<p>Suppose that <i>G</i> is a compact Hausdorff Abelian group. We say <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mu \in M(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mo>∈</mo> <mi>M</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is strongly continuous if <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(|\mu |(x+H)=0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">|</mo> <mi>μ</mi> <mo stretchy="false">|</mo> <mo stretchy="false">(</mo> <mi>x</mi> <mo>+</mo> <mi>H</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation> for any <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(x \in G\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>x</mi> <mo>∈</mo> <mi>G</mi> </mrow> </math></EquationSource> </InlineEquation> and any <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(H \le G\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>H</mi> <mo>≤</mo> <mi>G</mi> </mrow> </math></EquationSource> </InlineEquation> that is closed and of infinite index. We prove that for any sufficiently rapidly decreasing sequence <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\((a_{n})_{n=1}^{\infty }\in c_{0}(\mathbb {N})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mrow> <mo stretchy="false">(</mo> <msub> <mi>a</mi> <mi>n</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mrow> <mi>n</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>∞</mi> </msubsup> <mo>∈</mo> <msub> <mi>c</mi> <mn>0</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">N</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, for every strongly continuous <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\mu \in M(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mo>∈</mo> <mi>M</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> with <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\Vert \mu \Vert \le 1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">‖</mo> <mi>μ</mi> <mo stretchy="false">‖</mo> <mo>≤</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\widehat{\mu }(\widehat{G})\subset \{a_n: n \in \mathbb {N}\}\cup \{0\}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mi>μ</mi> <mo stretchy="true">^</mo> </mover> <mrow> <mo stretchy="false">(</mo> <mover accent="true"> <mi>G</mi> <mo stretchy="true">^</mo> </mover> <mo stretchy="false">)</mo> </mrow> <mo>⊂</mo> <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> <mo>∪</mo> <mrow> <mo stretchy="false">{</mo> <mn>0</mn> <mo stretchy="false">}</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, the measure <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\mu *\mu \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mrow /> <mo>∗</mo> <mi>μ</mi> </mrow> </math></EquationSource> </InlineEquation> is absolutely continuous with respect to Haar measure on <i>G</i>. This implies that <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\mu \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>μ</mi> </math></EquationSource> </InlineEquation> does not exhibit the so-called Wiener-Pitt phenomenon. The paper is a continuation of investigations started in the article ‘On the relationships between Fourier-Stieltjes coefficients and spectra of measures’ published in 2014.</p>

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Wiener-Pitt sets for compact Abelian groups

  • Przemysław Ohrysko,
  • Tom Sanders,
  • Michał Wojciechowski

摘要

Suppose that G is a compact Hausdorff Abelian group. We say \(\mu \in M(G)\) μ M ( G ) is strongly continuous if \(|\mu |(x+H)=0\) | μ | ( x + H ) = 0 for any \(x \in G\) x G and any \(H \le G\) H G that is closed and of infinite index. We prove that for any sufficiently rapidly decreasing sequence \((a_{n})_{n=1}^{\infty }\in c_{0}(\mathbb {N})\) ( a n ) n = 1 c 0 ( N ) , for every strongly continuous \(\mu \in M(G)\) μ M ( G ) with \(\Vert \mu \Vert \le 1\) μ 1 and \(\widehat{\mu }(\widehat{G})\subset \{a_n: n \in \mathbb {N}\}\cup \{0\}\) μ ^ ( G ^ ) { a n : n N } { 0 } , the measure \(\mu *\mu \) μ μ is absolutely continuous with respect to Haar measure on G. This implies that \(\mu \) μ does not exhibit the so-called Wiener-Pitt phenomenon. The paper is a continuation of investigations started in the article ‘On the relationships between Fourier-Stieltjes coefficients and spectra of measures’ published in 2014.