Inverse limits, entropy and weak isomorphism for discrete dynamical systems

Abstract

A categorical approach is taken to the study of a single measure-preserving transformation of a finite measure space and to inverse systems and inverse limits of such transformations. The questions of existence and uniqueness of inverse limits are settled. Sinaǐ’s theorem on generators is recast and slightly extended to say that entropy respects inverse limits, and various known results about entropy are obtained as immediate corollaries, e.g. systems with quasi-discrete or quasi-periodic spectrum have zero entropy. The inverse limit Φ \Phi of an inverse system { Φ α : α ∈ J } \{ {\Phi _\alpha }:\alpha \in J\} of dynamical systems is (1) ergodic, (2) weakly mixing, (3) mixing (of any order) iff each Φ α {\Phi _\alpha } has the same property. Finally, inverse limits are used to lift a weak isomorphism of dynamical systems Φ 1 {\Phi _1} and Φ 2 {\Phi _2} to an isomorphism of systems Φ ^ 1 {\hat \Phi _1} and Φ ^ 2 {\hat \Phi _2} with the same entropy.