lattice with an appropriate distributivity property deserved to be studied as a generalized topological space in its own right. He called the lattice a local lattice. Here is the distributivity property: x ∧ Vxα = V(x∧xα). A map of local lattices should preserve finite meets and arbitrary joins (and hence top and bottom elements). Dowker and Papert introduced the term frame for a local lattice and extended many results of topology to frame theory. At the 1981 international conference on categorical algebra and topology at Cape Town University a suggestion was made that a study of "uniform frames" (whatever they might be) would be an appropriate and useful start to a project concerned with examining, from a lattice theoretical point of view, the many topological structures which have gained acceptance in the topologist's arsenal of useful tools. It was felt that many of the pre-requisites for such a study had been established, and in fact one of the themes of the conference was the growing role of lattice theory in topology. The suggestion was eagerly accepted, and this thesis is the result.
- ItemOpen AccessSyntopogenous structures and real-compactness(1972) Flax, Cyril Lee; Brümmer, Guillaume C LThe syntopogenous structures were introduced by Á. Császár. These are generalisations of classical continuity structures such as topologies, proximities and uniformities. In his book, Foundations of General Topology (1963) (Preceded by a French (1960) and a German (1963) edition), Császár treated many properties of syntopgenous structures. Among these properties were completeness and compactness, but not realcompactness. Our purpose was to extend the definition of realcompactness from uniformisable topologies to arbitrary syntopogenous structures and to produce a real compact reflection for arbitrary syntopogenous structures. We did not fully accomplish this purpose. We have, in fact, first defined a notion of quasirealcompactness for arbitrary syntopogenous structures. For uniformisable Hausdorff topologies, realcompactness implies quasirealcompactness; we could not prove or disprove the converse implication. Nevertheless, we were able to give a characterisation of realcompactness for a uniformisable Hausdorff topology in terms of quasirealcompactness of a certain induced proximity; moreover, we produced a double quasirealcompact reflection in the category of separated syntopogenous structures, and from this retrieved the classical Hewitt realcompact reflection.
- ItemOpen AccessTransitive quasi-uniform spaces(1974) Halpin, Michael Norman; Brümmer, Guillaume C LChapter 1 deals with basic properties of the category of quasi-uniform spaces and its full subcategory Qut of transitive quasi-uniform spaces. Chapter 2 concerns Fletcher's construction. We extend the class of covers to which this construction may be applied and study the functoriality of the construction. The major result is that every right inverse of the forgetful functor Qut--->Top is obtainable by the extended Fletcher construction. In Chapter 3 we characterize pairwise zero dimensional bitopological spaces as those admitting transitive quasi-uniformities. An initiality characterization of pairwise zero dimensional bitopological spaces suggested by Brümmer leads to a description of the coarsest right inverse of the forgetful functor. In Chapter 4 we discuss countably based transitive quasi-uniformities, in that they relate to quasi-metrization. We elaborate on a result of Fletcher and Lindgren (1972) and obtain a bitopological analogue. In Chapter 5 we bring together a number of topics which relate to our previous chapters and point to further questions.
- ItemOpen AccessA Wyler-type approach to categorical topology(1989) Vajner, Václav; Bargenda, Hubertus WChapter 0 contains a summary of well-known terminology which will subsequently be used in the thesis. In Chapter 1 we begin by describing how topological categories may be viewed as categories of models corresponding to theories into the category of complete lattices. This leads naturally to the study of categories of T-models corresponding to theories into categories other than the category of complete lattices. It is shown, for example, that a concrete category corresponds to a poset-valued theory just in the case that it is (co)fibration complete. This shows that a concrete category is of the form Mod(T) only for poset-valued theories T. We make some technical observations regarding the correspondence between transformations and concrete functors. In particular, the fact that natural transformations between theories are in a bijective correspondence to finality preserving concrete functors between their respective categories of models will be of importance in Chapter 2. A theoretic interpretation is given of those categories which are (co)reflective modifications of certain concrete categories. Chapter 2 deals with the theoretic interpretation of certain topological completions of concrete categories. These are described in abstract theoretic terms using the correspondence between transformations and concrete functors. We also consider how concrete categories are embedded into (co)fibration complete categories. These "weak" completions have the nice property that they are always legitimate. For an arbitrary concrete category, the relationship between its topological completions and the various order-theoretic completions of its fibres is rather weak. However, if one assumes some additional structure properties, such as (co)fibration completeness, then the concepts of a categorical completion and an order-theoretic completion are more closely related, as shown by the result that for certain kinds of cofibrations, taking the universal order-theoretic completion of each fibre even yields the universal final topological completion. Chapter 3 is entirely concerned with the main goal of this thesis. We study so-called "convenient" topological categories, i.e., topological categories with additional structure. The purpose is to characterise each such type of category as a category of T-models for some theory T which satisfies a special "preservation" property with respect to pullbacks. The cartesian closed topological categories are characterised as those categories of T-models where the associated theory T sends a pointwise pullback of any regular sink into product covering family of diagrams. The concretely cartesian closed topological categories are characterised as those for which the associated theory T sends the pointwise pullback of an arbitrary sink into a product covering family. We also characterise the concretely cartesian closed categories by means of a certain natural transformation, given by the product of two structures. Perhaps the most satisfactory result of this Chapter is the characterisation of the universally topological categories. The theories corresponding to these categories may be described in two ways : firstly, they are shown to be frame-valued, send pullbacks into covering diagrams, and send morphisms into downset-preserving, cover-reflecting maps; secondly, they are shown to send the pointwise pullback of any sink into an order-covering diagram. Similarly, the concrete quasitopoi may be characterised by those theories which send the pullback of any regular sink into an order-covering family of diagrams. Finally, we consider hereditary topological categories. These are characterised as categories of T-models for which the theory T preserves terminal objects and sends the pointwise pullback of an arbitrary sink along an embedding into a weakly covering diagram family. In this context, a notion of strong heredity is introduced and characterised by a frame-valued theory sending pullbacks along monomorphisms into order-covering diagrams.