English

Ideal convergent subseries in Banach spaces

Functional Analysis 2018-03-13 v1

Abstract

Assume that I\mathcal{I} is an ideal on N\mathbb{N}, and nxn\sum_n x_n is a divergent series in a Banach space XX. We study the Baire category, and the measure of the set A(I):={t{0,1}N ⁣:nt(n)xn is I-convergent}A(\mathcal{I}):=\left\{t \in \{0,1\}^{\mathbb{N}} \colon \sum_n t(n)x_n \textrm{ is } \mathcal{I}\textrm{-convergent}\right\}. In the category case, we assume that I\mathcal{I} has the Baire property and nxn\sum_n x_n is not unconditionally convergent, and we deduce that A(I)A(\mathcal{I}) is meager. We also study the smallness of A(I)A(\mathcal{I}) in the measure case when the Haar probability measure λ\lambda on {0,1}N\{0,1\}^{\mathbb{N}} is considered. If I\mathcal{I} is analytic or coanalytic, and nxn\sum_n x_n is I\mathcal{I}-divergent, then λ(A(I))=0\lambda(A(\mathcal{I}))=0 which extends the theorem of Dindo\v{s}, \v{S}al\'at and Toma. Generalizing one of their examples, we show that, for every ideal I\mathcal{I} on N\mathbb{N}, with the property of long intervals, there is a divergent series of reals such that λ(A(Fin))=0\lambda(A(Fin))=0 and λ(A(I))=1\lambda(A(\mathcal{I}))=1.

Keywords

Cite

@article{arxiv.1803.03699,
  title  = {Ideal convergent subseries in Banach spaces},
  author = {Marek Balcerzak and Michał Popławski and Artur Wachowicz},
  journal= {arXiv preprint arXiv:1803.03699},
  year   = {2018}
}