English

An unusual example of a universal automorphism group

Logic 2026-04-23 v1

Abstract

Let MM be a Fra\"{i}ss\'{e} structure (a countably infinite ultrahomogeneous structure). We refer to the class of structures embeddable in MM as the ω\omega-age of MM. We consider the following two properties of MM: we say that MM has a universal automorphism group if, for each AA in the ω\omega-age of MM, there is an embedding Aut(A)Aut(M)\textrm{Aut}(A) \to \textrm{Aut}(M), and we say that MM has group-extensible ω\omega-age if, for each AA in the ω\omega-age of MM, there is an embedding AMA \to M such that each automorphism of the image extends to an automorphism of MM and the extension map preserves group composition. It is immediate that if MM has group-extensible ω\omega-age, then MM has a universal automorphism group. We give an example of a Fra\"{i}ss\'{e} structure with a universal automorphism group whose ω\omega-age is not group-extensible, showing that the above two properties are not equivalent.

Keywords

Cite

@article{arxiv.2604.20501,
  title  = {An unusual example of a universal automorphism group},
  author = {Rob Sullivan and Jeroen Winkel},
  journal= {arXiv preprint arXiv:2604.20501},
  year   = {2026}
}