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 Title: Essential norms of the Neumann operator of the arithmetical mean (English) Author: Král, Josef Author: Medková, Dagmar Language: English Journal: Mathematica Bohemica ISSN: 0862-7959 (print) ISSN: 2464-7136 (online) Volume: 126 Issue: 4 Year: 2001 Pages: 669-690 Summary lang: English . Category: math . Summary: Let $K\subset \mathbb{R}^m$ ($m\ge 2$) be a compact set; assume that each ball centered on the boundary $B$ of $K$ meets $K$ in a set of positive Lebesgue measure. Let ${C}_0^{(1)}$ be the class of all continuously differentiable real-valued functions with compact support in $\mathbb{R}^m$ and denote by $\sigma _m$ the area of the unit sphere in $\mathbb{R}^m$. With each $\varphi \in {C}_0^{(1)}$ we associate the function $W_K\varphi (z)={1\over \sigma _m}\underset{\mathbb{R}^m \setminus K}{\rightarrow }\int \mathop {\mathrm grad}\nolimits \varphi (x)\cdot {z-x\over |z-x|^m}\ x$ of the variable $z\in K$ (which is continuous in $K$ and harmonic in $K\setminus B$). $W_K\varphi$ depends only on the restriction $\varphi |_B$ of $\varphi$ to the boundary $B$ of $K$. This gives rise to a linear operator $W_K$ acting from the space ${C}^{(1)}(B)=\lbrace \varphi |_B; \varphi \in {C}_0^{(1)}\rbrace$ to the space ${C}(B)$ of all continuous functions on $B$. The operator ${T}_K$ sending each $f\in {C}^{(1)}(B)$ to ${T}_Kf=2W_Kf-f \in {C}(B)$ is called the Neumann operator of the arithmetical mean; it plays a significant role in connection with boundary value problems for harmonic functions. If $p$ is a norm on ${C}(B)\supset {C}^{(1)}(B)$ inducing the topology of uniform convergence and $G$ is the space of all compact linear operators acting on ${C}(B)$, then the associated $p$-essential norm of ${T}_K$ is given by $\omega _p {T}_K=\underset{Q\in {G}}{\rightarrow }\inf \sup \bigl \lbrace p[({T}_K-Q)f]; \ f\in {C}^{(1)}(B), \ p(f)\le 1\bigr \rbrace .$ In the present paper estimates (from above and from below) of $\omega _p {T}_K$ are obtained resulting in precise evaluation of $\omega _p {T}_K$ in geometric terms connected only with $K$. (English) Keyword: double layer potential Keyword: Neumann’s operator of the arithmetical mean Keyword: essential norm MSC: 31B10 MSC: 45P05 MSC: 47A30 MSC: 47G10 idZBL: Zbl 0998.31003 idMR: MR1869461 DOI: 10.21136/MB.2001.134114 . Date available: 2009-09-24T21:55:56Z Last updated: 2020-07-29 Stable URL: http://hdl.handle.net/10338.dmlcz/134114 . 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