English

Convergent subseries of divergent series

Classical Analysis and ODEs 2020-11-24 v1 Functional Analysis General Topology

Abstract

Let X\mathscr{X} be the set of positive real sequences x=(xn)x=(x_n) such that the series nxn\sum_n x_n is divergent. For each xXx \in \mathscr{X}, let Ix\mathcal{I}_x be the collection of all ANA\subseteq \mathbf{N} such that the subseries nAxn\sum_{n \in A}x_n is convergent. Moreover, let A\mathscr{A} be the set of sequences xXx \in \mathscr{X} such that limnxn=0\lim_n x_n=0 and IxIy\mathcal{I}_x\neq \mathcal{I}_y for all sequences y=(yn)Xy=(y_n) \in \mathscr{X} with lim infnyn+1/yn>0\liminf_n y_{n+1}/y_n>0. We show that A\mathscr{A} is comeager and that contains uncountably many sequences xx which generate pairwise nonisomorphic ideals Ix\mathcal{I}_x. This answers, in particular, an open question recently posed by M. Filipczak and G. Horbaczewska.

Keywords

Cite

@article{arxiv.2011.10638,
  title  = {Convergent subseries of divergent series},
  author = {Marek Balcerzak and Paolo Leonetti},
  journal= {arXiv preprint arXiv:2011.10638},
  year   = {2020}
}

Comments

6 pp; comments are welcome