<p>Given any compact Hausdorff space <i>X</i>, we present a simple proof that a continuous function <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(g\in \mathcal {C}(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>g</mi> <mo>∈</mo> <mi mathvariant="script">C</mi> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> can be <i>uniformly</i> approximated on <i>X</i> by elements of some linear subspace <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mathcal {L}\subset \mathcal {C}(X)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">L</mi> <mo>⊂</mo> <mi mathvariant="script">C</mi> <mo stretchy="false">(</mo> <mi>X</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, if and only if <i>g</i> can be <i>pointwise</i> approximated on <i>X</i> by some equibounded sequence in <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathcal {L}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">L</mi> </math></EquationSource> </InlineEquation>. Moreover, given any compactum <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(K\subset \mathbb {C}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>K</mi> <mo>⊂</mo> <mi mathvariant="double-struck">C</mi> </mrow> </math></EquationSource> </InlineEquation>, we also show that every <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(f\in \mathcal {C}(K)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>f</mi> <mo>∈</mo> <mi mathvariant="script">C</mi> <mo stretchy="false">(</mo> <mi>K</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> can be <i>uniformly</i> approximated by rational functions (without poles on <i>K</i>), if and only if the complex conjugate <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(w\mapsto \overline{w}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>w</mi> <mo>↦</mo> <mover> <mi>w</mi> <mo>¯</mo> </mover> </mrow> </math></EquationSource> </InlineEquation> can be <i>pointwise</i> approximated by functions holomorphic on <i>K</i> (no equibounded hypothesis required).</p>

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Equibounded pointwise approximation implies the uniform one

  • Eduardo S. Zeron,
  • Jesús Emmanuel Castillo

摘要

Given any compact Hausdorff space X, we present a simple proof that a continuous function \(g\in \mathcal {C}(X)\) g C ( X ) can be uniformly approximated on X by elements of some linear subspace \(\mathcal {L}\subset \mathcal {C}(X)\) L C ( X ) , if and only if g can be pointwise approximated on X by some equibounded sequence in \(\mathcal {L}\) L . Moreover, given any compactum \(K\subset \mathbb {C}\) K C , we also show that every \(f\in \mathcal {C}(K)\) f C ( K ) can be uniformly approximated by rational functions (without poles on K), if and only if the complex conjugate \(w\mapsto \overline{w}\) w w ¯ can be pointwise approximated by functions holomorphic on K (no equibounded hypothesis required).