<p>We establish interpolation analogs of Lebesgue-type inequalities on the sets <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq1.gif" Format="GIF" Height="26" Rendition="HTML" Resolution="72" Type="Linedraw" Width="41" /> </InlineMediaObject> <EquationSource Format="TEX">\(C^{\psi }_{\beta }L_{1}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msubsup> <mi>C</mi> <mi>β</mi> <mi>ψ</mi> </msubsup> <msub> <mi>L</mi> <mn>1</mn> </msub> </mrow> </math></EquationSource> </InlineEquation> of <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq2.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\(2\pi \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>2</mn> <mi>π</mi> </mrow> </math></EquationSource> </InlineEquation>-periodic functions <i>f</i> defined as convolutions of the generating kernel <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq3.gif" Format="GIF" Height="43" Rendition="HTML" Resolution="72" Type="Linedraw" Width="262" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Psi _{\beta }(t) = \displaystyle \sum \nolimits _{k = 1}^{\infty }\psi (k)\cos \bigg (kt-\dfrac{\beta \pi }{2}\bigg ),\)</EquationSource> <EquationSource Format="MATHML"><math> <mstyle displaystyle="true" scriptlevel="0"> <mrow> <msub> <mi mathvariant="normal">Ψ</mi> <mi>β</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>t</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msubsup> <mo>∑</mo> <mrow> <mi>k</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>∞</mi> </msubsup> <mi>ψ</mi> <mrow> <mo stretchy="false">(</mo> <mi>k</mi> <mo stretchy="false">)</mo> </mrow> <mo>cos</mo> <mrow> <mo maxsize="2.047em" minsize="2.047em" stretchy="true">(</mo> </mrow> <mi>k</mi> <mi>t</mi> <mo>-</mo> <mstyle displaystyle="true" scriptlevel="0"> <mfrac> <mrow> <mi>β</mi> <mi>π</mi> </mrow> <mn>2</mn> </mfrac> </mstyle> <mrow> <mo maxsize="2.047em" minsize="2.047em" stretchy="true">)</mo> </mrow> <mo>,</mo> </mrow> </mstyle> </math></EquationSource> </InlineEquation> <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq4.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="72" /> </InlineMediaObject> <EquationSource Format="TEX">\(\psi (k)\ge 0,\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ψ</mi> <mo stretchy="false">(</mo> <mi>k</mi> <mo stretchy="false">)</mo> <mo>≥</mo> <mn>0</mn> <mo>,</mo> </mrow> </math></EquationSource> </InlineEquation> <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq5.gif" Format="GIF" Height="31" Rendition="HTML" Resolution="72" Type="Linedraw" Width="130" /> </InlineMediaObject> <EquationSource Format="TEX">\(\displaystyle \sum \nolimits _{k = 1}^{\infty }\psi (k)&lt;\infty ,\)</EquationSource> <EquationSource Format="MATHML"><math> <mstyle displaystyle="true" scriptlevel="0"> <mrow> <msubsup> <mo>∑</mo> <mrow> <mi>k</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>∞</mi> </msubsup> <mi>ψ</mi> <mrow> <mo stretchy="false">(</mo> <mi>k</mi> <mo stretchy="false">)</mo> </mrow> <mo>&lt;</mo> <mi>∞</mi> <mo>,</mo> </mrow> </mstyle> </math></EquationSource> </InlineEquation> <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq6.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="50" /> </InlineMediaObject> <EquationSource Format="TEX">\(\beta \in \mathbb {R},\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>β</mi> <mo>∈</mo> <mi mathvariant="double-struck">R</mi> <mo>,</mo> </mrow> </math></EquationSource> </InlineEquation> with functions <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq7.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="16" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>φ</mi> </math></EquationSource> </InlineEquation> from <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq8.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="25" /> </InlineMediaObject> <EquationSource Format="TEX">\(L_{1}.\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mn>1</mn> </msub> <mo>.</mo> </mrow> </math></EquationSource> </InlineEquation> In these inequalities, for each <InlineEquation ID="IEq9"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq9.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="48" /> </InlineMediaObject> <EquationSource Format="TEX">\(x\in \mathbb {R},\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>x</mi> <mo>∈</mo> <mi mathvariant="double-struck">R</mi> <mo>,</mo> </mrow> </math></EquationSource> </InlineEquation> the moduli of deviations <InlineEquation ID="IEq10"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq10.gif" Format="GIF" Height="22" Rendition="HTML" Resolution="72" Type="Linedraw" Width="141" /> </InlineMediaObject> <EquationSource Format="TEX">\(|f(x)- \tilde{S}_{n-1}(f;x)|\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mo stretchy="false">|</mo> <mi>f</mi> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo>-</mo> </mrow> <msub> <mover accent="true"> <mi>S</mi> <mo stretchy="false">~</mo> </mover> <mrow> <mi>n</mi> <mo>-</mo> <mn>1</mn> </mrow> </msub> <mrow> <mrow> <mo stretchy="false">(</mo> <mi>f</mi> <mo>;</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">|</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> of the interpolation Lagrange polynomials <InlineEquation ID="IEq11"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq11.gif" Format="GIF" Height="22" Rendition="HTML" Resolution="72" Type="Linedraw" Width="69" /> </InlineMediaObject> <EquationSource Format="TEX">\( \tilde{S}_{n-1}(f;\cdot )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mover accent="true"> <mi>S</mi> <mo stretchy="false">~</mo> </mover> <mrow> <mi>n</mi> <mo>-</mo> <mn>1</mn> </mrow> </msub> <mrow> <mo stretchy="false">(</mo> <mi>f</mi> <mo>;</mo> <mo>·</mo> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> are estimated via the best approximations of functions <InlineEquation ID="IEq12"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq7.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="16" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>φ</mi> </math></EquationSource> </InlineEquation> by trigonometric polynomials in <InlineEquation ID="IEq13"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq13.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">\(L_{1}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>L</mi> <mn>1</mn> </msub> </math></EquationSource> </InlineEquation>-metrics. If the sequences <InlineEquation ID="IEq14"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq14.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="36" /> </InlineMediaObject> <EquationSource Format="TEX">\(\psi (k)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ψ</mi> <mo stretchy="false">(</mo> <mi>k</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> decrease to zero faster than any power function, then the obtained inequalities are asymptotically exact in numerous important cases. In these cases, we also establish asymptotic equalities for the exact upper bounds of pointwise approximations by interpolation trigonometric polynomials in the classes of convolutions of the generating kernel <InlineEquation ID="IEq15"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq15.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="23" /> </InlineMediaObject> <EquationSource Format="TEX">\(\Psi _{\beta }\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi mathvariant="normal">Ψ</mi> <mi>β</mi> </msub> </math></EquationSource> </InlineEquation> with functions <InlineEquation ID="IEq16"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq7.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="16" /> </InlineMediaObject> <EquationSource Format="TEX">\(\varphi \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>φ</mi> </math></EquationSource> </InlineEquation> that belong to the unit ball in the space&#xa0;<InlineEquation ID="IEq17"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10958_2025_7877_Article_IEq8.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="25" /> </InlineMediaObject> <EquationSource Format="TEX">\(L_{1}.\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mn>1</mn> </msub> <mo>.</mo> </mrow> </math></EquationSource> </InlineEquation></p>

