arXiv subjects

Jerzy Kakol

Publications and source records attributed to Jerzy Kakol.

12 recordsLinked to original sources

arXiv2026

A survey on the Asplund property for spaces $C(X)$ of continuous functions

The Asplund property plays an important role in Banach space theory due to its connections with differentiability properties of continuous convex functions, optimization problems, and the weak topology of Banach spaces. Motivated by the variety of nonequivalent definitions proposed in the literature for locally convex spaces, in this survey we provide a unified framework for studying the Asplund property beyond the Banach space setting. The main object of our study is the locally convex space of continuous functions $C_k(X)$ endowed with the compact-open topology, where $X$ is an arbitrary Tychonoff space. We completely characterize the Asplund property for $C_k(X)$ in terms of topological properties of the underlying space $X$. As an essential step, we revisit the proof of several classical results, including the Namioka--Phelps theorem. Our approach is independent of differentiability techniques and relies solely on topological methods. The exposition is self-contained, and all major results are provided with complete proofs, making the paper accessible to both specialists and newcomers.

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arXiv2026

On weak*-basic sequences in duals and biduals of spaces C(X) and Quojections

We show that for infinite Tychonoff spaces X and Y the weak*-dual of Ck(X x Y) contains a basic sequence; moreover, the weak*-bidual of Ck(X) contains such a sequence as well. When X and Y are infinite compact spaces, we single out a concrete sequence (μn) of finitely supported signed measures on X x Y with quantitative small-rectangle estimates, and we prove that every subsequence of (μn) admits a further subsequence which is strongly normal and forms a weak*-basic sequence in the dual C(X x Y)* of the Banach space C(X x Y). We also study the weak*-basic sequence problem for Frechet locally convex spaces in the class of quojections, and prove that for every quojection E the bidual E** admits a weak*-basic sequence, while a long-standing open problem asks whether the dual of every infinite-dimensional Banach space admits a basic sequence in the weak*-topology. Several examples and open questions are included, in particular for spaces C(X) and for inductive limits of Frechet spaces.

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arXiv2025

An Elementary Proof Of The Josefson-Nissenzweig Theorem For Banach Spaces C(KxL)

In [8] probabilistic methods, in particular a variant of the Weak Law of Large Numbers related to the Bernoulli distribution, have been used to show that for every infinite compact spaces K and L there exists a sequence $(μ_n)$ of normalized signed measures on $K\times L$ with finite supports which converges to $0$ with respect to the weak topology of the dual Banach space $C(K\times L).$ In this paper, we return to this construction, limiting ourselves only to elementary combinatorial calculus. The main efects of this construction are additional information about the measures $μ_n$, this is particularly clearly seen (among the others) in the resulting inequalities $$\frac{1}{2\sqrtπ}\frac{1}{\sqrt{n}} <\sup_{A\times B\subset X\times Y} |μ_n(A\times B)|<\frac{2}{\sqrtπ}\frac{1}{\sqrt{n}},$$ $n\in\mathbb{N}$, with $μ_n(f) \to_n 0$ for every $f\in C(X \times Y);$ where X and Y are arbitrary Tychonoff spaces containing infinite compact subsets, respectively. As an application we explicitly describe for Banach spaces $C(X\times Y)$ some complemented subspaces isomorphic to $c_0$. This result generalizes the classical theorem of Cembranos and Freniche, which states that for every infinite compact spaces K and L, the Banach space $C(K\times L)$ contains a complemented copy of the Banach space $c_0.$

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arXiv2024

On the product of Weak Asplund locally convex spaces

For locally convex spaces, we systematize several known equivalent definitions of Fréchet (G\^ ateaux) Differentiability Spaces and Asplund (Weak Asplund) Spaces. As an application, we extend the classical Mazur's theorem as follows: Let $E$ be a separable Baire locally convex space and let $Y$ be the product $\prod_{α\in A} E_α$ of any family of separable Fréchet spaces; then the product $E \times Y$ is Weak Asplund. Also, we prove that the product $Y$ of any family of Banach spaces $(E_α)$ is an Asplund locally convex space if and only if each $E_α$ is Asplund. Analogues of both results are valid under the same assumptions, if $Y$ is the $Σ$-product of any family $(E_α)$.

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arXiv2022

When is a locally convex space Eberlein-Grothendieck?

