Eventual Nonsensitivity and Tame Dynamical Systems

In this paper we characterize tame dynamical systems and functions in terms of eventual non-sensitivity and eventual fragmentability. As a notable application we obtain a neat characterization of tame subshifts X ⊂ {0, 1}Z: for every infinite subset L ⊆ Z there exists an infinite subset K ⊆ L such that πK(X) is a countable subset of {0, 1}K . The notion of eventual fragmentability is one of the properties we encounter which indicate some “smallness” of a family. We investigate a “smallness hierarchy” for families of continuous functions on compact dynamical systems, and link the existence of a “small” family which separates points of a dynamical system (G,X) to the representability of X on “good” Banach spaces. For example, for metric dynamical systems the property of admitting a separating family which is eventually fragmented is equivalent to being tame. We give some sufficient conditions for coding functions to be tame and, among other applications, show that certain multidimensional analogues of Sturmian sequences are tame. We also show that linearly ordered dynamical systems are tame and discuss examples where some universal dynamical systems associated with certain Polish groups are tame.