$L$-orthogonal elements and $L$-orthogonal sequences
Abstract
Given a Banach space , we say that a sequence in the unit ball of is -orthogonal if for every . On the other hand, an element in the bidual sphere is said to be -orthogonal (to ) if for every . A result of V. Kadets, V. Shepelska and D. Werner asserts that a Banach space contains an isomorphic copy of if and only if there exists an equivalent renorming with an -orthogonal sequence, whereas a result of G. Godefroy claims that containing an isomorphic copy of is equivalent to the existence of an equivalent renorming with -orthogonals in the bidual. The aim of this paper is to clarify the relation between -orthogonal sequences and -orthogonal elements. Namely, we study whether every -orthogonal sequence contains -orthogonal elements in its weak*-closure. We provide an affirmative answer whenever the ambient space has small density character. Nevertheless, we show that, surprisingly, the general answer is independent of the usual axioms of set theory. We also prove that, even though the set of -orthogonals is not a vector space, this set contains infinite-dimensional Banach spaces when the surrounding space is separable.
Cite
@article{arxiv.2104.05535,
title = {$L$-orthogonal elements and $L$-orthogonal sequences},
author = {Antonio Avilés and Gonzalo Martínez-Cervantes and Abraham Rueda Zoca},
journal= {arXiv preprint arXiv:2104.05535},
year = {2021}
}