In this paper we undertake a systematic study of those locally convex spaces $E$ such that $(E, w)$ is (linearly) Eberlein-Grothendieck, where $w$ is the weak topology of $E$. Let $C_{k}(X)$ be the space of continuous real-valued functions on a Tychonoff space $X$ endowed with the compact-open topology. The main results of our paper are: (1) For a first-countable space $X$ (in particular, for a metrizable $X$) the locally convex space $(C_{k}(X), w)$ is Eberlein-Grothendieck if and only if $X$ is both $σ$-compact and locally compact; (2) $(C_{k}(X), w)$ is linearly Eberlein-Grothendieck if and only if $X$ is compact. We characterize $E$ such that $(E, w)$ is linearly Eberlein-Grothendieck for several other important classes of locally convex spaces $E$. Also, we show that the class of $E$ for which $(E, w)$ is linearly Eberlein-Grothendieck preserves linear continuous quotients. Various illustrating examples are provided.

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arXiv2021

On linear continuous operators between distinguished spaces $C_p(X)$

As proved in [16], for a Tychonoff space $X$, a locally convex space $C_{p}(X)$ is distinguished if and only if $X$ is a $Δ$-space. If there exists a linear continuous surjective mapping $T:C_p(X) \to C_p(Y)$ and $C_p(X)$ is distinguished, then $C_p(Y)$ also is distinguished [17]. Firstly, in this paper we explore the following question: Under which conditions the operator $T:C_p(X) \to C_p(Y)$ above is open? Secondly, we devote a special attention to concrete distinguished spaces $C_p([1,α])$, where $α$ is a countable ordinal number. A complete characterization of all $Y$ which admit a linear continuous surjective mapping $T:C_p([1,α]) \to C_p(Y)$ is given. We also observe that for every countable ordinal $α$ all closed linear subspaces of $C_p([1,α])$ are distinguished, thereby answering an open question posed in [17]. Using some properties of $Δ$-spaces we prove that a linear continuous surjection $T:C_p(X) \to C_k(X)_w$, where $C_k(X)_w$ denotes the Banach space $C(X)$ endowed with its weak topology, does not exist for every infinite metrizable compact $C$-space $X$ (in particular, for every infinite compact $X \subset \mathbb{R}^n$).

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arXiv2021

Basic properties of $X$ for which spaces $C_p(X)$ are distinguished

In our paper [18] we showed that a Tychonoff space $X$ is a $Δ$-space (in the sense of [20], [30]) if and only if the locally convex space $C_{p}(X)$ is distinguished. Continuing this research, we investigate whether the class $Δ$ of $Δ$-spaces is invariant under the basic topological operations. We prove that if $X \in Δ$ and $φ:X \to Y$ is a continuous surjection such that $φ(F)$ is an $F_σ$-set in $Y$ for every closed set $F \subset X$, then also $Y\in Δ$. As a consequence, if $X$ is a countable union of closed subspaces $X_i$ such that each $X_i\in Δ$, then also $X\in Δ$. In particular, $σ$-product of any family of scattered Eberlein compact spaces is a $Δ$-space and the product of a $Δ$-space with a countable space is a $Δ$-space. Our results give answers to several open problems posed in \cite{KL}. Let $T:C_p(X) \longrightarrow C_p(Y)$ be a continuous linear surjection. We observe that $T$ admits an extension to a linear continuous operator $\widehat{T}$ from $R^X$ onto $R^Y$ and deduce that $Y$ is a $Δ$-space whenever $X$ is. Similarly, assuming that $X$ and $Y$ are metrizable spaces, we show that $Y$ is a $Q$-set whenever $X$ is. Making use of obtained results, we provide a very short proof for the claim that every compact $Δ$-space has countable tightness. As a consequence, under Proper Forcing Axiom (PFA) every compact $Δ$-space is sequential. In the article we pose a dozen open questions.

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arXiv2020

A characterization of $X$ for which spaces $C_p(X)$ are distinguished and its applications

We prove that the locally convex space $C_{p}(X)$ of continuous real-valued functions on a Tychonoff space $X$ equipped with the topology of pointwise convergence is distinguished if and only if $X$ is a $Δ$-space in the sense of \cite {Knight}. As an application of this characterization theorem we obtain the following results: 1) If $X$ is a Čech-complete (in particular, compact) space such that $C_p(X)$ is distinguished, then $X$ is scattered. 2) For every separable compact space of the Isbell--Mrówka type $X$, the space $C_p(X)$ is distinguished. 3) If $X$ is the compact space of ordinals $[0,ω_1]$, then $C_p(X)$ is not distinguished. We observe that the existence of an uncountable separable metrizable space $X$ such that $C_p(X)$ is distinguished, is independent of ZFC. We explore also the question to which extent the class of $Δ$-spaces is invariant under basic topological operations.

