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Continuous extension of functions from a segment to functions in $\mathbb{R}^n$ with zero ball means
V. V. Volchkov,
Vit. V. Volchkov Donetsk National University, 24 Universitetskaya str., Donetsk, 283001 Republic of Ukraine
Abstract:
Let
$\mathbb{R}^n$ be an Euclidean space of dimension
$n\geq 2$. For a domain
$G\subset \mathbb{R}^n$, we denote by
$V_r(G)$ the set of functions
$f\in L_{\mathrm{loc}}(G)$ having zero integrals over all closed balls of radius
$r$ contained in
$G$ (if the domain
$G$ does not contain such balls, then we set
$V_r(G)=L_{\mathrm{loc}}(G)$). Let
$E$ be a nonempty subset of
$\mathbb{R}^n$. In this paper we study the following questions related to with the extension problem.
1) Under what conditions given on
$E$ continuous function can be extended to the whole space
$\mathbb{R}^n$ to a continuous function of class
$V_r(\mathbb{R}^n)$?
2) If the above extension exists, obtain growth estimates continued function at infinity.
Theorem 1 of this paper shows that for a wide class of continuous functions on segment
$E$ defined in terms of the modulus of continuity there exists extension to a bounded function of class
$(V_r\cap C)(\mathbb{R}^n)$ regardless of the length of segment
$E$. A similar result is not true for open sets
$E$ with a diameter greater than
$2r$ even without conditions for extension growth. Theorem 1 also contains an estimate of the velocity decrease of the extended function at infinity in directions orthogonal to the segment
$E$.
As Theorem 2 shows, in the case of a space with odd dimension
$n$ Theorem 1 holds for any function continuous on
$E$ with another growth estimate. The method of proving Theorems 1 and 2 allows one to obtain similar results for functions with zero integrals over all spheres of fixed radius (in this case, an analog of Theorem 2 holds for spaces with even dimension).
Keywords:
spherical and ball means, extension problem, trigonometric series.
UDC:
517.444 Received: 21.04.2020
Revised: 04.06.2020
Accepted: 29.06.2020
DOI:
10.26907/0021-3446-2021-3-3-14