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JOURNALS // Matematicheskie Zametki // Archive

Mat. Zametki, 1975 Volume 18, Issue 1, Pages 77–90 (Mi mzm7628)

This article is cited in 1 paper

Approximation of integrable functions by linear methods almost everywhere

T. V. Radoslavova

V. A. Steklov Mathematical Institute, USSR Academy of Sciences

Abstract: It is shown that $2\pi$ periodic functions whose $(r-1)$-th derivatives have bounded variation $(r>0)$ can be approximated by de La Vallée-Poussin $\sigma_{n,m}(an\le m=m(n)\le An, 0<a<A<1)$ at almost all points with a rate $o(n^{--r})$. For functions belonging to the class $\operatorname{Lip}(\alpha,L)(0<\alpha<1)$, any natural $N$, and a positive $\varepsilon>0$, we have almost everywhere
$$ |f(x)-\sigma_{n,m}(f;x)|\le c(f,x)n^{-\alpha}\ln n\dots\ln_N^{1+\varepsilon}n, $$
where $\ln_kx=\underbrace{\ln\dots\ln x}_k(k=1,2,\dots)$. For any triangular method of summation $T$ with bounded coefficients we construct functions belonging to $\operatorname{Lip}(\alpha,L)(0<\alpha<1)$ and such that almost everywhere,
$$ \varlimsup_{n\to\infty}|f(x)-\tau_n(f;x)|n^\alpha(\ln n\dots\ln_Nn)^{-\alpha}=\infty, $$
where the $\tau_n(f;x)$ are the means of the method $T$.

UDC: 517.5

Received: 04.11.1974


 English version:
Mathematical Notes, 1975, 18:1, 628–636

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