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Characterizations of L 1 -predual spaces by centerable subsets

Yanzheng Duan, Bor-Luh Lin (2007)

Commentationes Mathematicae Universitatis Carolinae

In this note, we prove that a real or complex Banach space X is an L 1 -predual space if and only if every four-point subset of X is centerable. The real case sharpens Rao’s result in [Chebyshev centers and centerable sets, Proc. Amer. Math. Soc. 130 (2002), no. 9, 2593–2598] and the complex case is closely related to the characterizations of L 1 -predual spaces by Lima [Complex Banach spaces whose duals are L 1 -spaces, Israel J. Math. 24 (1976), no. 1, 59–72].

Chebyshev centers in hyperplanes of c 0

Libor Veselý (2002)

Czechoslovak Mathematical Journal

We give a full characterization of the closed one-codimensional subspaces of c 0 , in which every bounded set has a Chebyshev center. It turns out that one can consider equivalently only finite sets (even only three-point sets) in our case, but not in general. Such hyperplanes are exactly those which are either proximinal or norm-one complemented.

Chebyshev coficients for L1-preduals and for spaces with the extension property.

José Manuel Bayod Bayod, María Concepción Masa Noceda (1990)

Publicacions Matemàtiques

We apply the Chebyshev coefficients λf and λb, recently introduced by the authors, to obtain some results related to certain geometric properties of Banach spaces. We prove that a real normed space E is an L1-predual if and only if λf(E) = 1/2, and that if a (real or complex) normed space E is a P1 space, then λb(E) equals λb(K), where K is the ground field of E.

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