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ESTIMATES OF APPROXIMATIONS BY INTERPOLATION TRIGONOMETRIC POLYNOMIALS ON THE CLASSES OF CONVOLUTIONS OF PERIODIC FUNCTIONS OF HIGH SMOOTHNESS

  • Anatolii Serdyuk,
  • Tetiana Stepaniuk

摘要

We establish interpolation analogs of Lebesgue-type inequalities on the sets \(C^{\psi }_{\beta }L_{1}\) C β ψ L 1 of \(2\pi \) 2 π -periodic functions f defined as convolutions of the generating kernel \(\Psi _{\beta }(t) = \displaystyle \sum \nolimits _{k = 1}^{\infty }\psi (k)\cos \bigg (kt-\dfrac{\beta \pi }{2}\bigg ),\) Ψ β ( t ) = k = 1 ψ ( k ) cos ( k t - β π 2 ) , \(\psi (k)\ge 0,\) ψ ( k ) 0 , \(\displaystyle \sum \nolimits _{k = 1}^{\infty }\psi (k)<\infty ,\) k = 1 ψ ( k ) < , \(\beta \in \mathbb {R},\) β R , with functions \(\varphi \) φ from \(L_{1}.\) L 1 . In these inequalities, for each \(x\in \mathbb {R},\) x R , the moduli of deviations \(|f(x)- \tilde{S}_{n-1}(f;x)|\) | f ( x ) - S ~ n - 1 ( f ; x ) | of the interpolation Lagrange polynomials \( \tilde{S}_{n-1}(f;\cdot )\) S ~ n - 1 ( f ; · ) are estimated via the best approximations of functions \(\varphi \) φ by trigonometric polynomials in \(L_{1}\) L 1 -metrics. If the sequences \(\psi (k)\) ψ ( k ) decrease to zero faster than any power function, then the obtained inequalities are asymptotically exact in numerous important cases. In these cases, we also establish asymptotic equalities for the exact upper bounds of pointwise approximations by interpolation trigonometric polynomials in the classes of convolutions of the generating kernel \(\Psi _{\beta }\) Ψ β with functions \(\varphi \) φ that belong to the unit ball in the space  \(L_{1}.\) L 1 .