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Roman Pol

Publications and source records attributed to Roman Pol.

15 recordsLinked to original sources
arXiv2025

On closed embeddings in $P^N \cup Q^N$

We prove that if a separable metrizable $X$ is a union of two disjoint 0-dimensional sets $E$, $F$, $E$ is absolutely $G_{\delta}$ and $F$ is absolutely $F_{\sigma\delta}$ then there is a closed embedding $h$ into the union of countable products of the irrationals and the rationals with $E$ being the preimage under $h$ of the countable product of the irrationals and $F$ being the preimage under $h$ of the countable product of the rationals. We prove also that for the set $H$ of points $x$ in the Hilbert cube such that for each $k$ there is $l$ with $x(2^k 3^l)=0$, whenever $A$ is an $F_{\sigma \delta}$ set in a compact one-dimensional space $X$, there is an embedding $h$ into the union of the countable product of the irrationals with added point $0$, and the countable product of the rationals, such that $A$ is the preimage under $h$ of the set $H$.

math.GN↗
arXiv2025

On Sierpi\'nski sets, Hurewicz spaces and Hilgers functions

The Hurewicz property is a classical generalization of $\sigma$-compactness and Sierpi\'nski sets (whose existence follows from CH) are standard examples of non-$\sigma$-compact Hurewicz spaces. We show, solving a problem stated by Szewczak and Tsaban, that for each Sierpi\'nski set S of cardinality at least $\mathfrak b$ there is a Hurewicz space H with $S\times H$ not Hurewicz. Some other questions in the literature concerning this topic are also answered.

math.GN↗
arXiv2025

A note on uniform continuity of monotone functions

We prove that it is consistent with ZFC that for every non-decreasing function $f:[0,1]\to [0,1]$, each subset of $[0,1]$ of cardinality $\mathfrak c$ contains a set of cardinality $\mathfrak c$ on which $f$ is uniformly continuous. We show that this statement follows from the assumptions that $\mathfrak d^* < \mathfrak c$ and $\mathfrak c$ is regular, where $\mathfrak d^*\leq \mathfrak d$ is the smallest cardinality $\kappa$ such that any two disjoint countable dense sets in the Cantor set can be separated by sets each of which is an intersection of at most $\kappa$-many open sets in the Cantor set. We establish also that $\mathfrak d^*=\min\{\mathfrak u, \mathfrak d\}=\min\{\mathfrak r, \mathfrak d\}$, thus giving an alternative proof of the latter equality established by J. Aubrey in 2004.

math.LO↗
arXiv2024

On two consequences of CH established by Sierpinski. II

We continue a study of the relations between two consequences of the Continuum Hypothesis discovered by Waclaw Sierpinski, concerning uniform continuity of continuous functions and uniform convergence of sequences of real-valued functions, defined on subsets of the real line of cardinality continuum.

math.LO↗
arXiv2023

On two consequences of CH established by Sierpi\'nski

We study the relations between two consequences of the Continuum Hypothesis discovered by Wac{\l}aw Sierpi\'nski, concerning uniform continuity of continuous functions and uniform convergence of sequences of real-valued functions, defined on subsets of the real line of cardinality continuum.

math.LO↗
arXiv2023

A remark on Fremlin-Miller theorem concerning the Menger property and Michael concentrated sets

The theorem we prove is a slight strengthening of some results by Just, Miller, Scheepers and Szeptycki [JMSS]. We use the Michael technique instead of the combinatorial approach in the literature. Comments by the submitter: This short unpublished paper, written in 2002, is a cornerstone in the study of selection principles and classic covering properties. It is the first to solve the Hurewicz Problem, by showing that there is, in ZFC, a real set with Menger's covering property (indeed, in all finite powers) but without Hurewicz's covering property. Unfortunately, web searches begin to miss this paper, and I sought, and received, permission from the authors to store this paper in the ArXiv, providing it a permanent link. The author's decision to not publish this paper, in light of some later stronger results by other authors (including myself), is a true act of generosity. Hopefully, this submission does some justice to their achievement.

