Topological 2-generation of automorphism groups of countable ultrahomogeneous graphs

Julius Jonusas, J. D. Mitchell*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)


A countable graph is ultrahomogeneous if every isomorphism between finite induced subgraphs can be extended to an automorphism. Woodrow and Lachlan showed that there are essentially four types of such countably infinite graphs: The random graph, infinite disjoint unions of complete graphs Kn with n ò Nvertices, the Kn-free graphs, finite unions of the infinite complete graph K, and duals of such graphs. The groups Aut(λ) of automorphisms of such graphs λ have a natural topology, which is compatible with multiplication and inversion, i.e. The groupsAut(λ) are topological groups.We consider the problem of finding minimally generated dense subgroups of the groups Aut(λ) where λ is ultrahomogeneous.We show that if λ is ultrahomogeneous, then Aut(λ) has 2-generated dense subgroups, and that under certain conditions given f ò Aut(λ) there exists g ò Aut(λ) such that the subgroup generated by f and g is dense. We also show that, roughly speaking, g can be chosen with a high degree of freedom. For example, if λ is either an infinite disjoint union of Kn or a finite union of Kω, then g can be chosen to have any given finite set of orbit representatives.

Original languageEnglish
Pages (from-to)905-939
Number of pages35
JournalForum Mathematicum
Issue number4
Early online date14 Sept 2016
Publication statusPublished - Jul 2017


  • Automorphism groups
  • Fraïssé limits
  • Polish groups
  • topological generation


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