<p>In this article, we study the ccs-Daugavet, ccs-<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="15" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Delta \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Δ</mi> </math></EquationSource> </InlineEquation>, super-Daugavet, super-<InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="15" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Delta \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Δ</mi> </math></EquationSource> </InlineEquation>, Daugavet, <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq1.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="15" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Delta \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">Δ</mi> </math></EquationSource> </InlineEquation>, and <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq4.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">\(\nabla \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">∇</mi> </math></EquationSource> </InlineEquation> points in the unit balls of vector-valued function spaces <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq5.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="67" /> </InlineMediaObject> <EquationSource Format="TEX">\(C_0(L, X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>C</mi> <mn>0</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>L</mi> <mo>,</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, <i>A</i>(<i>K</i>,&#xa0;<i>X</i>), <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq6.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="71" /> </InlineMediaObject> <EquationSource Format="TEX">\(L_\infty (\mu , X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>μ</mi> <mo>,</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, and <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq7.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="65" /> </InlineMediaObject> <EquationSource Format="TEX">\(L_1(\mu , X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mn>1</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>μ</mi> <mo>,</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>. To partially or fully characterize these diametral points, we first provide improvements of several stability results under <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq8.gif" Format="GIF" Height="15" Rendition="HTML" Resolution="72" Type="Linedraw" Width="29" /> </InlineMediaObject> <EquationSource Format="TEX">\(\oplus _\infty \)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mo>⊕</mo> <mi>∞</mi> </msub> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq9.gif" Format="GIF" Height="15" Rendition="HTML" Resolution="72" Type="Linedraw" Width="22" /> </InlineMediaObject> <EquationSource Format="TEX">\(\oplus _1\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mo>⊕</mo> <mn>1</mn> </msub> </math></EquationSource> </InlineEquation>-sums shown in the literature. For complex Banach spaces, <InlineEquation ID="IEq10"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq4.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">\(\nabla \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">∇</mi> </math></EquationSource> </InlineEquation> points are identical to Daugavet points, and so the study of <InlineEquation ID="IEq11"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq4.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">\(\nabla \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">∇</mi> </math></EquationSource> </InlineEquation> points only makes sense when a Banach space is real. Consequently, we obtain that the seven notions of diametral points are equivalent for <InlineEquation ID="IEq12"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="13398_2025_1757_Article_IEq12.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="48" /> </InlineMediaObject> <EquationSource Format="TEX">\(L_\infty (\mu )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mi>∞</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>μ</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and uniform algebra when <i>K</i> is infinite.</p>

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On various diametral notions of points in the unit ball of some vector-valued function spaces

  • Han Ju Lee,
  • Óscar Roldán,
  • Hyung-Joon Tag

摘要

In this article, we study the ccs-Daugavet, ccs- \(\Delta \) Δ , super-Daugavet, super- \(\Delta \) Δ , Daugavet, \(\Delta \) Δ , and \(\nabla \) points in the unit balls of vector-valued function spaces \(C_0(L, X)\) C 0 ( L , X ) , A(KX), \(L_\infty (\mu , X)\) L ( μ , X ) , and \(L_1(\mu , X)\) L 1 ( μ , X ) . To partially or fully characterize these diametral points, we first provide improvements of several stability results under \(\oplus _\infty \) and \(\oplus _1\) 1 -sums shown in the literature. For complex Banach spaces, \(\nabla \) points are identical to Daugavet points, and so the study of \(\nabla \) points only makes sense when a Banach space is real. Consequently, we obtain that the seven notions of diametral points are equivalent for \(L_\infty (\mu )\) L ( μ ) and uniform algebra when K is infinite.