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On Bárány's theorems of Carathéodory and Helly type

Ehrhard Behrends (2000)

Studia Mathematica

The paper begins with a self-contained and short development of Bárány’s theorems of Carathéodory and Helly type in finite-dimensional spaces together with some new variants. In the second half the possible generalizations of these results to arbitrary Banach spaces are investigated. The Carathéodory-Bárány theorem has a counterpart in arbitrary dimensions under suitable uniform compactness or uniform boundedness conditions. The proper generalization of the Helly-Bárány theorem reads as follows:...

On having a countable cover by sets of small local diameter

Nadezhda Ribarska (2000)

Studia Mathematica

A characterization of topological spaces admitting a countable cover by sets of small local diameter close in spirit to known characterizations of fragmentability is obtained. It is proved that if X and Y are Hausdorff compacta such that C(X) has an equivalent p-Kadec norm and C p ( Y ) has a countable cover by sets of small local norm diameter, then C p ( X × Y ) has a countable cover by sets of small local norm diameter as well.

On r -reflexive Banach spaces

Iryna Banakh, Taras O. Banakh, Elena Riss (2009)

Commentationes Mathematicae Universitatis Carolinae

A Banach space X is called r -reflexive if for any cover 𝒰 of X by weakly open sets there is a finite subfamily 𝒱 𝒰 covering some ball of radius 1 centered at a point x with x r . We prove that an infinite-dimensional separable Banach space X is -reflexive ( r -reflexive for some r ) if and only if each ε -net for X has an accumulation point (resp., contains a non-trivial convergent sequence) in the weak topology of X . We show that the quasireflexive James space J is r -reflexive for no r . We do not know...

On the Bishop-Phelps-Bollobás theorem for operators and numerical radius

Sun Kwang Kim, Han Ju Lee, Miguel Martín (2016)

Studia Mathematica

We study the Bishop-Phelps-Bollobás property for numerical radius (for short, BPBp-nu) of operators on ℓ₁-sums and -sums of Banach spaces. More precisely, we introduce a property of Banach spaces, which we call strongly lush. We find that if X is strongly lush and X ⊕₁ Y has the weak BPBp-nu, then (X,Y) has the Bishop-Phelps-Bollobás property (BPBp). On the other hand, if Y is strongly lush and X Y has the weak BPBp-nu, then (X,Y) has the BPBp. Examples of strongly lush spaces are C(K) spaces, L₁(μ)...

On the structure of non-dentable subsets of C ( ω ω k )

Pericles D. Pavlakos, Minos Petrakis (2011)

Studia Mathematica

It is shown that there is no closed convex bounded non-dentable subset K of C ( ω ω k ) such that on subsets of K the PCP and the RNP are equivalent properties. Then applying the Schachermayer-Rosenthal theorem, we conclude that every non-dentable K contains a non-dentable subset L so that on L the weak topology coincides with the norm topology. It follows from known results that the RNP and the KMP are equivalent on subsets of C ( ω ω k ) .

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