Quotients of $\mathbb{N}^*$, $ω$-limit sets, and chain transitivity

Abstract

$\mathbb{N}^* = β\mathbb{N} \setminus \mathbb{N}$ has a canonical dynamical structure provided by the shift map, the unique continuous extension to $β\mathbb{N}$ of the map $n \mapsto n+1$ on $\mathbb{N}$. Here we investigate the question of what dynamical systems can be written as quotients of $\mathbb{N}^*$. We prove that a dynamical system is a quotient of $\mathbb{N}^*$ if and only if it is isomorphic to the $ω$-limit set of some point in some larger system. This provides a full external characterization of the quotients of $\mathbb{N}^*$. We also prove, assuming MA$_{σ\text{-centered}}(κ)$, that a dynamical system of weight $κ$ is a quotient of $\mathbb{N}^*$ if and only if it is chain transitive. This provides a consistent partial internal characterization of the quotients of $\mathbb{N}^*$, and a full internal characterization for metrizable systems.