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Uniform G-Convexity for Vector-Valued Lp Spaces

Boyko, NataliiaKadets, Vladimir — 2009

Serdica Mathematical Journal

2000 Mathematics Subject Classification: 46B20. Uniform G-convexity of Banach spaces is a recently introduced natural generalization of uniform convexity and of complex uniform convexity. We study conditions under which uniform G-convexity of X passes to the space of X-valued functions Lp (m,X).

Convexity around the Unit of a Banach Algebra

Kadets, VladimirKatkova, OlgaMartín, MiguelVishnyakova, Anna — 2008

Serdica Mathematical Journal

2000 Mathematics Subject Classification: Primary: 46B20. Secondary: 46H99, 47A12. We estimate the (midpoint) modulus of convexity at the unit 1 of a Banach algebra A showing that inf {max±||1 ± x|| − 1 : x ∈ A, ||x||=ε} ≥ (π/4e)ε²+o(ε²) as ε → 0. We also give a characterization of two-dimensional subspaces of Banach algebras containing the identity in terms of polynomial inequalities.

Quotients of Banach Spaces with the Daugavet Property

Vladimir KadetsVarvara ShepelskaDirk Werner — 2008

Bulletin of the Polish Academy of Sciences. Mathematics

We consider a general concept of Daugavet property with respect to a norming subspace. This concept covers both the usual Daugavet property and its weak* analogue. We introduce and study analogues of narrow operators and rich subspaces in this general setting and apply the results to show that a quotient of L₁[0,1] by an ℓ₁-subspace need not have the Daugavet property. The latter answers in the negative a question posed to us by A. Pełczyński.

Metric spaces with the small ball property

Ehrhard BehrendsVladimir M. Kadets — 2001

Studia Mathematica

A metric space (M,d) is said to have the small ball property (sbp) if for every ε₀ > 0 it is possible to write M as the union of a sequence (B(xₙ,rₙ)) of closed balls such that the rₙ are smaller than ε₀ and lim rₙ = 0. We study permanence properties and examples of sbp. The main results of this paper are the following: 1. Bounded convex closed sets in Banach spaces have sbp only if they are compact. 2. Precisely the finite-dimensional Banach spaces have sbp. (More generally: a complete metric...

Properties of lush spaces and applications to Banach spaces with numerical index 1

Kostyantyn BoykoVladimir KadetsMiguel MartínJavier Merí — 2009

Studia Mathematica

The concept of lushness, introduced recently, is a Banach space property, which ensures that the space has numerical index 1. We prove that for Asplund spaces lushness is actually equivalent to having numerical index 1. We prove that every separable Banach space containing an isomorphic copy of c₀ can be renormed equivalently to be lush, and thus to have numerical index 1. The rest of the paper is devoted to the study of lushness just as a property of Banach spaces. We prove that lushness is separably...

Narrow operators and rich subspaces of Banach spaces with the Daugavet property

Vladimir M. KadetsRoman V. ShvidkoyDirk Werner — 2001

Studia Mathematica

Let X be a Banach space. We introduce a formal approach which seems to be useful in the study of those properties of operators on X which depend only on the norms of the images of elements. This approach is applied to the Daugavet equation for norms of operators; in particular we develop a general theory of narrow operators and rich subspaces of spaces X with the Daugavet property previously studied in the context of the classical spaces C(K) and L₁(μ).

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