Automaton ranks of some self-similar groups

Abstract

Given a group G and a positive integer d ≥ 2 we introduce the notion of an automaton rank of a group G with respect to its self-similar actions on a d-ary tree of words as the minimal number of states in an automaton over a d-letter alphabet which generates this group (topologically if G is closed). We construct minimal automata generating free abelian groups of finite ranks, which completely determines automaton ranks of free abelian groups. We also provide naturally defined 3-state automaton realizations for profinite groups which are infinite wreath powers ...\H\H for some 2-generated finite perfect groups H. This determines the topological rank and improves the estimation for the automaton rank of these wreath powers. We show that we may take H as alternating groups and projective special linear groups. © 2012 Springer-Verlag.