An unusual example of a universal automorphism group
Abstract
Let be a Fra\"{i}ss\'{e} structure (a countably infinite ultrahomogeneous structure). We refer to the class of structures embeddable in as the -age of . We consider the following two properties of : we say that has a universal automorphism group if, for each in the -age of , there is an embedding , and we say that has group-extensible -age if, for each in the -age of , there is an embedding such that each automorphism of the image extends to an automorphism of and the extension map preserves group composition. It is immediate that if has group-extensible -age, then has a universal automorphism group. We give an example of a Fra\"{i}ss\'{e} structure with a universal automorphism group whose -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}
}