arXiv subjects

Kyriakos Keremedis

Publications and source records attributed to Kyriakos Keremedis.

13 recordsLinked to original sources

arXiv2021

$P$-spaces in the absence of the Axiom of Choice

A $P$-space is a topological space whose every $G_δ$-set is open. In this article, basic properties of $P$-spaces are investigated in the absence of the Axiom of Choice. New weaker forms of the Axiom of Choice, all relevant to $P$-spaces or to countable intersections of $G_δ$-sets, are introduced. Several independence results are obtained and open problems are posed. It is shown that a zero-dimensional subspace of the real line may fail to be strongly zero-dimensional in $\mathbf{ZF}$. Among the open problems there is the question whether it is provable in $\mathbf{ZF}$ that every finite product of $P$-spaces is a $P$-space. A partial answer to this question is given.

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arXiv2021

Countable products and countable sums of compact metrizable spaces in the absence of the Axiom of Choice

The main aim of the article is to show, in the absence of the Axiom of Choice, relationships between the following, independent of $\mathbf{ZF}$, statements: "Every countable product of compact metrizable spaces is separable (respectively, compact)" and "Every countable product of compact metrizable spaces is metrizable". Statements related to the above-mentioned ones are also studied. Permutation models (among them new ones) are shown in which a countable sum (also a countable product) of metrizable spaces need not be metrizable, countable unions of countable sets are countable and there is a countable family of non-empty sets of size at most $2^{\aleph_0}$ which does not have a choice function. A new permutation model is constructed in which every uncountable compact metrizable space is of size at least $2^{\aleph_0}$ but a denumerable family of denumerable sets need not have a multiple choice function.

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arXiv2021

$k$-spaces, sequential spaces and related topics in the absence of the axiom of choice

In the absence of the axiom of choice, new results concerning sequential, Fréchet-Urysohn, $k$-spaces, very $k$-spaces, Loeb and Cantor completely metrizable spaces are shown. New choice principles are introduced. Among many other theorems, it is proved in $\mathbf{ZF}$ that every Loeb, $T_3$-space having a base expressible as a countable union of finite sets is a metrizable second-countable space whose every $F_σ$-subspace is separable; moreover, every $G_δ$-subspace of a second-countable, Cantor completely metrizable space is Cantor completely metrizable, Loeb and separable. It is also noticed that Arkhangel'skii's statement that every very $k$-space is Fréchet-Urysohn is unprovable in $\mathbf{ZF}$ but it holds in $\mathbf{ZF}$ that every first-countable, regular very $k$-space whose family of all non-empty compact sets has a choice function is Fréchet-Urysohn. That every second-countable metrizable space is a very $k$-space is equivalent to the axiom of countable choice for $\mathbb{R}$.

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arXiv2021

On subcompactness and countable subcompactness of metrizable spaces in ZF

We show in ZF that: (i) Every subcompact metrizable space is completely metrizable, and every completely metrizable space is countably subcompact. (ii) A metrizable space X=(X,T) is countably compact iff it is countably subcompact relative to T. (iii) For every metric space X=(X,d), X is compact iff it is subcompact relative to T. We also show: (iv) The negation of each of the statements, (a) every countably subcompact metrizable space is completely metrizable, (b) every countably subcompact metrizable space is subcompact, (c) every complete metrizable space is subcompact is relatively consistent with ZF.

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arXiv2021

On iso-dense and scattered spaces in $\mathbf{ZF}$

A topological space is iso-dense if it has a dense set of isolated points. A topological space is scattered if each of its non-empty subspaces has an isolated point. In $\mathbf{ZF}$, in the absence of the axiom of choice, basic properties of iso-dense spaces are investigated. A new permutation model is constructed in which a discrete weakly Dedekind-finite space can have the Cantor set as a remainder. A metrization theorem for a class of quasi-metric spaces is deduced. The statement "every compact scattered metrizable space is separable" and several other statements about metric iso-dense spaces are shown to be equivalent to the countable axiom of choice for families of finite sets. Results concerning the problem of whether it is provable in $\mathbf{ZF}$ that every non-discrete compact metrizable space contains an infinite compact scattered subspace are also included.

