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Convergence of Trigonometric Fourier Series of Functions with a Constraint on the Fractality of Their Graphs
Proceedings of the Steklov Institute of Mathematics ( IF 0.4 ) Pub Date : 2020-05-28 , DOI: 10.1134/s008154382002008x
M. L. Gridnev

For a function f continuous on a closed interval, its modulus of fractality ν(f, ε) is defined as the function that maps any ε > 0 to the smallest number of squares of size ε that cover the graph of f. The following condition for the uniform convergence of the Fourier series of f is obtained in terms of the modulus of fractality and the modulus of continuity ω(f, δ): if$$\begin{array}{*{20}{c}} {\omega (f,\pi /n)\ln \left( {\frac{{v(f,\pi /n)}}{n}} \right) \to 0}&{\text{as}}&{n \to + \infty ,} \end{array}$$then the Fourier series of f converges uniformly. This condition refines the known Dini–Lipschitz test. In addition, for the growth order of the partial sums Sn(f, x) of a continuous function f, we derive an estimate that is uniform in x ∈ [0, 2π]:$${S_n}(f,x) = o\left( {\ln \left( {\frac{{v(f,\pi /n)}}{n}} \right)} \right).$$.The optimality of this estimate is shown.

中文翻译:

函数三角图傅里叶级数的收敛性

对于在闭合区间上连续的函数f,其分形模数νf,ε)定义为将ε > 0映射到覆盖f的图的最小大小为ε的平方的函数。根据分形模量和连续模量ωf,δ得到f的傅里叶级数的均匀收敛的以下条件:if $$ \ begin {array} {* {20} {c} } {\ omega(f,\ pi / n)\ ln \ left({\ frac {{v(f,\ pi / n)}} {n}} \ right)\ to 0}&{\ text {as }}&{n \ to + \ infty,} \ end {array} $$然后是f的傅立叶级数均匀收敛。此条件完善了已知的Dini–Lipschitz检验。另外,对于部分和的生长顺序小号ÑF,X的连续函数的)˚F,我们导出的估计是在均匀的X ∈[0,2 π ]:$$ {S_N}(F,X )= o \ left({\ ln \ left({\ frac {{v(f,\ pi / n)}} {n}} \ right)} \ right)。$$表示此估计的最优性。
更新日期:2020-05-28
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