CARTESIAN CLOSEDNESS AND THE MCNEILLE COMPLETION OF AN INITIALLY STRUCTURED CATEGORY

Abstract

An adaptation of a theorem by Herrlich [5] shows that every initially structured category A can be fully embedded in a topological category A_C, which is, in fact, a MacNeille completion of A. It is then shown that A is Cartesian closed if and only if A_C is.

Also developed is the notion of a Cartesian closed initially structured (CCIS) hull of a category. The theory of the CCIS hull is analogous to that of the Cartesian closed topological (CCT) hull. It is proved that a category has a CCT hull-if and only if it has a CCIS hull; and this allows the list of conditions equivalent to the existence of a CCT hull to be supplemented.

Examples are given, drawn mainly from the various categories of binary relations.     

CARTESIAN CLOSEDNESS,EXPONENTIALITY, AND FINAL HULLS IN PSEUDOTOPOLOGICAL SPACES

Abstract

We show that each non-trivial epireflective subcategory of the topological or pretopological spaces fails to be cartesian closed. Motivated by this “negative” result, we consider the supercategory of pseudotopological spaces and obtain: An epireflective subcategory of the pseudotopological spaces which contains a finite non-indiscrete space is cartesian closed iff it is closed with respect to powers in the pseudotopological spaces. Here the density property that every pseudotopological space is a final epi-sink of free ultraspaces is essential.     

TWO FAMILIES OF CARTESIAN CLOSED TOPOLOGICAL CATEGORIES

Abstract

In this paper two ordered families of topological categories are studied. The first family includes the category of all abstract simplicial complexes and the subcategories of all abstract simplicial complexes of dimension less than or equal to n. The categories of the second family are bireflective subcategories of the category of all bornological spaces. All these categories are cartesian closed and have other nice properties.     

CONCRETE CATEGORIES FOR WHICH FIBRE COMPLETIONS INDUCE TOPOLOGICAL COMPLETIONS

Abstract

Every topological category over an arbitrary base category X may be considered as a category of T-models with respect to some theory (i.e., functor) T from X into a category of complete lattices. Using this model-theoretic correspondence as our basic tool, we study initial and final completions of (co)fibration complete categories. For an arbitrary concrete category (A, U) over X, the process of order-theoretically completing each fibre does not usually yield an initial/final completion of (A, U). It is shown in this paper that for concrete categories which are assumed to be fibration and/or cofibration complete, initial and final completions can be constructed by completing the fibres. These completions are further shown to exhibit some interesting external properties.     

CARTESIAN CLOSED COREFLECTIVE HULLS

Conditions are established under which a given class of objects in a topological category will have a Cartesian closed coreflective hull. The main theorem is used to discover new Cartesian closed topological categories and to unify a diversity of known special results. It also provides a mild criterion for the existence of Cartesian closed topological hulls.     

POWERS AND EXPONENTIAL OBJECTS IN INITIALLY STRUCTURED CATEGORIES AND APPLICATIONS TO CATEGORIES OF LIMIT SPACES

Abstract

Generalizing results of Herrlich and Nel, the author characterizes by means of smallest proper structures those objects X of an initially structured category for which X x—has a right adjoint, and describes the corresponding function spaces. It is shown that reduction to finally and initially dense classes is possible. The results are applied to epireflective subcategories of the category of limit spaces containing a finite non-indiscrete space, in particular to epireflective subcategories of TOP.     

TOPOLOGICAL CATEGORIES AND CLOSURE OPERATORS

Abstract

It is shown that the category CS of closure spaces is a topological category. For each epireflective subcategory A of a topological category X a functor F _A :X → X is defined and used to extend to the general case of topological categories some results given in [4], [5] and [10] for epireflective subcategories of the category Top of topological spaces.     

NILPOTENCY,SOLVABILITY AND RADICALS IN CATEGORIES

Abstract

Nilpotent and solvable ideals are defined and investigated in categories. The relation between the prime radical and the sum of the solvable ideals (which is also a radical) is discussed in categories. For example: If an object satisfies the maximal condition for ideals, then the prime radical is equal to the sum of the solvable ideals. Certain generalizations of theorems in rings, groups, Lie algebras, etc. are also proven, for example: An ideal α: I → A is semiprime if and only if A/I contains no non-zero nilpotent ideals.     

NETS AND GRAPHS AS PINAL COMPLETIONS AND THE RELATIONS BETWEEN THEM

Abstract

Nets and graphs, both used in Computer Science, are studied from a categorical point of view. It is shown that they may be constructed via final completions of very simple small concrete categories and that their nice properties, namely to form topological categories which are quasitopoi with concrete powers such that products of final maps are final, depend on this fact. Furthermore, the relations between them can be described by means of adjoint functors.     

