Currently displaying 1 – 20 of 20

Showing per page

Order by Relevance | Title | Year of publication

On spaces with the property of weak approximation by points

Angelo Bella — 1994

Commentationes Mathematicae Universitatis Carolinae

A sufficient condition that the product of two compact spaces has the property of weak approximation by points (briefly WAP) is given. It follows that the product of the unit interval with a compact WAP space is also a WAP space.

Sequential compactness vs. countable compactness

Angelo BellaPeter Nyikos — 2010

Colloquium Mathematicae

The general question of when a countably compact topological space is sequentially compact, or has a nontrivial convergent sequence, is studied from the viewpoint of basic cardinal invariants and small uncountable cardinals. It is shown that the small uncountable cardinal 𝔥 is both the least cardinality and the least net weight of a countably compact space that is not sequentially compact, and that it is also the least hereditary Lindelöf degree in most published models. Similar results, some definitive,...

Some properties of perfect metric spaces

Angelo BellaBiagio Ricceri — 1983

Atti della Accademia Nazionale dei Lincei. Classe di Scienze Fisiche, Matematiche e Naturali. Rendiconti Lincei. Matematica e Applicazioni

In questa Nota, dati uno spazio metrico perfetto X ed un suo sottoinsieme K chiuso e raro, si dimostra l'esistenza di una funzione continua f : X [ 0 , 1 ] tale che i n t ( f - 1 ( t ) ) = per ogni t [ 0 , 1 ] , f ( x ) = 0 per ogni x K e f ( y ) = 1 per qualche y X K . In particolare, ciò permette di dare risposta simultaneamente a due questioni poste in [2]. Si mettono in evidenza, poi, ulteriori conseguenze di tale risultato.

Some properties of perfect metric spaces

Angelo BellaBiagio Ricceri — 1983

Atti della Accademia Nazionale dei Lincei. Classe di Scienze Fisiche, Matematiche e Naturali. Rendiconti

In questa Nota, dati uno spazio metrico perfetto X ed un suo sottoinsieme K chiuso e raro, si dimostra l'esistenza di una funzione continua f : X [ 0 , 1 ] tale che i n t ( f - 1 ( t ) ) = per ogni t [ 0 , 1 ] , f ( x ) = 0 per ogni x K e f ( y ) = 1 per qualche y X K . In particolare, ciò permette di dare risposta simultaneamente a due questioni poste in [2]. Si mettono in evidenza, poi, ulteriori conseguenze di tale risultato.

About remainders in compactifications of homogeneous spaces

D. BasileAngelo Bella — 2009

Commentationes Mathematicae Universitatis Carolinae

We prove a dichotomy theorem for remainders in compactifications of homogeneous spaces: given a homogeneous space X , every remainder of X is either realcompact and meager or Baire. In addition we show that two other recent dichotomy theorems for remainders of topological groups due to Arhangel’skii cannot be extended to homogeneous spaces.

Further remarks on KC and related spaces

Angelo BellaCamillo Costantini — 2011

Commentationes Mathematicae Universitatis Carolinae

A topological space is KC when every compact set is closed and SC when every convergent sequence together with its limit is closed. We present a complete description of KC-closed, SC-closed and SC minimal spaces. We also discuss the behaviour of the finite derived set property in these classes.

Sequential + separable vs sequentially separable and another variation on selective separability

Angelo BellaMaddalena BonanzingaMikhail Matveev — 2013

Open Mathematics

A space X is sequentially separable if there is a countable D ⊂ X such that every point of X is the limit of a sequence of points from D. Neither “sequential + separable” nor “sequentially separable” implies the other. Some examples of this are presented and some conditions under which one of the two implies the other are discussed. A selective version of sequential separability is also considered.

Local cardinal functions of H-closed spaces

Angelo BellaJack R. Porter — 1996

Commentationes Mathematicae Universitatis Carolinae

The cardinal functions of pseudocharacter, closed pseudocharacter, and character are used to examine H-closed spaces and to contrast the differences between H-closed and minimal Hausdorff spaces. An H-closed space X is produced with the properties that | X | > 2 2 ψ ( X ) and ψ ¯ ( X ) > 2 ψ ( X ) .

Countable fan-tightness versus countable tightness

Aleksander V. Arhangel'skiiAngelo Bella — 1996

Commentationes Mathematicae Universitatis Carolinae

Countable tightness is compared to the stronger notion of countable fan-tightness. In particular, we prove that countable tightness is equivalent to countable fan-tightness in countably compact regular spaces, and that countable fan-tightness is preserved by pseudo-open compact mappings. We also discuss the behaviour of countable tightness and of countable fan-tightness under the product operation.

On AP and WAP spaces

Angelo BellaIvan V. Yashchenko — 1999

Commentationes Mathematicae Universitatis Carolinae

Several remarks on the properties of approximation by points (AP) and weak approximation by points (WAP) are presented. We look in particular at their behavior in product and at their relationships with radiality, pseudoradiality and related concepts. For instance, relevant facts are: (a) There is in ZFC a product of a countable WAP space with a convergent sequence which fails to be WAP. (b) C p over σ -compact space is AP. Therefore AP does not imply even pseudoradiality in function spaces, while...

Tightness and resolvability

Angelo BellaViacheslav I. Malykhin — 1998

Commentationes Mathematicae Universitatis Carolinae

We prove resolvability and maximal resolvability of topological spaces having countable tightness with some additional properties. For this purpose, we introduce some new versions of countable tightness. We also construct a couple of examples of irresolvable spaces.

Algebras and spaces of dense constancies

Angelo BellaJorge MartinezScott D. Woodward — 2001

Czechoslovak Mathematical Journal

A DC-space (or space of dense constancies) is a Tychonoff space X such that for each f C ( X ) there is a family of open sets { U i i I } , the union of which is dense in X , such that f , restricted to each U i , is constant. A number of characterizations of DC-spaces are given, which lead to an algebraic generalization of the concept, which, in turn, permits analysis of DC-spaces in the language of archimedean f -algebras. One is led naturally to the notion of an almost DC-space (in which the densely constant functions...

Page 1 Next

Download Results (CSV)