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JOURNALS // Matematicheskii Sbornik // Archive

Mat. Sb., 2003 Volume 194, Number 10, Pages 77–106 (Mi sm774)

This article is cited in 17 papers

On the $L^p_\mu$-strong property of orthonormal systems

M. G. Grigoryan

Yerevan State University

Abstract: Let $\{\varphi_n(x)\}$ be a system of bounded functions complete and orthonormal in $L^2_{[0,1]}$ and assume that $\|\varphi_n\|_{p_0}\leqslant\mathrm{const}$, $n\geqslant 1$, for some $p_0>2$. Then the elements of the system can be rearranged so that the resulting system has the $L^p_\mu$-strong property: for each $\varepsilon>0$ there exists a (measurable) subset $E\subset[0,1]$ of measure $|E|>1-\varepsilon$ and a measurable function $\mu(x)$, $0<\mu(x)\leqslant 1$, $\mu(x)=1$ on $E$ such that for all $p>2$ and $f(x)\in L^p_\mu[0,1]$ one can find a function $g(x)\in L^1_{[0,1]}$ coinciding with $f(x)$ on $E$ such that its Fourier series in the system $\{\varphi_{\sigma(k)}(x)\}$ converges to $g(x)$ in the $L^p_\mu[0,1]$-norm and the sequence of Fourier coefficients of this function belongs to all spaces $l^q$, $q>2$.

UDC: 517.51

MSC: 42C15, 42C20

Received: 24.10.2002

DOI: 10.4213/sm774


 English version:
Sbornik: Mathematics, 2003, 194:10, 1503–1532

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