We prove that if a countable group contains a copy of F 2 , then it admits uncountably many non orbit equivalent actions.
IntroductionThroughout this paper we consider free, ergodic, measure preserving (m.p.) actions (X, μ) of countable, discrete groups on standard probability spaces (X, μ). Measurable group theory is roughly the study of such group actions from the viewpoint of the induced orbit equivalence relation. A basic question in measurable group theory is to find groups which admit many non-orbit equivalent actions (see the survey [35]). In this respect, recall that two free, ergodic, m.p. actions (X, μ) and (Y, ν) are said to be orbit equivalent (OE) if they induce isomorphic equivalence relations, i.e. if there exists a measure space isomorphism θ :The striking lack of rigidity manifested by amenable groups (any two free, ergodic m.p. actions of any two infinite amenable groups and are orbit equivalent-a result proved by Dye in the case and are Abelian [8] and by Ornstein-Weiss in general ([29], see also [6])) implies that the above question is well-posed only for non-amenable groups. For a non-amenable group , it is known that admits at least two non-OE actions [5,18,36]. Moreover, recently, the following classes of non-amenable groups have been shown to admit uncountably many non-OE actions: property (T) groups [18], free groups A. Ioana ( )