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JOURNALS // Matematicheskie Zametki // Archive

Mat. Zametki, 1975 Volume 18, Issue 4, Pages 527–539 (Mi mzm9967)

Local properties of functions and approximation by trigonometric polynomials

T. V. Radoslavova

V. A. Steklov Mathematics Institute, Academy of Sciences of the USSR

Abstract: Suppose $\Phi_{p,E}$ ($p>0$ an integer, $E\subset[0,2\pi]$) is a family of positive nondecreasing functions $\varphi_x(t)$ ($t>0$, $x\in E$) such that $\varphi_x(nt)\leqslant n^p\varphi_x(t)$ ($n=0,1,\dots$), $t_n$ is a trigonometric polynomial of order at most $n$, and $\Delta_h^l(f,x)$ ($l>0$ an integer) is the finite difference of order $l$ with step $h$ of the function $f$. \underline{THEOREM.} Suppose $f(x)$ is a function which is measurable, finite almost everywhere on $[0, 2\pi]$, and integrable in some neighborhood of each point $x\in E$, $\varphi_x\in\Phi_{p,E}$ and
$$ \varlimsup_{\delta\to\infty}\left|(2\delta)^{-1}\int_{-\delta}^\delta\Delta_u^l(f,x)\,du\right|\varphi_x^{-1}(\delta)\leqslant C(x)<\infty\qquad(x\in E). $$
Then there exists a sequence $\{t_n\}_{n=1}^\infty$, which converges to $f(x)$ almost everywhere, such that for $x\in E$
$$ \varlimsup_{n\to\infty}|f(x)-t_n(x)|\varphi_x^{-1}(1/n)\leqslant AC(x), $$
where $A$ depends on $p$ and $l$.

UDC: 517.5

Received: 18.06.1975


 English version:
Mathematical Notes, 1975, 18:4, 903–910

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