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

William R. Brian

Published 2014-12-31Version 1

$\mathbb{N}^* = \beta\mathbb{N} \setminus \mathbb{N}$ has a canonical dynamical structure provided by the shift map, the unique continuous extension to $\beta\mathbb{N}$ 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 $\omega$-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$_{\sigma\text{-centered}}(\kappa)$, that a dynamical system of weight $\kappa$ 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.