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Evgenii Reznichenko

Publications and source records attributed to Evgenii Reznichenko.

At least 19 recordsLinked to original sources
arXiv2026

Homogeneous linearly ordered spaces

Every compact subset of a homogeneous generalized ordered (GO) space has character at most $ω_1$ and cardinality at most $2^{ω_1}$; if such a subset has uncountable character, then the character of the whole space equals $ω_1$ and its $π$-character is countable. We construct a homogeneous $σ$-compact linearly ordered space (LOTS) $\mathbf{H}$ containing a compact subset $\mathbf{S}$ of cardinality $2^{ω_1}$ whose character is $ω_1$ at every point and whose weight and Souslin number are both $2^{ω_1}$; thus both bounds obtained are sharp. We prove that a semitopological group that is a GO space is hereditarily paracompact; if, in addition, it is not a $P$-space, then it is submetrizable, has countable character, and its compact subsets are metrizable. Every linearly ordered semitopological group (and, more generally, every GO semitopological group) is either metrizable or is a $P$-space; the same holds for topological groups. We also show that in an order-homogeneous LOTS every compact subset is first countable.

math.GN↗
arXiv2026

Baire-type properties of topological vector spaces

Burzyk, Kliś and Lipecki proved that every topological vector space (tvs) $E$ with the property $(K)$ is a Baire space. Kcakol and Sánchez Ruiz proved that every sequentially complete Fréchet--Urysohn locally convex space (lcs) is Baire. Being motivated by the property $(K)$ and the notion of a Mackey null sequence we introduce a property $(MK)$ which is strictly weaker than the property $(K)$, and show that any locally complete lcs has the property $(MK)$. We prove that any $κ$-Fréchet--Urysohn tvs with the property $(MK)$ is a Baire space; consequently, each locally complete $κ$-Fréchet--Urysohn lcs is a Baire space. This generalizes both the aforementioned results. We construct a feral Baire space $E$ with the property $(K)$ and which is not $κ$-Fréchet--Urysohn. Although a $κ$-Fréchet--Urysohn lcs $E$ can be not a Baire space, we show that $E$ is always $b$-Baire-like in the sense of Ruess. Applications to spaces of Baire functions and $C_k$-spaces are given.

math.FA↗
arXiv2026

On $κ$-Frechet-Urysohn topological groups

We characterize $κ$-Fréchet--Urysohn topological groups. Using this characterization we show that: (1) a hemicompact topological group is $κ$-Fréchet--Urysohn iff it is locally compact, and (2) if $F$ is a closed metrizable subspace of a topological vector space (tvs) $E$ such that the quotient $E/F$ is a $κ$-Fréchet--Urysohn space, then also $E$ is a $κ$-Fréchet--Urysohn space. Consequently, the product of a $κ$-Fréchet--Urysohn tvs and a metrizable tvs is a $κ$-Fréchet--Urysohn space. Under Martin's Axiom, we construct a countable Boolean $κ$-Fréchet--Urysohn group which is not a $k_{\mathbb R}$-space.

math.GN↗
arXiv2026

$\mathbb R^{ω_1}$-Factorizable Spaces and Groups

A topological space $X$ is $\mathbb R^{ω_1}$-factorizable if any continuous function $f\colon X\to \mathbb R^{ω_1}$ factors through a continuous function from $X$ to a second-countable space. It is shown that a Tychonoff space $X$ is $\mathbb R^{ω_1}$-factorizable if and only if $X\times D(ω_1)$, where $D(ω_1)$ is a discrete space of cardinality $ω_1$, is $z$-embedded in the product $βX\times βD(ω_1)$ of the Stone--Cech compactifications. It is also proved that $\mathbb R^{ω_1}$-factorizability is hereditary and countably multiplicative, that any $\mathbb R^{ω_1}$-factorizable space is hereditarily Lindelöf and hereditarily separable, and that the existence of nonmetrizable $\mathbb R^{ω_1}$-factorizable topological spaces and groups is independent of ZFC: under CH, all $\mathbb R^{ω_1}$-factorizable spaces are second-countable, while under MA + $\lnot$CH, the countable Fréchet--Urysohn fan is $\mathbb R^{ω_1}$-factorizable.

math.GN↗
arXiv2026

Completeness and reflexivity type properties of $B_1(X)$

For a Tychonoff space $X$, $B_1(X)$ denotes the space of all Baire-one functions on $X$ endowed with the pointwise topology. We prove that the following assertions are equivalent: (1) $B_1(X)$ is a (semi-)Montel space, (2) $B_1(X)$ is a (semi-)reflexive space, (3) $B_1(X)$ is a (quasi-)complete space, (4) $B_1(X)=\mathbb{R}^X$, (5) $X$ is a $Q_f$-space. It is proved that $B_1(X)$ is sequentially complete iff $B_1(X)$ is locally complete iff $X$ is a $CZ$-space. In the case when $K$ is a compact space, we show that $B_1(K)$ is locally complete iff $K$ is scattered. We thoroughly study the case when $X$ is a separable metrizable space. Numerous distinguished examples are given.

