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Remarks on star countable discrete closed spaces

Yan-Kui Song (2013)

Czechoslovak Mathematical Journal

In this paper, we prove the following statements: (1) There exists a Tychonoff star countable discrete closed, pseudocompact space having a regular-closed subspace which is not star countable. (2) Every separable space can be embedded into an absolutely star countable discrete closed space as a closed subspace. (3) Assuming 2 0 = 2 1 , there exists a normal absolutely star countable discrete closed space having a regular-closed subspace which is not star countable.

Remarks on star covering properties in pseudocompact spaces

Yan-Kui Song (2013)

Mathematica Bohemica

Let P be a topological property. A space X is said to be star P if whenever 𝒰 is an open cover of X , there exists a subspace A X with property P such that X = St ( A , 𝒰 ) , where St ( A , 𝒰 ) = { U 𝒰 : U A } . In this paper, we study the relationships of star P properties for P { Lindel ö f , compact , countablycompact } in pseudocompact spaces by giving some examples.

Remarks on Star-Hurewicz Spaces

Yan-Kui Song (2013)

Bulletin of the Polish Academy of Sciences. Mathematics

A space X is star-Hurewicz if for each sequence (𝒰ₙ: n ∈ ℕ) of open covers of X there exists a sequence (𝓥ₙ: n ∈ ℕ) such that for each n, 𝓥ₙ is a finite subset of 𝒰ₙ, and for each x ∈ X, x ∈ St(⋃ 𝓥ₙ,𝒰ₙ) for all but finitely many n. We investigate the relationship between star-Hurewicz spaces and related spaces, and also study topological properties of star-Hurewicz spaces.

Remarks on strongly star-Menger spaces

Yan-Kui Song (2013)

Commentationes Mathematicae Universitatis Carolinae

A space X is strongly star-Menger if for each sequence ( 𝒰 n : n ) of open covers of X , there exists a sequence ( K n : n N ) of finite subsets of X such that { S t ( K n , 𝒰 n ) : n } is an open cover of X . In this paper, we investigate the relationship between strongly star-Menger spaces and related spaces, and also study topological properties of strongly star-Menger spaces.

Remarks on the cardinality of a power homogeneous space

Angelo Bella (2005)

Commentationes Mathematicae Universitatis Carolinae

We provide a further estimate on the cardinality of a power homogeneous space. In particular we show the consistency of the formula | X | 2 π χ ( X ) for any regular power homogeneous ccc space.

Remarks on the region of attraction of an isolated invariant set

Konstantin Athanassopoulos (2006)

Colloquium Mathematicae

We study the complexity of the flow in the region of attraction of an isolated invariant set. More precisely, we define the instablity depth, which is an ordinal and measures how far an isolated invariant set is from being asymptotically stable within its region of attraction. We provide upper and lower bounds of the instability depth in certain cases.

Remarks on the sobriety of Scott topology and weak topology on posets

He Wei, Jiang Shouli (2002)

Commentationes Mathematicae Universitatis Carolinae

We give some necessary and sufficient conditions for the Scott topology on a complete lattice to be sober, and a sufficient condition for the weak topology on a poset to be sober. These generalize the corresponding results in [1], [2] and [4].

Remarks on the Stone Spaces of the Integers and the Reals without AC

Horst Herrlich, Kyriakos Keremedis, Eleftherios Tachtsis (2011)

Bulletin of the Polish Academy of Sciences. Mathematics

In ZF, i.e., the Zermelo-Fraenkel set theory minus the Axiom of Choice AC, we investigate the relationship between the Tychonoff product 2 ( X ) , where 2 is 2 = 0,1 with the discrete topology, and the Stone space S(X) of the Boolean algebra of all subsets of X, where X = ω,ℝ. We also study the possible placement of well-known topological statements which concern the cited spaces in the hierarchy of weak choice principles.

Remarks on Yu’s ‘property A’ for discrete metric spaces and groups

Jean-Louis Tu (2001)

Bulletin de la Société Mathématique de France

Guoliang Yu has introduced a property on discrete metric spaces and groups, which is a weak form of amenability and which has important applications to the Novikov conjecture and the coarse Baum–Connes conjecture. The aim of the present paper is to prove that property in particular examples, like spaces with subexponential growth, amalgamated free products of discrete groups having property A and HNN extensions of discrete groups having property A.

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