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On the structure of the complements of Chebyshev sets. (English. Russian original) Zbl 1099.41501

Funct. Anal. Appl. 35, No. 3, 176-182 (2001); translation from Funkts. Anal. Prilozh. 35, No. 3, 19-27 (2001).
Summary: A set is called a Chebyshev set if it contains a unique best approximation element. We study the structure of the complements of Chebyshev sets, in particular considering the following question: How many connected components can the complement of a Chebyshev set in a finite-dimensional normed or nonsymmetrically normed linear space have? We extend some results from [A. R. Alimov, East J. Approx, 2, No. 2, 215–232 (1996; Zbl 0866.46006)]. A. L. Brown’s characterization [A. L. Brown, Proc. Lond. Math. Soc. (3) 41, No. 2, 297–339 (1980; Zbl 0419.41025)] of four-dimensional normed linear spaces in which every Chebyshev set is convex is extended to the nonsymmetric setting. A characterization of finite-dimensional spaces that contain a strict sun whose complement has a given number of connected components is established.

MSC:

41A50 Best approximation, Chebyshev systems
41A65 Abstract approximation theory (approximation in normed linear spaces and other abstract spaces)
46B20 Geometry and structure of normed linear spaces
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