Group-extensive embeddings into Fraïssé structures and stationary weak independence relations

Abstract

Let $M$ be a Fraïssé structure (a countably infinite ultrahomogeneous structure). We call an embedding $f:A\to M$ group-extensive if each automorphism of its image extends to an automorphism of $M$, where the extension map respects composition. We say that $M$ has group-extensible $\omega$-age if each substructure admits a group-extensive embedding into $M$. We investigate the relationship between the following two properties: the presence of a stationary weak independence relation (SWIR) on $M$, and group-extensibility of the $\omega$-age of $M$. We show that linearly ordered Fraïssé structures with a SWIR have group-extensible $\omega$-age, but also we give examples of Fraïssé structures where only one of the two properties holds. Finally, we consider whether a wide range of examples of Fraïssé structures have group-extensible $\omega$-age or a finite SWIR expansion, including all countably infinite ultrahomogeneous oriented graphs (with one exception).

final version, accepted to Journal of Algebra