Complexities of Theories of Profinite Subgroups of S_ω via Tree Presentations

Although S_ω (the group of all permutations of N) is size continuum, both it and its closed subgroups can be presented as the set of paths through a countable tree. The subgroups of S_ω that can be presented this way with finite branching trees are exactly the profinite ones. We use these tree presentations to find upper bounds on the complexity of the existential theories of profinite subgroups of S_ω, as well as to prove sharpness for these bounds. These complexity results enable us to distinguish a simple subclass of profinite groups, those with orbit independence, for which we find an upper bound on the complexity of the entire first order theory.