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arXiv2020

Second-countable compact Hausdorff spaces as remainders in $\mathbf{ZF}$ and two new notions of infiniteness

In the absence of the Axiom of Choice, necessary and sufficient conditions for a locally compact Hausdorff space to have all non-empty second-countable compact Hausdorff spaces as remainders are given in $\mathbf{ZF}$. Among other independence results, the characterization of locally compact Hausdorff spaces having all non-empty metrizable compact spaces as remainders, obtained by Hatzenhuhler and Mattson in $\mathbf{ZFC}$, is proved to be independent of $\mathbf{ZF}$. Urysohn's Metrization Theorem is generalized to the following theorem: every $T_3$-space which admits a base expressible as a countable union of finite sets is metrizable. Applications to solutions of problems concerning the existence of some special metrizable compactifications in $\mathbf{ZF}$ are shown. New concepts of a strongly filterbase infinite set and a dyadically filterbase infinite set are introduced, both stemming from the investigations on compactifications. Set-theoretic and topological definitions of the new concepts are given, and their relationship with certain known notions of infinite sets is investigated in $\mathbf{ZF}$. A new permutation model is introduced in which there exists a strongly filterbase infinite set which is weakly Dedekind-finite. All $\mathbf{ZFA}$-independence results of this article are transferable to $\mathbf{ZF}$.

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arXiv2020

Several amazing discoveries about compact metrizable spaces in ZF

In the absence of the axiom of choice, the set-theoretic status of many natural statements about metrizable compact spaces is investigated. Some of the statements are provable in $\mathbf{ZF}$, some are shown to be independent of $\mathbf{ZF}$. For independence results, distinct models of $\mathbf{ZF}$ and permutation models of $\mathbf{ZFA}$ with transfer theorems of Pincus are applied. New symmetric models are constructed in each of which the power set of $\mathbb{R}$ is well-orderable, the Continuum Hypothesis is satisfied but a denumerable family of non-empty finite sets can fail to have a choice function, and a compact metrizable space need not be embeddable into the Tychonoff cube $[0, 1]^{\mathbb{R}}$.

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arXiv2020

Cuf products and cuf sums of (quasi)-metrizable spaces in $\mathbf{ZF}$

A cuf space (set, resp.) is a space (set, resp.) which is a countable union of finite subspaces (subsets, resp.). It is proved in $\mathbf{ZF}$ (with the absence of the axiom of choice) that all countable unions of cuf (denumerable, resp.) sets are cuf sets iff all countable products of cofinite cuf (denumerable, resp.) spaces are quasi-metrizable iff all countable products of one-point Hausdorff compactifications of infinite cuf (denumerable, resp.) spaces are quasi-metrizable. A countable product of one-point Hausdorff compactifications of denumerable discrete spaces is first-countable iff it is quasi-metrizable. A model of $\mathbf{ZF}$ is shown in which a countable product two-point Hausdorff compactifications of denumerable discrete spaces is first-countable without being quasi-metrizable. Other relevant independence results are also proved.

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arXiv2020

Denumerable cellular families in Hausdorff spaces and towers of Boolean algebras in $\mathbf{ZF}$

A denumerable cellular family of a topological space $\mathbf{X}$ is an infinitely countable collection of pairwise disjoint non-empty open sets of $ \mathbf{X}$. It is proved that the following statements are equivalent in $\mathbf{ZF}$: (i) For every infinite set $X,[X]^{<ω}\mathbf{\ }$has a denumerable subset. (ii) Every infinite $0$-dimensional Hausdorff space admits a denumerable cellular family. It is also proved that (i) implies the following: (iii) Every infinite Hausdorff Baire space has a denumerable cellular family. Among other results, the following theorems are also proved in $\mathbf{ZF}$: (iv) Every countable collection of non-empty subsets of $\mathbb{R}$ has a choice function iff, for every infinite second-countable Hausdorff space $ \mathbf{X}$, it holds that every base of $\mathbf{X}$ contains a denumerable cellular family of $\mathbf{X}$. (v) If every Cantor cube is pseudocompact, then every non-empty countable collection of non-empty finite sets has a choice function. (vi) If all Cantor cubes are countably paracompact, then (i) holds. Moreover, among other forms independent of $\mathbf{ZF}$, a partial Kinna-Wagner selection principle for families expressible as countable unions of finite families of finite sets is introduced. It is proved that if this new selection principle and (i) hold, then every infinite Boolean algebra has a tower and every infinite Hausdorff space has a denumerable cellular family.

