English

Macphail's Theorem revisited

Functional Analysis 2020-12-03 v2

Abstract

In 1947, M. S. Macphail constructed a series in 1\ell_{1} that converges unconditionally but does not converge absolutely. According to the literature, this result helped Dvoretzky and Rogers to finally answer a long standing problem of Banach Space Theory, by showing that in all infinite-dimensional Banach spaces, there exists an unconditionally summable sequence that fails to be absolutely summable. More precisely, the Dvoretzky--Rogers Theorem asserts that in every infinite-dimensional Banach space EE there exists an unconditionally convergent series x(j){\textstyle\sum}x^{(j)} such that x(j)2ε={\textstyle\sum}\Vert x^{(j)}\Vert^{^{2-\varepsilon}}=\infty for all ε>0.\varepsilon>0. Their proof is non-constructive and Macphail's result for E=1E=\ell_{1} provides a constructive proof just for ε1.\varepsilon\geq1. In this note we revisit Machphail's paper and present two alternative constructions that work for all ε>0.\varepsilon>0.

Keywords

Cite

@article{arxiv.2006.07626,
  title  = {Macphail's Theorem revisited},
  author = {Daniel Pellegrino and Janiely Silva},
  journal= {arXiv preprint arXiv:2006.07626},
  year   = {2020}
}
R2 v1 2026-06-23T16:17:55.168Z