English

Many symmetrically indivisible structures

Logic 2019-09-04 v2 Combinatorics

Abstract

A structure M\mathcal{M} in a first-order language L\mathcal{L} is \emph{indivisible} if for every coloring of MM in two colors, there is a monochromatic MM\mathcal{M}^{\prime} \subseteq \mathcal{M} such that MM\mathcal{M}^{\prime}\cong\mathcal{M}. Additionally, we say that M\mathcal{M} is symmetrically indivisible if M\mathcal{M}^{\prime} can be chosen to be \emph{symmetrically embedded} in M\mathcal{M} (that is, every automorphism of M\mathcal{M}^{\prime} can be extended to an automorphism of M\mathcal{M}). In the following paper we give a general method for constructing new symmetrically indivisible structures out of existing ones. Using this method, we construct 202^{\aleph_0} many non-isomorphic symmetrically indivisible countable structures in given (elementary) classes and answer negatively the following question asked by A. Hasson, M. Kojman and A. Onshuus in "On symmetric indivisibility of countable structures" (Cont. Math. 558(1):453--466): Let M\mathcal{M} be a symmetrically indivisible structure in a language L\mathcal{L}. Let L0L\mathcal{L}_0 \subseteq \mathcal{L}. Is ML0 \mathcal{M} \upharpoonright \mathcal{L}_0 symmetrically indivisible?

Keywords

Cite

@article{arxiv.1411.1202,
  title  = {Many symmetrically indivisible structures},
  author = {Nadav Meir},
  journal= {arXiv preprint arXiv:1411.1202},
  year   = {2019}
}

Comments

10 pages

R2 v1 2026-06-22T06:48:44.919Z