<p>Let (<i>a</i>(<i>j</i>)) be an unbounded convex sequence of natural numbers. In 1983, Carleson [<CitationRef CitationID="CR7">7</CitationRef>] proved that a necessary and sufficient condition for the (<i>C</i>,&#xa0;1) means of <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\((S_{a(j)}f)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>S</mi> <mrow> <mi>a</mi> <mo stretchy="false">(</mo> <mi>j</mi> <mo stretchy="false">)</mo> </mrow> </msub> <mi>f</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> (partial Fourier sums) to converge uniformly to <i>f</i> in the supremum norm is <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\sup _j j^{-1/2}\log a(j) &lt;+\infty \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mo movablelimits="true">sup</mo> <mi>j</mi> </msub> <msup> <mi>j</mi> <mrow> <mo>-</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </msup> <mo>log</mo> <mi>a</mi> <mrow> <mo stretchy="false">(</mo> <mi>j</mi> <mo stretchy="false">)</mo> </mrow> <mo>&lt;</mo> <mo>+</mo> <mi>∞</mi> </mrow> </math></EquationSource> </InlineEquation>. In this paper, we prove that if we consider Riesz logarithmic means instead of (<i>C</i>,&#xa0;1) means, then almost everywhere convergence holds for any integrable function and for any convex (<i>a</i>(<i>j</i>)). That is (see Corollary <InternalRef RefID="FPar3">1.3</InternalRef>): Let (<i>a</i>(<i>j</i>)) be any unbounded convex sequence of natural numbers and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(f\in L^1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>f</mi> <mo>∈</mo> <msup> <mi>L</mi> <mn>1</mn> </msup> </mrow> </math></EquationSource> </InlineEquation>. Then the following convergence holds almost everywhere: <Equation ID="Equ10"> <EquationSource Format="TEX">\( \frac{1}{\log n}\sum _{j=1}^{n}\frac{S_{a(j)}f}{j} \rightarrow f. \)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <mfrac> <mn>1</mn> <mrow> <mo>log</mo> <mi>n</mi> </mrow> </mfrac> <munderover> <mo>∑</mo> <mrow> <mi>j</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <mfrac> <mrow> <msub> <mi>S</mi> <mrow> <mi>a</mi> <mo stretchy="false">(</mo> <mi>j</mi> <mo stretchy="false">)</mo> </mrow> </msub> <mi>f</mi> </mrow> <mi>j</mi> </mfrac> <mo stretchy="false">→</mo> <mi>f</mi> <mo>.</mo> </mrow> </math></EquationSource> </Equation>In fact, we verify a more general statement (see Theorem <InternalRef RefID="FPar1">1.1</InternalRef>).</p>

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Riesz logarithmic means of subsequences of partial sums of trigonometric Fourier series

  • György Gát

摘要

Let (a(j)) be an unbounded convex sequence of natural numbers. In 1983, Carleson [7] proved that a necessary and sufficient condition for the (C, 1) means of \((S_{a(j)}f)\) ( S a ( j ) f ) (partial Fourier sums) to converge uniformly to f in the supremum norm is \(\sup _j j^{-1/2}\log a(j) <+\infty \) sup j j - 1 / 2 log a ( j ) < + . In this paper, we prove that if we consider Riesz logarithmic means instead of (C, 1) means, then almost everywhere convergence holds for any integrable function and for any convex (a(j)). That is (see Corollary 1.3): Let (a(j)) be any unbounded convex sequence of natural numbers and \(f\in L^1\) f L 1 . Then the following convergence holds almost everywhere: \( \frac{1}{\log n}\sum _{j=1}^{n}\frac{S_{a(j)}f}{j} \rightarrow f. \) 1 log n j = 1 n S a ( j ) f j f . In fact, we verify a more general statement (see Theorem 1.1).