A ξ-weak Grothendieck compactness principle

2021 ◽  
pp. 1-16
Author(s):  
KEVIN BEANLAND ◽  
RYAN M. CAUSEY
Keyword(s):  
Banach Space ◽  
Convex Hull ◽  
Banach Spaces ◽  
Compact Set ◽  
Closed Convex Hull ◽  
Compact Sets ◽  
Weakly Compact ◽  
Schur Property ◽  
Compactness Principle ◽  
Weakly Compact Sets

Abstract For 0 ≤ ξ ≤ ω1, we define the notion of ξ-weakly precompact and ξ-weakly compact sets in Banach spaces and prove that a set is ξ-weakly precompact if and only if its weak closure is ξ-weakly compact. We prove a quantified version of Grothendieck’s compactness principle and the characterisation of Schur spaces obtained in [7] and [9]. For 0 ≤ ξ ≤ ω1, we prove that a Banach space X has the ξ-Schur property if and only if every ξ-weakly compact set is contained in the closed, convex hull of a weakly null (equivalently, norm null) sequence. The ξ = 0 and ξ= ω1 cases of this theorem are the theorems of Grothendieck and [7], [9], respectively.

Banach Spaces that are Uniformly Rotund in Weakly Compact Sets of Directions

1977 ◽  
Vol 29 (5) ◽  
pp. 963-970 ◽  
Author(s):  
Mark A. Smith
Keyword(s):  
Banach Space ◽  
Unit Ball ◽  
Banach Spaces ◽  
Nonzero Element ◽  
Bounded Subset ◽  
Compact Sets ◽  
Weakly Compact ◽  
Uniform Rotundity ◽  
Minimum Depth ◽  
Weakly Compact Sets

In a Banach space, the directional modulus of rotundity, δ (ϵ, z), measures the minimum depth at which the midpoints of all chords of the unit ball which are parallel to z and of length at least ϵ are buried beneath the surface. A Banach space is uniformly rotund in every direction (URED) if δ (ϵ, z) is positive for every positive ϵ and every nonzero element z. This concept of directionalized uniform rotundity was introduced by Garkavi [6] to characterize those Banach spaces in which every bounded subset has at most one Čebyšev center.

scholarly journals Farthest points and the farthest distance map

2005 ◽  
Vol 71 (3) ◽  
pp. 425-433 ◽  
Author(s):  
Pradipta Bandyopadhyay ◽  
S. Dutta
Keyword(s):  
Banach Space ◽  
Maximal Monotone ◽  
Convex Banach Space ◽  
Closed Convex Hull ◽  
Compact Sets ◽  
Weakly Compact ◽  
Distance Map ◽  
Farthest Points ◽  
Bounded Set ◽  
Weakly Compact Sets

In this paper, we consider farthest points and the farthest distance map of a closed bounded set in a Banach space. We show, inter alia, that a strictly convex Banach space has the Mazur intersection property for weakly compact sets if and only if every such set is the closed convex hull of its farthest points, and recapture a classical result of Lau in a broader set-up. We obtain an expression for the subdifferential of the farthest distance map in the spirit of Preiss' Theorem which in turn extends a result of Westphal and Schwartz, showing that the subdifferential of the farthest distance map is the unique maximal monotone extension of a densely defined monotone operator involving the duality map and the farthest point map.

scholarly journals Weakly compact sets and smooth norms in Banach spaces

2002 ◽  
Vol 65 (2) ◽  
pp. 223-230 ◽  
Author(s):  
Marián Fabian ◽  
Vicente Montesinos ◽  
Václav Zizler
Keyword(s):  
Banach Spaces ◽  
Lower Semicontinuous ◽  
Compact Sets ◽  
Weakly Compact ◽  
Weakly Compact Sets

Two smoothness characterisations of weakly compact sets in Banach spaces are given. One that involves pointwise lower semicontinuous norms and one that involves projectional resolutions of identity.

scholarly journals Delta- and Daugavet points in Banach spaces

2020 ◽  
Vol 63 (2) ◽  
pp. 475-496
Author(s):  
T. A. Abrahamsen ◽  
R. Haller ◽  
V. Lima ◽  
K. Pirk
Keyword(s):  
Banach Space ◽  
Convex Hull ◽  
Unit Ball ◽  
Banach Spaces ◽  
Unit Sphere ◽  
Closed Convex Hull ◽  
Müntz Spaces ◽  
Daugavet Property ◽  
Diameter Two Property

