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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 $l$.
Received: 18.06.1975
Citation:
T. V. Radoslavova, “Local properties of functions and approximation by trigonometric polynomials”, Mat. Zametki, 18:4 (1975), 527–539; Math. Notes, 18:4 (1975), 903–910
Linking options:
https://www.mathnet.ru/eng/mzm9967 https://www.mathnet.ru/eng/mzm/v18/i4/p527
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Abstract page: | 203 | Full-text PDF : | 94 | First page: | 1 |
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