<p>For an N-extremal solution <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mu \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>μ</mi> </math></EquationSource> </InlineEquation> to an indeterminate moment problem it is known by a theorem of M. Riesz that the measure <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\((1+x^2)^{-1}d\mu (x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mi>d</mi> <mi>μ</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is determinate. For <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(0&lt;\alpha &lt;1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>0</mn> <mo>&lt;</mo> <mi>α</mi> <mo>&lt;</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> we show by contradiction that there exist indeterminate N-extremal solutions <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\mu \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>μ</mi> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\((1+x^2)^{-\alpha }d\mu (x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mi>α</mi> </mrow> </msup> <mi>d</mi> <mi>μ</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is determinate, and there exist also indeterminate N-extremal solutions <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\mu \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>μ</mi> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\((1+x^2)^{-\alpha }d\mu (x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mi>α</mi> </mrow> </msup> <mi>d</mi> <mi>μ</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is indeterminate. Explicit examples of such measures are so far only known when <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\alpha =1/2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>α</mi> <mo>=</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation>. For indeterminate Stieltjes moment problems and for N-extremal solutions <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\mu \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>μ</mi> </math></EquationSource> </InlineEquation>, we show that <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\((1+x^2)^{-1/2}d\mu (x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </msup> <mi>d</mi> <mi>μ</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is indeterminate except when <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\mu =\mu _F\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>μ</mi> <mo>=</mo> <msub> <mi>μ</mi> <mi>F</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is the Friedrichs solution in case of which <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\((1+x^2)^{-1/2}d\mu _F(x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </msup> <mi>d</mi> <msub> <mi>μ</mi> <mi>F</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> is determinate. We identify the Friedrichs and Krein solutions for some indeterminate Stieltjes moment problems.</p>

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Special N-Extremal Solutions to Indeterminate Moment Problems

  • Christian Berg,
  • Ryszard Szwarc

摘要

For an N-extremal solution \(\mu \) μ to an indeterminate moment problem it is known by a theorem of M. Riesz that the measure \((1+x^2)^{-1}d\mu (x)\) ( 1 + x 2 ) - 1 d μ ( x ) is determinate. For \(0<\alpha <1\) 0 < α < 1 we show by contradiction that there exist indeterminate N-extremal solutions \(\mu \) μ such that \((1+x^2)^{-\alpha }d\mu (x)\) ( 1 + x 2 ) - α d μ ( x ) is determinate, and there exist also indeterminate N-extremal solutions \(\mu \) μ such that \((1+x^2)^{-\alpha }d\mu (x)\) ( 1 + x 2 ) - α d μ ( x ) is indeterminate. Explicit examples of such measures are so far only known when \(\alpha =1/2\) α = 1 / 2 . For indeterminate Stieltjes moment problems and for N-extremal solutions \(\mu \) μ , we show that \((1+x^2)^{-1/2}d\mu (x)\) ( 1 + x 2 ) - 1 / 2 d μ ( x ) is indeterminate except when \(\mu =\mu _F\) μ = μ F is the Friedrichs solution in case of which \((1+x^2)^{-1/2}d\mu _F(x)\) ( 1 + x 2 ) - 1 / 2 d μ F ( x ) is determinate. We identify the Friedrichs and Krein solutions for some indeterminate Stieltjes moment problems.