Rank-one nonsingular actions of countable groups and their odometer factors

Abstract

For an arbitrary countable discrete infinite group $G$, nonsingular rank-one actions are introduced. It is shown that the class of nonsingular rank-one actions coincides with the class of nonsingular $(C,F)$-actions. Given a decreasing sequence $Γ_1\supsetneqΓ_2\supsetneq\cdots$ of cofinite subgroups in $G$ with $\bigcap_{n=1}^\infty\bigcap_{g\in G}gΓ_ng^{-1}=\{1_G\}$, the projective limit of the homogeneous $G$-spaces $G/Γ_n$ as $n\to\infty$ is a $G$-space. Endowing this $G$-space with an ergodic nonsingular nonatomic measure we obtain a dynamical system which is called a nonsingular odometer. Necessary and sufficient conditions are found for a rank-one nonsingular $G$-action to have a finite factor and a nonsingular odometer factor in terms of the underlying $(C,F)$-parameters. Similar conditions are also found for a rank-one nonsingular $G$-action to be isomorphic to an odometer. Minimal Radon uniquely ergodic locally compact Cantor models are constructed for the nonsingular rank-one extensions of odometers. Several concrete examples are constructed and several facts are proved that illustrate a sharp difference of the nonsingular noncommutative case from the classical finite measure preserving one: odometer actions which are not of rank one, factors of rank-one systems which are not of rank-one, however each probability preserving odometer is a factor of an infinite measure preserving rank-one system, etc.