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arXiv2016

Topological properties of function spaces over ordinals

A topological space $X$ is said to be an Ascoli space if any compact subset $K$ of $C_k(X)$ is evenly continuous. This definition is motivated by the classical Ascoli theorem. We study the $k_R$-property and the Ascoli property of $C_p(κ)$ and $C_k(κ)$ over ordinals $κ$. We prove that $C_p(κ)$ is always an Ascoli space, while $C_p(κ)$ is a $k_R$-space iff the cofinality of $κ$ is countable. In particular, this provides the first $C_p$-example of an Ascoli space which is not a $k_R$-space, namely $C_p(ω_1)$. We show that $C_k(κ)$ is Ascoli iff $cf(κ)$ is countable iff $C_k(κ)$ is metrizable.

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arXiv2016

The Ascoli property for function spaces

The paper deals with Ascoli spaces $C_p(X)$ and $C_k(X)$ over Tychonoff spaces $X$. The class of Ascoli spaces $X$, i.e. spaces $X$ for which any compact subset $K$ of $C_k(X)$ is evenly continuous, essentially includes the class of $k_{\mathbb R}$-spaces. First we prove that if $C_p(X)$ is Ascoli, then it is $κ$-Fréchet-Urysohn. If $X$ is cosmic, then $C_p(X)$ is Ascoli iff it is $κ$-Fr'echet-Urysohn. This leads to the following extension of a result of Morishita: If for a Čech-complete space $X$ the space $C_p(X)$ is Ascoli, then $X$ is scattered. If $X$ is scattered and stratifiable, then $C_p(X)$ is an Ascoli space. Consequently: (a) If $X$ is a complete metrizable space, then $C_p(X)$ is Ascoli iff $X$ is scattered. (b) If $X$ is a Čech-complete Lindelöf space, then $C_p(X)$ is Ascoli iff $X$ is scattered iff $C_p(X)$ is Fréchet-Urysohn. Moreover, we prove that for a paracompact space $X$ of point-countable type the following conditions are equivalent: (i) $X$ is locally compact. (ii) $C_k(X)$ is a $k_{\mathbb R}$-space. (iii) $C_k(X)$ is an Ascoli space. The Asoli spaces $C_k(X,[0,1])$ are also studied.

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arXiv2016

Free locally convex spaces with a small base

The paper studies the free locally convex space $L(X)$ over a Tychonoff space $X$. Since for infinite $X$ the space $L(X)$ is never metrizable (even not Fréchet-Urysohn), a possible applicable generalized metric property for $L(X)$ is welcome. We propose a concept (essentially weaker than first-countability) which is known under the name a $\mathfrak{G}$-base. A space $X$ has a {\em $\mathfrak{G}$-base} if for every $x\in X$ there is a base $\{ U_α: α\in\mathbb{N}^\mathbb{N}\}$ of neighborhoods at $x$ such that $U_β\subseteq U_α$ whenever $α\leqβ$ for all $α,β\in\mathbb{N}^\mathbb{N}$, where $α=(α(n))_{n\in\mathbb{N}}\leq β=(β(n))_{n\in\mathbb{N}}$ if $α(n)\leqβ(n)$ for all $n\in\mathbb{N}$. We show that if $X$ is an Ascoli $σ$-compact space, then $L(X)$ has a $\mathfrak{G}$-base if and only if $X$ admits an Ascoli uniformity $\mathcal{U}$ with a $\mathfrak{G}$-base. We prove that if $X$ is a $σ$-compact Ascoli space of $\mathbb{N}^\mathbb{N}$-uniformly compact type, then $L(X)$ has a $\mathfrak{G}$-base. As an application we show: (1) if $X$ is a metrizable space, then $L(X)$ has a $\mathfrak{G}$-base if and only if $X$ is $σ$-compact, and (2) if $X$ is a countable Ascoli space, then $L(X)$ has a $\mathfrak{G}$-base if and only if $X$ has a $\mathfrak{G}$-base.

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arXiv2015

On topological properties of the weak topology of a Banach space

Being motivated by the famous Kaplansky theorem we study various sequential properties of a Banach space $E$ and its closed unit ball $B$, both endowed with the weak topology of $E$. We show that $B$ has the Pytkeev property if and only if $E$ in the norm topology contains no isomorphic copy of $\ell_1$, while $E$ has the Pytkeev property if and only if it is finite-dimensional. We extend Schlüchtermann and Wheeler's result by showing that $B$ is a (separable) metrizable space if and only if it has countable $cs^\ast$-character and is a $k$-space. As a corollary we obtain that $B$ is Polish if and only if it has countable $cs^\ast$-character and is Čech-complete, that supplements a result of Edgar and Wheeler.

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