<p>For a Tychonoff space <i>X</i> by <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq3.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> and <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq4.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="48" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_k(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>C</mi> <mi>k</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> we denote the space <i>C</i>(<i>X</i>) of continuous real valued functions on <i>X</i> endowed with the pointwise topology <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq5.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="16" /> </InlineMediaObject> <EquationSource Format="TEX">\(\tau _{p}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>τ</mi> <mi>p</mi> </msub> </math></EquationSource> </InlineEquation> and the compact-open topology <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq6.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">\(\tau _{k}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>τ</mi> <mi>k</mi> </msub> </math></EquationSource> </InlineEquation>, respectively. If <i>X</i> is a pseudocompact space, then the uniform topology <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq7.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="22" /> </InlineMediaObject> <EquationSource Format="TEX">\(\tau _{\infty }\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>τ</mi> <mi>∞</mi> </msub> </math></EquationSource> </InlineEquation> on <i>C</i>(<i>X</i>) is generated by the sup norm <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq8.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="46" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Vert \cdot \Vert _{\infty }\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mrow> <mo stretchy="false">‖</mo> <mo>·</mo> <mo stretchy="false">‖</mo> </mrow> <mi>∞</mi> </msub> </math></EquationSource> </InlineEquation>; clearly <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq9.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="281" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_{\infty }(X)=(C(X), \tau _{\infty })=(C(X), \Vert \cdot \Vert _{\infty })\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>C</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mrow> <mo stretchy="false">(</mo> <mi>C</mi> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <msub> <mi>τ</mi> <mi>∞</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msub> <mrow> <mo stretchy="false">(</mo> <mi>C</mi> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mo stretchy="false">‖</mo> <mo>·</mo> <mo stretchy="false">‖</mo> </mrow> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is a Banach space. We characterize <InlineEquation ID="IEq10"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq1.gif" Format="GIF" Height="10" Rendition="HTML" Resolution="72" Type="Linedraw" Width="14" /> </InlineMediaObject> <EquationSource Format="TEX">\(\omega \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>ω</mi> </math></EquationSource> </InlineEquation>-bounded (hence also compact) scattered spaces in terms of <InlineEquation ID="IEq11"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq11.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="54" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_{\infty }(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>C</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq12"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq3.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> (supplementing results characterizing compact scattered spaces <i>X</i> due to Namioka-Phelps, Gerlits, Pytkeev, Pełczyński-Semadeni, Lotz-Peck-Porta and Ka̧kol-Kurka). Applications yielding a <InlineEquation ID="IEq13"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq13.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_p\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>C</mi> <mi>p</mi> </msub> </math></EquationSource> </InlineEquation>-version of the Pełczyński-Semadeni theorem are studied. We prove (among other results): A compact space <i>X</i> is scattered if and only if every infinite-dimensional <InlineEquation ID="IEq14"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq7.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="22" /> </InlineMediaObject> <EquationSource Format="TEX">\(\tau _{\infty }\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>τ</mi> <mi>∞</mi> </msub> </math></EquationSource> </InlineEquation>-closed subspace of <InlineEquation ID="IEq15"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq3.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> contains an isomorphic copy of <InlineEquation ID="IEq16"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq16.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="15" /> </InlineMediaObject> <EquationSource Format="TEX">\(c_0\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>c</mi> <mn>0</mn> </msub> </math></EquationSource> </InlineEquation> (with the pointwise topology) if and only if every infinite-dimensional subspace of <InlineEquation ID="IEq17"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq3.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> contains an isomorphic copy of <InlineEquation ID="IEq18"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1745_Article_IEq18.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(c_{00}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>c</mi> <mn>00</mn> </msub> </math></EquationSource> </InlineEquation> (with the pointwise topology). Illustrating examples and several open problems are also provided.</p>

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A characterization of scattered compact (and \(\omega \)-bounded) spaces

  • Jerzy Ka̧kol,
  • Salvador López-Alfonso,
  • Wiesław Śliwa

摘要

For a Tychonoff space X by \(C_p(X)\) C p ( X ) and \(C_k(X)\) C k ( X ) we denote the space C(X) of continuous real valued functions on X endowed with the pointwise topology \(\tau _{p}\) τ p and the compact-open topology \(\tau _{k}\) τ k , respectively. If X is a pseudocompact space, then the uniform topology \(\tau _{\infty }\) τ on C(X) is generated by the sup norm \(\Vert \cdot \Vert _{\infty }\) · ; clearly \(C_{\infty }(X)=(C(X), \tau _{\infty })=(C(X), \Vert \cdot \Vert _{\infty })\) C ( X ) = ( C ( X ) , τ ) = ( C ( X ) , · ) is a Banach space. We characterize \(\omega \) ω -bounded (hence also compact) scattered spaces in terms of \(C_{\infty }(X)\) C ( X ) and \(C_p(X)\) C p ( X ) (supplementing results characterizing compact scattered spaces X due to Namioka-Phelps, Gerlits, Pytkeev, Pełczyński-Semadeni, Lotz-Peck-Porta and Ka̧kol-Kurka). Applications yielding a \(C_p\) C p -version of the Pełczyński-Semadeni theorem are studied. We prove (among other results): A compact space X is scattered if and only if every infinite-dimensional \(\tau _{\infty }\) τ -closed subspace of \(C_p(X)\) C p ( X ) contains an isomorphic copy of \(c_0\) c 0 (with the pointwise topology) if and only if every infinite-dimensional subspace of \(C_p(X)\) C p ( X ) contains an isomorphic copy of \(c_{00}\) c 00 (with the pointwise topology). Illustrating examples and several open problems are also provided.