English

On cohesive powers of linear orders

Logic 2023-02-23 v3

Abstract

Cohesive powers of computable structures are effective analogs of ultrapowers, where cohesive sets play the role of ultrafilters. Let ω\omega, ζ\zeta, and η\eta denote the respective order-types of the natural numbers, the integers, and the rationals when thought of as linear orders. We investigate the cohesive powers of computable linear orders, with special emphasis on computable copies of ω\omega. If L\mathcal{L} is a computable copy of ω\omega that is computably isomorphic to the usual presentation of ω\omega, then every cohesive power of L\mathcal{L} has order-type ω+ζη\omega + \zeta\eta. However, there are computable copies of ω\omega, necessarily not computably isomorphic to the usual presentation, having cohesive powers not elementarily equivalent to ω+ζη\omega + \zeta\eta. For example, we show that there is a computable copy of ω\omega with a cohesive power of order-type ω+η\omega + \eta. Our most general result is that if XN{0}X \subseteq \mathbb{N} \setminus \{0\} is a Boolean combination of Σ2\Sigma_2 sets, thought of as a set of finite order-types, then there is a computable copy of ω\omega with a cohesive power of order-type ω+σ(X{ω+ζη+ω})\omega + \sigma(X \cup \{\omega + \zeta\eta + \omega^*\}), where σ(X{ω+ζη+ω})\sigma(X \cup \{\omega + \zeta\eta + \omega^*\}) denotes the shuffle of the order-types in XX and the order-type ω+ζη+ω\omega + \zeta\eta + \omega^*. Furthermore, if XX is finite and non-empty, then there is a computable copy of ω\omega with a cohesive power of order-type ω+σ(X)\omega + \sigma(X).

Keywords

Cite

@article{arxiv.2009.00340,
  title  = {On cohesive powers of linear orders},
  author = {Rumen Dimitrov and Valentina Harizanov and Andrey Morozov and Paul Shafer and Alexandra A. Soskova and Stefan V. Vatev},
  journal= {arXiv preprint arXiv:2009.00340},
  year   = {2023}
}
R2 v1 2026-06-23T18:14:04.881Z