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Displaying similar documents to “More on κ -Ohio completeness”

On the cardinality of Hausdorff spaces and Pol-Šapirovskii technique

Alejandro Ramírez-Páramo (2005)

Commentationes Mathematicae Universitatis Carolinae

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In this paper we make use of the Pol-Šapirovskii technique to prove three cardinal inequalities. The first two results are due to Fedeli [2] and the third theorem of this paper is a common generalization to: (a) (Arhangel’skii [1]) If X is a T 1 space such that (i) L ( X ) t ( X ) κ , (ii) ψ ( X ) 2 κ , and (iii) for all A [ X ] 2 κ , A ¯ 2 κ , then | X | 2 κ ; and (b) (Fedeli [2]) If X is a T 2 -space then | X | 2 aql ( X ) t ( X ) ψ c ( X ) .

Initially κ -compact spaces for large κ

Stavros Christodoulou (1999)

Commentationes Mathematicae Universitatis Carolinae

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This work presents some cardinal inequalities in which appears the closed pseudo-character, ψ c , of a space. Using one of them — ψ c ( X ) 2 d ( X ) for T 2 spaces — we improve, from T 3 to T 2 spaces, the well-known result that initially κ -compact T 3 spaces are λ -bounded for all cardinals λ such that 2 λ κ . And then, using an idea of A. Dow, we prove that initially κ -compact T 2 spaces are in fact compact for κ = 2 F ( X ) , 2 s ( X ) , 2 t ( X ) , 2 χ ( X ) , 2 ψ c ( X ) or κ = max { τ + , τ < τ } , where τ > t ( p , X ) for all p X .

Weak extent in normal spaces

Ronnie Levy, Mikhail Matveev (2005)

Commentationes Mathematicae Universitatis Carolinae

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If X is a space, then the we ( X ) of X is the cardinal min { α : If 𝒰 is an open cover of X , then there exists A X such that | A | = α and St ( A , 𝒰 ) = X } . In this note, we show that if X is a normal space such that | X | = 𝔠 and we ( X ) = ω , then X does not have a closed discrete subset of cardinality 𝔠 . We show that this result cannot be strengthened in ZFC to get that the extent of X is smaller than 𝔠 , even if the condition that we ( X ) = ω is replaced by the stronger condition that X is separable.