English

On the set of limit points of conditionally convergent series

Functional Analysis 2016-04-22 v1

Abstract

Let n=1xn\sum_{n=1}^\infty x_n be a conditionally convergent series in a Banach space and let τ\tau be a permutation of natural numbers. We study the set LIM(n=1xτ(n))\operatorname{LIM}(\sum_{n=1}^\infty x_{\tau(n)}) of all limit points of a sequence (n=1pxτ(n))p=1(\sum_{n=1}^p x_{\tau(n)})_{p=1}^\infty of partial sums of a rearranged series n=1xτ(n)\sum_{n=1}^\infty x_{\tau(n)}. We give full characterization of limit sets in finite dimensional spaces. Namely, a limit set in Rm\mathbb{R}^m is either compact and connected or it is closed and all its connected components are unbounded. On the other hand each set of one of these types is a limit set of some rearranged conditionally convergent series. Moreover, this characterization does not hold in infinite dimensional spaces. We show that if n=1xn\sum_{n=1}^\infty x_n has the Rearrangement Property and AA is a closed subset of the closure of the n=1xn\sum_{n=1}^\infty x_n sum range and it is ε\varepsilon-chainable for every ε>0\varepsilon>0, then there is a permutation τ\tau such that A=LIM(n=1xτ(n))A=\operatorname{LIM}(\sum_{n=1}^\infty x_{\tau(n)}). As a byproduct of this observation we obtain that series having the Rearrangement Property have closed sum ranges.

Keywords

Cite

@article{arxiv.1604.06255,
  title  = {On the set of limit points of conditionally convergent series},
  author = {Szymon Głab and Jacek Marchwicki},
  journal= {arXiv preprint arXiv:1604.06255},
  year   = {2016}
}