math.GN↗
arXiv2020

Countably perfectly meager sets

We study a strengthening of the notion of a perfectly meager set. We say that that a subset $A$ of a perfect Polish space $X$ is countably perfectly meager in $X$, if for every sequence of perfect subsets $\{P_n: n \in {\mathbb N}\}$ of $X$, there exists an $F_\sigma$-set $F$ in $X$ such that $A \subseteq F$ and $F\cap P_n$ is meager in $P_n$ for each $n$. We give various characterizations and examples of countably perfectly meager sets. We prove that not every universally meager set is countably perfectly meager correcting an earlier result of Bartoszy\'nski.

math.LO↗
arXiv2019

On Mazurkiewicz's sets, thin {\sigma}-ideals of compact sets and the space of probability measures on the rationals

We shall establish some properties of thin $\sigma$-ideals of compact sets in compact metric spaces (in particular, the $\sigma$-ideals of compact null-sets for thin subadditive capacities), and we shall refine the celebrated theorem of David Preiss that there exist compact non-uniformly tight sets of probability measures on the rationals. Both topics will be based on a construction of Stefan Mazurkiewicz from his 1927 paper containing a solution of a Urysohn's problem in dimension theory.

math.GN↗
arXiv2018

On a problem of Talagrand concerning separately continuous functions

We construct a separately continuous function $e:E\times K\to\{0,1\}$ on the product of a Baire space $E$ and a zero-dimensional compact space $K$ such that no restriction of $e$ to any non-meager Borel set in $E\times K$ is continuous. The function $e$ provides a negative solution of Talagrand's problem in \cite{T}.

math.GN↗
arXiv2017

On Borel maps, calibrated $\sigma$-ideals and homogeneity

Let $\mu$ be a Borel measure on a compactum $X$. The main objects in this paper are $\sigma$-ideals $I(dim)$, $J_0(\mu)$, $J_f(\mu)$ of Borel sets in $X$ that can be covered by countably many compacta which are finite-dimensional, or of $\mu$-measure null, or of finite $\mu$-measure, respectively. Answering a question of J. Zapletal, we shall show that for the Hilbert cube, the $\sigma$-ideal $I(dim)$ is not homogeneous in a strong way. We shall also show that in some natural instances of measures $\mu$ with non-homogeneous $\sigma$-ideals $J_0(\mu)$ or $J_f(\mu)$, the completions of the quotient Boolean algebras $Borel(X)/J_0(\mu)$ or $Borel(X)/J_f(\mu)$ may be homogeneous. We discuss the topic in a more general setting, involving calibrated $\sigma$-ideals.

math.LO↗
arXiv2017

Isometric embeddings and continuous maps onto the irrationals

Let f be a continuous map of a complete separable metric space E onto the irrationals. We show that if a complete separable metric space M contains isometric copies of every closed relatively discrete set in E, then M contains also an isometric copy of some fiber of f. We shall show also that if all fibers of f have positive dimension, then the collection of closed zero-dimensional sets in E is non-analytic in the Wijsman hyperspace of E.

math.GN↗
arXiv2017

On Closed Mappings of Sigma-Compact Spaces and Dimension

We prove that if K is a remainder of the Hilbert space (i.e., K is the complement of the Hilbert space in its metrizable compactification) then every non-one-point closed image of K either contains a compact set with no transfinite dimension or contains compact sets of arbitrarily high inductive transfinite dimension ind. We construct also for each natural n a sigma-compact metrizable n-dimensional space whose image under any non-constant closed map has dimension at least n, and analogous examples for the transfinite dimension ind.

math.GN↗
arXiv2000

Remarks on hereditarily indecomposable continua

We recall a characterization of hereditary indecomposability originally obtained by Krasinkiewicz and Minc, and show how it may be used to give unified constructions of various hereditarily indecomposable continua. In particular we answer a question asked by Mackowiak and Tymchatyn by showing that any continuum of arbitrary weight is a weakly confluent image of a hereditarily indecomposable continuum of the same weight. We present two methods of constructing these preimages: (a) by model-theoretic means, using the compactness and completeness theorems from first-order logic to derive these results for continua of uncountable weight from their metric counterparts; and (b) by constructing essential mappings from hereditarily indecomposable continua onto Tychonoff cubes. We finish by reviving an argument due to Kelley about hyperspaces of hereditarily indecomposable continua and show how it leads to a point-set argument that reduces Brouwer's Fixed-point theorem to its three-dimensional version.

math.GN↗