<p>In this paper we consider the one-parameter family of weight functions <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mu _m(\theta )=1+\cos (m\theta )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>μ</mi> <mi>m</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>θ</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mn>1</mn> <mo>+</mo> <mo>cos</mo> <mrow> <mo stretchy="false">(</mo> <mi>m</mi> <mi>θ</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\theta \in [-\pi ,\pi ]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>θ</mi> <mo>∈</mo> <mo stretchy="false">[</mo> <mo>-</mo> <mi>π</mi> <mo>,</mo> <mi>π</mi> <mo stretchy="false">]</mo> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(m \in \mathbb {N}\cup \{ 0 \}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>m</mi> <mo>∈</mo> <mi mathvariant="double-struck">N</mi> <mo>∪</mo> <mo stretchy="false">{</mo> <mn>0</mn> <mo stretchy="false">}</mo> </mrow> </math></EquationSource> </InlineEquation>, and we obtain explicit expressions for the corresponding Szegő and para-orthogonal polynomials of the first and second kind. The associated Szegő quadrature formulas for the estimation of integrals of the form <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\int _{-\pi }^{\pi } F\left( e^{i\theta }\right) \mu _m(\theta )d\theta \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mo>∫</mo> <mrow> <mo>-</mo> <mi>π</mi> </mrow> <mi>π</mi> </msubsup> <mi>F</mi> <mfenced close=")" open="("> <msup> <mi>e</mi> <mrow> <mi>i</mi> <mi>θ</mi> </mrow> </msup> </mfenced> <msub> <mi>μ</mi> <mi>m</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>θ</mi> <mo stretchy="false">)</mo> </mrow> <mi>d</mi> <mi>θ</mi> </mrow> </math></EquationSource> </InlineEquation> are characterized, their efficient computation based on an eigenvalue problem related to CMV matrices is studied, and error bounds are obtained from the error when approximating the Herglotz-Riesz transform of <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\mu _m\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>μ</mi> <mi>m</mi> </msub> </math></EquationSource> </InlineEquation> by certain rational approximants. Most of these results are illustrated with several numerical experiments. By making use of the Joukowsky transformation, we apply the above results to develop a new procedure for the estimation of Christoffel transformations of the Chebyshev weight function on the interval, namely <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\int _{-1}^{1} f(x)\omega (x)dx\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mo>∫</mo> <mrow> <mo>-</mo> <mn>1</mn> </mrow> <mn>1</mn> </msubsup> <mi>f</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mi>ω</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mi>d</mi> <mi>x</mi> </mrow> </math></EquationSource> </InlineEquation> with <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\omega (x)=P(x)\cdot \left( 1-x^2\right) ^{-1/2}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ω</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>P</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo>·</mo> <msup> <mfenced close=")" open="("> <mn>1</mn> <mo>-</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mfenced> <mrow> <mo>-</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </msup> </mrow> </math></EquationSource> </InlineEquation> where <i>P</i> is an arbitrary real polynomial not necessarily positive on <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\([-1,1]\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">[</mo> <mo>-</mo> <mn>1</mn> <mo>,</mo> <mn>1</mn> <mo stretchy="false">]</mo> </mrow> </math></EquationSource> </InlineEquation>, by means of certain related Gauss-type quadrature formulas. Some more numerical experiments are finally carried out along with some conclusions.</p>

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Szegő quadrature formulas associated with the weight function \(1+\cos (m\theta )\) with application to the approximation of certain Christoffel transformations on the interval

  • Ruymán Cruz-Barroso,
  • Michael Díaz-Bautista

摘要

In this paper we consider the one-parameter family of weight functions \(\mu _m(\theta )=1+\cos (m\theta )\) μ m ( θ ) = 1 + cos ( m θ ) , \(\theta \in [-\pi ,\pi ]\) θ [ - π , π ] , \(m \in \mathbb {N}\cup \{ 0 \}\) m N { 0 } , and we obtain explicit expressions for the corresponding Szegő and para-orthogonal polynomials of the first and second kind. The associated Szegő quadrature formulas for the estimation of integrals of the form \(\int _{-\pi }^{\pi } F\left( e^{i\theta }\right) \mu _m(\theta )d\theta \) - π π F e i θ μ m ( θ ) d θ are characterized, their efficient computation based on an eigenvalue problem related to CMV matrices is studied, and error bounds are obtained from the error when approximating the Herglotz-Riesz transform of \(\mu _m\) μ m by certain rational approximants. Most of these results are illustrated with several numerical experiments. By making use of the Joukowsky transformation, we apply the above results to develop a new procedure for the estimation of Christoffel transformations of the Chebyshev weight function on the interval, namely \(\int _{-1}^{1} f(x)\omega (x)dx\) - 1 1 f ( x ) ω ( x ) d x with \(\omega (x)=P(x)\cdot \left( 1-x^2\right) ^{-1/2}\) ω ( x ) = P ( x ) · 1 - x 2 - 1 / 2 where P is an arbitrary real polynomial not necessarily positive on \([-1,1]\) [ - 1 , 1 ] , by means of certain related Gauss-type quadrature formulas. Some more numerical experiments are finally carried out along with some conclusions.