2016/02/18 by Julius Jonušas, J. Jonušas, Jonušas, J. +3
Biochemistry, Genetics and Molecular Biology · Mathematics · #Advanced Topology and Set Theory #Amino Acid Enzymes and Metabolism #Finite Group Theory Research #math.GR
paper · pdf · doi:10.48550/arxiv.1602.05766
Fixed some typos and a couple inaccuracies
arxiv created 2017/01/27 · arxiv updated 2017/01/30
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∈ ℕ vertices; the Kn-free graphs; finite unions of the infinite complete graph Kω; and duals of such graphs. The groups Aut(Γ) of automorphisms of such graphs Γ have a natural topology, which is compatible with multiplication and inversion, i.e. the groups Aut(Γ) are topological groups. We consider the problem of finding minimally generated dense subgroups of the groups Aut(Γ) where Γ is ultrahomogeneous. We show that if Γ is ultrahomogeneous, then Aut(Γ) has 2-generated dense subgroups, and that under certain conditions given f ∈ Aut(Γ) there exists g∈ Aut(Γ) 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 Γ is either an infinite disjoint unions of Kn or a finite union of Kω, then g can be chosen to have any given finite set of orbit representatives.