English

Cohesive Powers of Linear Orders

Logic 2019-08-28 v1

Abstract

Cohesive powers of computable structures can be viewed as effective ultraproducts over effectively indecomposable sets called cohesive sets. We investigate the isomorphism types of cohesive powers ΠC\Pi _{C}% \mathcal{L} for familiar computable linear orders L\mathcal{L}. If % \mathcal{L} is isomorphic to the ordered set of natural numbers N\mathbb{N} and has a computable successor function, then ΠCL\Pi _{C}\mathcal{L} is isomorphic to N+Q×Z.\mathbb{N}+\mathbb{Q}\times \mathbb{Z}. Here, ++ stands for the sum and ×\times for the lexicographical product of two orders. We construct computable linear orders L1\mathcal{L}_{1} and L2\mathcal{L}_{2} isomorphic to N,\mathbb{N}, both with noncomputable successor functions, such that ΠCL1 \Pi _{C}\mathcal{L}_{1}\mathbb{\ }is isomorphic to N+\mathbb{N}+% \mathbb{Q}\times \mathbb{Z}, while ΠCL2\Pi _{C}\mathcal{L}_{2} is not.. While cohesive powers preserve all Π20\Pi _{2}^{0} and Σ20\Sigma _{2}^{0} sentences, we provide new examples of Π30\Pi _{3}^{0} sentences Φ\Phi and computable structures % \mathcal{M} such that MΦ\mathcal{M}\vDash \Phi while ΠCM\Pi _{C}\mathcal{M}% \vDash \urcorner \Phi .

Keywords

Cite

@article{arxiv.1901.04786,
  title  = {Cohesive Powers of Linear Orders},
  author = {Rumen Dimitrov and Valentina Harizanov and Andrey Morozov and Paul Shafer and Alexandra Soskova and Stefan Vatev},
  journal= {arXiv preprint arXiv:1901.04786},
  year   = {2019}
}
R2 v1 2026-06-23T07:12:15.067Z