CARTESIAN CLOSED TOPOLOGICAL HULLS AS INJECTIVE HULLS

Abstract

It is shown that (concretely) Cartesian closed topological hulls can be characterized as injective hulls in a rather natural setting. The characterization of locale hulls as injective hulls in the category of (meet-) semilattices by Bruns & Lakser and Born & Kimura constitutes a special case.     

ON CATEGORIES OF MONOID-ACTIONS

Abstract

Given a monoidal category B and a category S of monoids in B we study the category MOD_S of all actions of monoids from S on B-objects. This is mainly done by investigation of the underlying functor V: MOD_S → SxB. In particular V creates limits; filtered colimits and arbitrary colimits are detected, provided the monoidal structure behaves nicely with respect to these constructions. Moreover MOD_S contains B as a full coreflective subcategory; S is contained as a full reflective (and coreflective) one provided B has a terminal (zero) object. Monadicity of MOD_S over B is discussed as well.     

GENERALIZATIONS AND CARTESIAN CLOSED SUBCATEGORIES OF SEMICONTINUOUS LATTICES

In this article, the authors mainly study how to obtain new semicontinuous lattices from the given semicontinuous lattices and discuss the conditions under which the image of a semicontinuous projection operator is also semicontinuous. Moreover, the authors investigate the relation between semicontinuous lattices and completely distributive lattices. Finally, it is proved that the strongly semicontinuous lattice category is a Cartesian closed category.     

ON EPIDENSE SUBCATEGORIES OF TOPOLOGICAL CATEGORIES

Dense subcategories were introduced by S. Marde?i? for an inverse system approach to (categorical) shape theory.

In this paper some internal characterizations of (epi,bi)dense subcategories of a topological category are given. We also show that if K ? A is a bidense subcategory then the “best approximation” of an A-object X by a K-inverse system is obtained by “modifications” of the structure of X.     

CATEGORIES OF TOPOLOGICAL GROUPS

Abstract

This paper is a survey of recent (and some not so recent, results concerning categorical constructions on topological groups, with particular emphasis on free topological groups and coproducts (free products) of topological groups. An extensive bibliography is included.     

T0-SEPARATION IN TOPOLOGICAL CATEGORIES

R.-E. Hoffmann [5,6] has introduced the notion of an (E,M)-universally topological functor, which provides a categorical characterization of the T₀-separation axiom of general topology. In this paper, we characterise these functors in terms of the unique extension of structure functors defined on the subcategory of “separated” objects (of the domain category). This, in turn, leads to a solution of some problems due to G.C.L. Brümmer [1,2]. Other results include a generalization of L. Skula's characterization of the bireflective subcategories of Top [10].     

COMPLETIONS AND CATEGORICAL COMPACTNESS FOR NILPOTENT GROUPS

Abstract

We consider reflection functors in the category of nilpotent groups satisfying certain exactness properties for which the Mal'cev completion functor and the p-cotorsion completion functors are prototypical examples. Each of these functors defines a generalized torsion theory, which in turn defines a closure operator on subgroups. This gives rise to the notion of a categorically compact group with respect to the closure operator which we characterize. This approach provides a unified treatment for the categorically compact groups with respect to the Mal'cev completion and with respect to the p-cotorsion completion, the latter being new. We also consider the p-pro-finite completion, suitably restricted to obtain a reflection functor, and characterize the compact groups so arising.     

SOME RECENT RESULTS ON CATEGORIES OF R-SPACES AND PROBABILISTIC SPACES AND ON CATEGORICAL SPECTRA

The purpose of this paper is twofold: first, to present some recent results obtained by Romanian mathematicians in the field of general and categorical topology; second, to present some current research results obtained by the author in what may be called the topological study of a category. Accordingly, the paper is divided into two parts.

The author wishes to express his gratitude to the organizing committee of this Symposium for the kind invitation to present' this paper.     

AN AXIOMATIC APPROACH TO CATEGORIES OF TOPOLOGICAL ALGEBRAS

Abstract

Consider a commuting square of functors TV = GU where G is an algebraic functor over sets (in the sense of Herrlich), and T and U are (regular epi, monosource)—topological and fibre small. Such a square is called a Topological Algebraic Situation (TAS) when the following two conditions are satisfied:

1. if h: UA → UB and g: VA → VB are morphisms with Gh = Tg, there exists a morphism f: A → B such that Uf = h and Vf = g;

2. V carries U-initial monosources into T-initial mono-sources.

The functor V has many nice properties which shed light on the blending of the “topology” and “algebra”; e.g., V is a topologically algebraic functor in the sense of Y.H. Hong. An ([Etilde],[Mtilde]) version of O. Wyler's “Taut Lift Theorem” is used to show that the existence of a left adjoint to V is related to Condition (ii). It is also shown that certain topological algebraic reflections arise as Topological Algebraic Situations from algebraic and topological surjective reflections.