English

$L$-orthogonal elements and $L$-orthogonal sequences

Functional Analysis 2021-04-13 v1 General Topology

Abstract

Given a Banach space XX, we say that a sequence {xn}\{x_n\} in the unit ball of XX is LL-orthogonal if x+xn1+x\Vert x+x_n\Vert\rightarrow 1+\Vert x\Vert for every xXx\in X. On the other hand, an element xx^{**} in the bidual sphere is said to be LL-orthogonal (to XX) if x+x=1+x\|x+x^{**}\|= 1+\Vert x\Vert for every xXx\in X. A result of V. Kadets, V. Shepelska and D. Werner asserts that a Banach space contains an isomorphic copy of 1\ell_1 if and only if there exists an equivalent renorming with an LL-orthogonal sequence, whereas a result of G. Godefroy claims that containing an isomorphic copy of 1\ell_1 is equivalent to the existence of an equivalent renorming with LL-orthogonals in the bidual. The aim of this paper is to clarify the relation between LL-orthogonal sequences and LL-orthogonal elements. Namely, we study whether every LL-orthogonal sequence contains LL-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 LL-orthogonals is not a vector space, this set contains infinite-dimensional Banach spaces when the surrounding space is separable.

Keywords

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}
}
R2 v1 2026-06-24T01:05:02.912Z