<p>The purpose of this article is to study the anti-coproximinal and strongly anti-coproximinal subspaces of the Banach space of all bounded (continuous) functions. We obtain a tractable necessary condition for a subspace to be stronsgly anti-coproximinal. We prove that for a subspace <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mathbb {Y}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">Y</mi> </math></EquationSource> </InlineEquation> of a Banach space <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mathbb {X}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">X</mi> </math></EquationSource> </InlineEquation> to be strongly anti-coproximinal, <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathbb {Y}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">Y</mi> </math></EquationSource> </InlineEquation> must contain all w-ALUR points of <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\mathbb {X}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">X</mi> </math></EquationSource> </InlineEquation> and intersect every maximal face of <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(B_{\mathbb {X}}.\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>B</mi> <mi mathvariant="double-struck">X</mi> </msub> <mo>.</mo> </mrow> </math></EquationSource> </InlineEquation> We also observe that the subspace <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\mathbb {K}(\mathbb {X}, \mathbb {Y})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">K</mi> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">X</mi> <mo>,</mo> <mi mathvariant="double-struck">Y</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> of all compact operators between the Banach spaces <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\( \mathbb {X} \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">X</mi> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\( \mathbb {Y}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">Y</mi> </math></EquationSource> </InlineEquation> is strongly anti-coproximinal in the space <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\mathbb {L}(\mathbb {X}, \mathbb {Y})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">L</mi> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">X</mi> <mo>,</mo> <mi mathvariant="double-struck">Y</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> of all bounded linear operators between <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\( \mathbb {X} \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">X</mi> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\( \mathbb {Y}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="double-struck">Y</mi> </math></EquationSource> </InlineEquation>, whenever <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\mathbb {K}(\mathbb {X}, \mathbb {Y})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">K</mi> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">X</mi> <mo>,</mo> <mi mathvariant="double-struck">Y</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is a proper subset of <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\mathbb {L}(\mathbb {X}, \mathbb {Y}),\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="double-struck">L</mi> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">X</mi> <mo>,</mo> <mi mathvariant="double-struck">Y</mi> <mo stretchy="false">)</mo> <mo>,</mo> </mrow> </math></EquationSource> </InlineEquation> and the unit ball <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(B_{\mathbb {X}}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>B</mi> <mi mathvariant="double-struck">X</mi> </msub> </math></EquationSource> </InlineEquation> is the closed convex hull of its strongly exposed points.</p>

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On anti-coproximinal and strongly anti-coproximinal subspaces of function spaces

  • Shamim Sohel,
  • Souvik Ghosh,
  • Debmalya Sain,
  • Kallol Paul

摘要

The purpose of this article is to study the anti-coproximinal and strongly anti-coproximinal subspaces of the Banach space of all bounded (continuous) functions. We obtain a tractable necessary condition for a subspace to be stronsgly anti-coproximinal. We prove that for a subspace \(\mathbb {Y}\) Y of a Banach space \(\mathbb {X}\) X to be strongly anti-coproximinal, \(\mathbb {Y}\) Y must contain all w-ALUR points of \(\mathbb {X}\) X and intersect every maximal face of \(B_{\mathbb {X}}.\) B X . We also observe that the subspace \(\mathbb {K}(\mathbb {X}, \mathbb {Y})\) K ( X , Y ) of all compact operators between the Banach spaces \( \mathbb {X} \) X and \( \mathbb {Y}\) Y is strongly anti-coproximinal in the space \(\mathbb {L}(\mathbb {X}, \mathbb {Y})\) L ( X , Y ) of all bounded linear operators between \( \mathbb {X} \) X and \( \mathbb {Y}\) Y , whenever \(\mathbb {K}(\mathbb {X}, \mathbb {Y})\) K ( X , Y ) is a proper subset of \(\mathbb {L}(\mathbb {X}, \mathbb {Y}),\) L ( X , Y ) , and the unit ball \(B_{\mathbb {X}}\) B X is the closed convex hull of its strongly exposed points.