math.GN↗
arXiv2025

New classes of compact-type spaces

Being motivated by the notions of $κ$-Fréchet--Urysohn spaces and $k'$-spaces introduced by Arhangel'skii, the notion of sequential spaces and the study of Ascoli spaces, we introduce three new classes of compact-type spaces. They are defined by the possibility to attain each or some of boundary points $x$ of an open set $U$ by a sequence in $U$ converging to $x$ or by a relatively compact subset $A\subseteq U$ such that $x\in \overline{A}$. Relationships of the introduced classes with the classical classes (as, for example, the classes of $κ$-Fréchet--Urysohn spaces, (sequentially) Ascoli spaces, $k_{\mathbb R}$-spaces, $s_{\mathbb R}$-spaces etc.) are given. We characterize these new classes of spaces and study them with respect to taking products, subspaces and quotients. In particular, we give new characterizations of $κ$-Fréchet--Urysohn spaces and show that each feathered topological group is $κ$-Fréchet--Urysohn. We describe locally compact abelian groups which endowed with the Bohr topology belong to one of the aforementioned classes. Numerous examples are given.

math.GN↗
arXiv2025

$κ$-spaces

We say that a Tychonoff space $X$ is a $κ$-space if it is homeomorphic to a closed subspace of $C_p(Y)$ for some locally compact space $Y$. The class of $κ$-spaces is strictly between the class of Dieudonné complete spaces and the class of $μ$-spaces. We show that the class of $κ$-spaces has nice stability properties, that allows us to define the $κ$-completion $κX$ of $X$ as the smallest $κ$-space in the Stone--Čech compactification $βX$ of $X$ containing $X$. For a point $z\inβX$, we show that (1) if $z\in\upsilon X$, then the Dirac measure $δ_z$ at $z$ is bounded on each compact subset of $C_p(X)$, (2) $z\in κX$ iff $δ_z$ is continuous on each compact subset of $C_p(X)$ iff $δ_z$ is continuous on each compact subset of $C_p^b(X)$, (3) $z\in\upsilon X$ iff $δ_z$ is bounded on each compact subset of $C_p^b(X)$. It is proved that $κX$ is the largest subspace $Y$ of $βX$ containing $X$ for which $C_p(Y)$ and $C_p(X)$ have the same compact subsets, this result essentially generalizes a known result of R.~Haydon.

math.GN↗
arXiv2025

Functions on products $X \times Y$ with applications to Ascoli spaces, $k_{\mathbb{R}}$-spaces and $s_{\mathbb{R}}$-spaces

We prove that a Tychonoff space $X$ is (sequentially) Ascoli iff for every compact space $K$ (resp., for a convergent sequence $\mathbf{s}$), each separately continuous $k$-continuous function $Φ:X\times K\to \mathbb{R}$ is continuous. We apply these characterizations to show that an open subspace of a (sequentially) Ascoli space is (sequentially) Ascoli, and that the $μ$-completion and the Dieudonné completion of a (sequentially) Ascoli space are (sequentially) Ascoli. We give also cover-type characterizations of Ascoli spaces and suggest an easy method of construction of pseudocompact Ascoli spaces which are not $k_\mathbb{R}$-spaces and show that each space $X$ can be closely embedded into such a space. Using a different method we prove Hušek's theorem: a Tychonoff space $Y$ is a locally pseudocompact $k_\mathbb{R}$-space iff $X\times Y$ is a $k_\mathbb{R}$-space for each $k_\mathbb{R}$-space $X$. It is proved that $X$ is an $s_\mathbb{R}$-space iff for every locally compact sequential space $K$, each $s$-continuous function $f:X\times K\to\mathbb{R}$ is continuous.

math.GN↗
arXiv2025

Weird $\mathbb R$-Factorizable Groups

The problem of the existence of non-pseudo-$\aleph_1$-compact $\mathbb R$-factorizable groups is studied. It is proved that any such group is submetrizable and has weight larger than $ω_1$. Closely related results concerning the $\mathbb R$-factorizability of products of topological groups and spaces are also obtained (a product $X\times Y$ of topological spaces is said to be $\mathbb R$-factorizable if any continuous function $X\times Y\to \mathbb R$ factors through a product of maps from $X$ and $Y$ to second-countable spaces). In particular, it is proved that the square $G\times G$ of a topological groups $G$ is $\mathbb R$-factorizable as a group if and only if it is $\mathbb R$-factorizable as a product of spaces, in which case $G$ is pseudo-$\aleph_1$-compact. It is also proved that if the product of a space $X$ and an uncountable discrete space is $\mathbb R$-factorizable, then $X^ω$ is heredirarily separable and heredirarily Lindelöf.