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arXiv2019

On Loeb and sequential spaces in $\mathbf{ZF}$

A topological space is called Loeb if the collection of all its non-empty closed sets has a choice function. In this article, in the absence of the axiom of choice, connections between Loeb and sequential spaces are investigated. Among other results, it is proved in $\mathbf{ZF}$ that if $\mathbf{X}$ is a Cantor completely metrizable second-countable space, then $\mathbf{X}^ω$ is Loeb. If a sequential, sequentially locally compact space $\mathbf{X}$ has the property that every infinitely countable family of non-empty closed subsets of $\mathbf{X}$ has a choice function, then the Cartesian product $\mathbf{X}\times\mathbf{Y}$ of $\mathbf{X}$ with any sequential space $\mathbf{Y}$ is sequential. In consequence, it holds true in $\mathbf{ZF}$ that the Cartesian product of a sequential locally countably compact space with any sequential space is sequential. If $\mathbb{R}$ is sequential, then every second-countable compact Hausdorff space is sequential. It is also proved that, in some models of $\mathbf{ZF}$, a countable product of Cantor completely metrizable second-countable spaces can fail to be Loeb and it is independent of $\mathbf{ZF}$ that every sequential subspace of $% \mathbb{R}$ is Loeb. Some other sentences are shown to be independent of $% \mathbf{ZF}$. Several open problems are posed, among them, the following question: is $\mathbb{R}^ω$ sequential if $\mathbb{R}$ is sequential?

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arXiv2019

On densely complete metric spaces and extensions of uniformly continuous functions in $\mathbf{ZF}$

A metric space $\mathbf{X}$ is called densely complete if there exists a dense set $D$ in $\mathbf{X}$ such that every Cauchy sequence of points of $D $ converges in $\mathbf{X}$. One of the main aims of this work is to prove that the countable axiom of choice, $\mathbf{CAC}$ for abbreviation, is equivalent with the following statements:\smallskip (i) Every densely complete (connected) metric space $\mathbf{X}$ is complete.\smallskip\ (ii) For every pair of metric spaces $\mathbf{X}$ and $\mathbf{Y}$, if $% \mathbf{Y}$ is complete and $\mathbf{S}$ is a dense subspace of $\mathbf{X}$% , while $f:\mathbf{S}\rightarrow \mathbf{Y}$ is a uniformly continuous function, then there exists a uniformly continuous extension $F:\mathbf{X}\to% \mathbf{Y}$ of $f$.\smallskip (iii) Complete subspaces of metric spaces have complete closures.\smallskip (iv) Complete subspaces of metric spaces are closed.\smallskip It is also shown that the restriction of (i) to subsets of the real line is equivalent to the restriction $\mathbf{CAC}(\mathbb{R})$ of $\mathbf{CAC}$ to subsets of $\mathbb{R}$. However, the restriction of (ii) to subsets of $% \mathbb{R}$ is strictly weaker than $\mathbf{CAC}(\mathbb{R})$ because it is equivalent with the statement that $\mathbb{R}$ is sequential. Moreover, among other relevant results, it is proved that, for every positive integer $% n$, the space $\mathbb{R}^n$ is sequential if and only if $\mathbb{R}$ is sequential. It is also shown that $\mathbb{R}\times\mathbb{Q}$ is not densely complete if and only if $\mathbf{CAC}(\mathbb{R})$ holds.

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arXiv2018

Compact complement topologies and k-spaces

Let $(X,τ)$ be a Hausdorff space, where $X$ is an infinite set. The compact complement topology $τ^{\star}$ on $X$ is defined by: $τ^{\star}=\{\emptyset\} \cup \{X\setminus M, \text{where $M$ is compact in $(X,τ)$}\}$. In this paper, properties of the space $(X, τ^{\star})$ are studied in $\mathbf{ZF}$ and applied to a characterization of $k$-spaces, to the Sorgenfrey line, to some statements independent of $\mathbf{ZF}$, as well as to partial topologies that are among Delfs-Knebusch generalized topologies. Among other results, it is proved that the axiom of countable multiple choice (\textbf{CMC}) is equivalent with each of the following two sentences: (i) every Hausdorff first countable space is a $k$-space, (ii) every metrizable space is a $k$-space. A \textbf{ZF}-example of a countable metrizable space whose compact complement topology is not first countable is given.

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arXiv2018

Hausdorff compactifications in ZF

For a compactification $αX$ of a Tychonoff space $X$, the algebra of all functions $f\in C(X)$ that are continuously extendable over $% αX$ is denoted by $C_α(X)$. It is shown that, in a model of $\textbf{ZF}$, it may happen that a discrete space $X$ can have non-equivalent Hausdorff compactifications $αX$ and $γX$ such that $% C_α(X)=C_γ(X)$. Amorphous sets are applied to a proof that Glicksberg's theorem that $βX\times βY$ is the Cech-Stone compactification of $X\times Y$ when $X\times Y$ is a Tychonoff pseudocompact space is false in some models of $\mathbf{ZF}$. It is noticed that if all Tychonoff compactifications of locally compact spaces had $C^{\ast}$-embedded remainders, then van Douwen's choice principle would be satisfied. Necessary and sufficient conditions for a set of continuous bounded real functions on a Tychonoff space $X$ to generate a compactification of $X$ are given in $\mathbf{ZF}$. A concept of a functional Čech-Stone compactification is investigated in the absence of the axiom of choice.

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