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A new form of fuzzy α -compactness

Fu Gui Shi (2006)

Mathematica Bohemica

A new form of α -compactness is introduced in L -topological spaces by α -open L -sets and their inequality where L is a complete de Morgan algebra. It doesn’t rely on the structure of the basis lattice L . It can also be characterized by means of α -closed L -sets and their inequality. When L is a completely distributive de Morgan algebra, its many characterizations are presented and the relations between it and the other types of compactness are discussed. Countable α -compactness and the α -Lindelöf property...

A new Lindelöf space with points G δ

Alan S. Dow (2015)

Commentationes Mathematicae Universitatis Carolinae

We prove that ♦ * implies there is a zero-dimensional Hausdorff Lindelöf space of cardinality 2 ℵ 1 which has points G δ . In addition, this space has the property that it need not be Lindelöf after countably closed forcing.

A new look at pointfree metrization theorems

Bernhard Banaschewski, Aleš Pultr (1998)

Commentationes Mathematicae Universitatis Carolinae

We present a unified treatment of pointfree metrization theorems based on an analysis of special properties of bases. It essentially covers all the facts concerning metrization from Engelking [1] which make pointfree sense. With one exception, where the generalization is shown to be false, all the theorems extend to the general pointfree context.

A new metrization theorem

F. G. Arenas, M. A. Sánchez-Granero (2002)

Bollettino dell'Unione Matematica Italiana

We give a new metrization theorem on terms of a new structure introduced by the authors in [2] and called fractal structure. As a Corollary we obtain Nagata-Smirnov’s and Uryshon’s metrization Theorems.

A nice class extracted from C p -theory

Vladimir Vladimirovich Tkachuk (2005)

Commentationes Mathematicae Universitatis Carolinae

We study systematically a class of spaces introduced by Sokolov and call them Sokolov spaces. Their importance can be seen from the fact that every Corson compact space is a Sokolov space. We show that every Sokolov space is collectionwise normal, ω -stable and ω -monolithic. It is also established that any Sokolov compact space X is Fréchet-Urysohn and the space C p ( X ) is Lindelöf. We prove that any Sokolov space with a G δ -diagonal has a countable network and obtain some cardinality restrictions on subsets...

A nice subclass of functionally countable spaces

Vladimir Vladimirovich Tkachuk (2018)

Commentationes Mathematicae Universitatis Carolinae

A space X is functionally countable if f ( X ) is countable for any continuous function f : X → ℝ . We will call a space X exponentially separable if for any countable family ℱ of closed subsets of X , there exists a countable set A ⊂ X such that A ∩ ⋂ 𝒢 ≠ ∅ whenever 𝒢 ⊂ ℱ and ⋂ 𝒢 ≠ ∅ . Every exponentially separable space is functionally countable; we will show that for some nice classes of spaces exponential separability coincides with functional countability. We will also establish that the class of exponentially separable spaces has...

A non-archimedean Dugundji extension theorem

Jerzy Kąkol, Albert Kubzdela, Wiesƚaw Śliwa (2013)

Czechoslovak Mathematical Journal

We prove a non-archimedean Dugundji extension theorem for the spaces C * ( X , 𝕂 ) of continuous bounded functions on an ultranormal space X with values in a non-archimedean non-trivially valued complete field 𝕂 . Assuming that 𝕂 is discretely valued and Y is a closed subspace of X we show that there exists an isometric linear extender T : C * ( Y , 𝕂 ) → C * ( X , 𝕂 ) if X is collectionwise normal or Y is Lindelöf or 𝕂 is separable. We provide also a self contained proof of the known fact that any metrizable compact subspace Y of an ultraregular...

A nonlinear Banach-Steinhaus theorem and some meager sets in Banach spaces

Jacek Jachymski (2005)

Studia Mathematica

We establish a Banach-Steinhaus type theorem for nonlinear functionals of several variables. As an application, we obtain extensions of the recent results of Balcerzak and Wachowicz on some meager subsets of L¹(μ) × L¹(μ) and c₀ × c₀. As another consequence, we get a Banach-Mazurkiewicz type theorem on some residual subset of C[0,1] involving Kharazishvili's notion of Φ-derivative.

A non-metrizable collectionwise Hausdorff tree with no uncountable chains and no Aronszajn subtrees

Akira Iwasa, Peter J. Nyikos (2006)

Commentationes Mathematicae Universitatis Carolinae

It is independent of the usual (ZFC) axioms of set theory whether every collectionwise Hausdorff tree is either metrizable or has an uncountable chain. We show that even if we add “or has an Aronszajn subtree,” the statement remains ZFC-independent. This is done by constructing a tree as in the title, using the set-theoretic hypothesis ♦ * , which holds in Gödel’s Constructible Universe.

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