math.GN↗
arXiv2025

On $k_\mathbb{R}$-spaces and $s_\mathbb{R}$-spaces

We give new characterizations of spaces $X$ which are $k_\mathbb{R}$-spaces or $s_\mathbb{R}$-spaces. Applying the obtained results we provide some sufficient and necessary conditions on $X$ for which $C_p(X)$ is a $k_\mathbb{R}$-space or an $s_\mathbb{R}$-space. It is proved that $C_p(X)$ is a $k_\mathbb{R}$-space for any space $X$ with one non-isolated point; if, in addition, $|X|$ is not sequential, then $C_p(X)$ is even an $s_\mathbb{R}$-space. Under $(CH)$, it is shown that there exists a separable metrizable space $X$ such that $C_p(X)$ is an Ascoli space but not a $k_\mathbb{R}$-space.

math.GN↗
arXiv2025

Regular rigid Korovin orbits

An example of an infinite regular feebly compact quasitopological group is presented such that all continuous real-valued functions on the group are constant. The example is based on the use of Korovin orbits in $X^G$, where $X$ is a special regular countably compact space constructed by S.Bardyla and L.Zdomskyy and $G$ is an abstract Abelian group of an appropriate cardinality. Also, we study the interplay between the separation properties of the space $X$ and Korovin orbits in $X^G$. We show in particular that if $X$ contains two nonempty disjoint open subsets, then every Korovin orbit in $X^G$ is Hausdorff.

math.GN↗
arXiv2025

On identities in connected topological groups

In 1957, Nemytskii proved the following fact: if in a locally compact or in an Abelian connected group there is a neighborhood of the identity in which some identity holds, then it holds in the entire group. The following question was also posed there: Let G be a connected topological group. In some neighborhood of the identity of the group G the identity $x^3=1$ holds. Is it true that then the identity $x^3=1$ holds in the entire group $G$? The same question is posed for the identity $gx^2 = x^2g$, where $g$ is a fixed element of the group. Platonov formulated the following generalized formulation of the Mytselsky problem: for a topological connected group, is it true that if the identity holds in a neighborhood of the identity, then the identity holds everywhere? In this paper, a negative answer to Platonov's question is given, the following theorem is proven: if $n > 10^{10}$ is odd, then there exists a connected topological group in which the identity $x^n=1$ holds in some neighborhood of unity, but not in the entire group.

math.GN↗
arXiv2024

Classes of Baire spaces defined by semi-neighborhoods of the diagonal

With the help of semi-neighborhoods of the diagonal, classes of Baire spaces are defined: $Δ$, $Δ_h$ and $Δ_s$ Baire spaces. These classes of spaces are studied with the help of topological games. They are useful in studying continuity in groups: paratopological $Δ$-Baire groups, quasi-topological $Δ_h$-Baire groups, and semitopological $Δ_s$-Baire groups are topological groups.

math.GN↗
arXiv2023

Almost paratopological groups

A class of almost paratopological groups is introduced, which (1) contains paratopological groups and Hausdorff quasitopological groups; (2) is closed under products; (3) subgroups. Almost paratopological $T_1$ groups $G$ are characterized by the fact that $\{(x,y)\in G^2: xy=e\}$ is closed in $G^2$. A compact almost paratopological group is topological. A regular $Σ$-space with a countable extend and a separately continuous Mal'tsev operation is $ω$-cellular (and ccc). A $σ$-compact regular almost paratopological group is ccc. In particular, a $σ$-compact regular quasitopological group is ccc.

math.GN↗
arXiv2023

Metrizability of CHART groups

For compact Hausdorff admissible right topological (CHART) group $G$, we prove $w(G)=πχ(G)$. This equality is well known for compact topological groups. This implies the criteria for the metrizability of CHART groups: if $G$ is first-countable (2013, Moors, Namioka) or $G$ is Fréchet (2013, Glasner, Megrelishvili), or $G$ has countable $π$-character (2022, Reznichenko) then $G$ is metrizable. Under the continuum hypothesis (CH) assumption, a sequentially compact CHART group is metrizable. Namioka's theorem that metrizable CHART groups are topological groups extends to CHART groups with small weight.

math.GR↗
arXiv2022

Algebraic structures on the Cantor set

Below, by space we mean a separable metrizable zero-dimensional space. It is studied when the space can be embedded in a Cantor set while maintaining the algebraic structure. Main results of the work: every space is an open retract of a Boolean precompact group; every strongly homogeneous space is rectifiable. In this case, the space can be embedded in the Cantor set with the preservation of the algebraic structure. An example of a strongly homogeneous space is constructed which do not admit the structure of a right topological group.

math.GN↗