English

Locally finite trees and the topological minor relation

Combinatorics 2017-05-16 v1 General Topology

Abstract

A well-known theorem of Nash-Williams shows that the collection of locally finite trees under the topological minor relation results in a BQO. Set theoretically, two very natural questions arise: (1) What is the number λ\lambda of topological types of locally finite trees? (2) What are the possible sizes of an equivalence class of locally finite trees? For (1), clearly, ωλc\omega \leq \lambda \leq \mathfrak{c} and Matthiesen refined it to ω1λc\omega_1 \leq \lambda \leq \mathfrak{c}. Thus, this question becomes non-trivial when the Continuum Hypothesis is not assumed. In this paper we address both questions by showing that - entirely within ZFC - for a large collection of locally finite trees that includes those with countably many rays: the answer for (1) is λ=ω1\lambda = \omega_1, and that for (2) the size of an equivalence class can only be either 11 or c\mathfrak{c}.

Keywords

Cite

@article{arxiv.1705.04937,
  title  = {Locally finite trees and the topological minor relation},
  author = {Jorge Bruno and Paul J. Szeptycki},
  journal= {arXiv preprint arXiv:1705.04937},
  year   = {2017}
}