<p>A topological space <i>Y</i> has the property (B) of Banakh if there is a countable family <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1739_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="100" /> </InlineMediaObject> <EquationSource Format="TEX">\(\{A_n:n\in \mathbb {N}\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <msub> <mi>A</mi> <mi>n</mi> </msub> <mo>:</mo> <mi>n</mi> <mo>∈</mo> <mi mathvariant="double-struck">N</mi> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation> of closed nowhere dense subsets of <i>Y</i> absorbing all compact subsets of <i>Y</i>. In this note we show that the space <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1739_Article_IEq2.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="48" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_p(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>C</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> of continuous real–valued functions on a Tychonoff space <i>X</i> with the topology of pointwise convergence, fails to satisfy the property (B) if and only if the space <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1739_Article_IEq2.gif" Format="GIF" Height="20" Rendition="HTML" Resolution="72" Type="Linedraw" Width="48" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_p(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>C</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> satisfies the <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1739_Article_IEq4.gif" Format="GIF" Height="10" Rendition="HTML" Resolution="72" Type="Linedraw" Width="13" /> </InlineMediaObject> <EquationSource Format="TEX">\(\kappa \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>κ</mi> </math></EquationSource> </InlineEquation>–Fréchet–Urysohn property. Additionally, we provide an analogous characterization for the compact–open topology on <i>C</i>(<i>X</i>). Finally, we give examples of Tychonoff spaces <i>X</i> whose all bounded subsets are finite, yet <i>X</i> fails to have the property <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1739_Article_IEq5.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="24" /> </InlineMediaObject> <EquationSource Format="TEX">\((\kappa )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <mi>κ</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. This answers a question of Tkachuk.</p>

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Characterizing function spaces which have the property (B) of Banakh

  • Mikołaj Krupski,
  • Kacper Kucharski,
  • Witold Marciszewski

摘要

A topological space Y has the property (B) of Banakh if there is a countable family \(\{A_n:n\in \mathbb {N}\}\) { A n : n N } of closed nowhere dense subsets of Y absorbing all compact subsets of Y. In this note we show that the space \(C_p(X)\) C p ( X ) of continuous real–valued functions on a Tychonoff space X with the topology of pointwise convergence, fails to satisfy the property (B) if and only if the space \(C_p(X)\) C p ( X ) satisfies the \(\kappa \) κ –Fréchet–Urysohn property. Additionally, we provide an analogous characterization for the compact–open topology on C(X). Finally, we give examples of Tychonoff spaces X whose all bounded subsets are finite, yet X fails to have the property \((\kappa )\) ( κ ) . This answers a question of Tkachuk.