AbstractA Δ-point x of a Banach space is a norm-one element that is arbitrarily close to convex combinations of elements in the unit ball that are almost at distance 2 from x. If, in addition, every point in the unit ball is arbitrarily close to such convex combinations, x is a Daugavet point. A Banach space X has the Daugavet property if and only if every norm-one element is a Daugavet point. We show that Δ- and Daugavet points are the same in L1-spaces, in L1-preduals, as well as in a big class of Müntz spaces. We also provide an example of a Banach space where all points on the unit sphere are Δ-points, but none of them are Daugavet points. We also study the property that the unit ball is the closed convex hull of its Δ-points. This gives rise to a new diameter-two property that we call the convex diametral diameter-two property. We show that all C(K) spaces, K infinite compact Hausdorff, as well as all Müntz spaces have this property. Moreover, we show that this property is stable under absolute sums.

scholarly journals On some new characterizations of weakly compact sets in Banach spaces

2010 ◽  
Vol 201 (2) ◽  
pp. 155-166 ◽  
Author(s):  
Lixin Cheng ◽  
Qingjin Cheng ◽  
Zhenghua Luo
Keyword(s):  
Banach Spaces ◽  
Compact Sets ◽  
Weakly Compact ◽  
Weakly Compact Sets

Some more characterizations of Banach spaces containing l1

1976 ◽  
Vol 80 (2) ◽  
pp. 269-276 ◽  
Author(s):  
Richard Haydon
Keyword(s):  
Banach Space ◽  
Convex Hull ◽  
Banach Spaces ◽  
Convex Subset ◽  
Compact Convex ◽  
Extreme Points ◽  
Compact Convex Subset ◽  
Closed Convex Hull ◽  
Separability Assumption

In a series of recent papers ((10), (9) and (11)) Rosenthal and Odell have given a number of characterizations of Banach spaces that contain subspaces isomorphic (that is, linearly homeomorphic) to the space l1 of absolutely summable series. The methods of (9) and (11) are applicable only in the case of separable Banach spaces and some of the results there were established only in this case. We demonstrate here, without the separability assumption, one of these characterizations:a Banach space B contains no subspace isomorphic to l1 if and only if every weak* compact convex subset of B* is the norm closed convex hull of its extreme points.

scholarly journals A description of relatively (p; r)-compact sets

2012 ◽  
Vol 16 (2) ◽  
pp. 227-232
Author(s):  
Kati Ain ◽  
Eve Oja
Keyword(s):  
Banach Space ◽  
Convex Hull ◽  
Null Sequence ◽  
Closed Convex Hull ◽  
Compact Sets ◽  
Straightforward Proof

We introduce the notion of (p; r)-null sequences in a Banach space and we prove a Grothendieck-like result: a subset of a Banach space is relatively (p; r)-compact if and only if it is contained in the closed convex hull of a (p; r)-null sequence. This extends a recent description of relatively p-compact sets due to Delgado and Piñeiro, providing to it an alternative straightforward proof.

The Structure of Weakly Compact Sets in Banach Spaces

1968 ◽  
Vol 88 (1) ◽  
pp. 35 ◽  
Author(s):  
D. Amir ◽  
J. Lindenstrauss
Keyword(s):  
Banach Spaces ◽  
Compact Sets ◽  
Weakly Compact ◽  
Weakly Compact Sets

scholarly journals DOMINATION BY POSITIVE WEAK DUNFORD–PETTIS OPERATORS ON BANACH LATTICES

2014 ◽  
Vol 90 (2) ◽  
pp. 311-318 ◽  
Author(s):  
JIN XI CHEN ◽  
ZI LI CHEN ◽  
GUO XING JI
Keyword(s):  
Banach Spaces ◽  
Acta Math ◽  
Positive Operators ◽  
Banach Lattices ◽  
Compact Sets ◽  
Weakly Compact ◽  
Domination Problem ◽  
Weakly Compact Sets

AbstractRecently, H’michane et al. [‘On the class of limited operators’, Acta Math. Sci. (submitted)] introduced the class of weak$^*$ Dunford–Pettis operators on Banach spaces, that is, operators which send weakly compact sets onto limited sets. In this paper, the domination problem for weak$^*$ Dunford–Pettis operators is considered. Let $S, T:E\to F$ be two positive operators between Banach lattices $E$ and $F$ such that $0\leq S\leq T$. We show that if $T$ is a weak$^{*}$ Dunford–Pettis operator and $F$ is $\sigma $-Dedekind complete, then $S$ itself is weak$^*$ Dunford–Pettis.

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