On Subspaces of Indecomposable Banach Spaces

TL;DR

This paper characterizes the class of Banach spaces that can be embedded into indecomposable Banach spaces, revealing that many spaces, including some with $ ext{"}ell_ ext{"}infty$ quotients, are included, advancing understanding of Banach space structure.

Contribution

It identifies broad classes of Banach spaces embeddable into indecomposable spaces and introduces a modified construction method for such spaces.

Findings

Includes all Banach spaces of density ≤ continuum without $ ext{"}ell_ ext{"}infty$ as a quotient.

Includes some Banach spaces with $ ext{"}ell_ ext{"}infty$ as a quotient.

Provides new insights into embedding properties related to indecomposable Banach spaces.

Abstract

We address the following question: what is the class of Banach spaces isomorphic to subspaces of indecomposable Banach spaces? We show that this class includes all Banach spaces of density not bigger than the continuum which do not admit $\ell_\infty$ as a quotient (equivalently do not admit a subspace isomorphic to $\ell_1(\cc)$). This includes all Asplund spaces and all weakly Lindel\"of determined Banach spaces of density not bigger than the continuum. However, we also show that this class includes some Banach spaces admitting $\ell_\infty$ as a quotient. This sheds some light on the question asked in [S. Argyros, R. Haydon, \emph{Bourgain-Delbaen $L^\infty$-spaces, the scalar-plus-compact property and related problems}, Proceedings of the International Congress of Mathematicians (ICM 2018), Vol. III, 1477--1510. Page 1502] whether all Banach spaces not containing $\ell_\infty$ embed in some indecomposable Banach spaces. Our method of constructing indecomposable Banach spaces above a given Banach space is a considerable modification of the method of constructing Banach spaces of continuous functions with few$^*$ operators developed before by the first-named author.