Multiplicity structure of preimages of invariant measures under finite-to-one factor maps

Abstract

Given a finite-to-one factor map $\pi: (X, T) \to (Y, S)$ between topological dynamical systems, we look into the pushforward map $\pi_*: M(X, T) \to M(Y,T)$ between sets of invariant measures. We investigate the structure of the measure fiber $\pi_*^{-1}(\nu)$ for an arbitrary ergodic measure $\nu$ on the factor system $Y$. We define the degree $d_{\pi,\nu}$ of the factor map $\pi$ relative to $\nu$ and the multiplicity of each ergodic measure $\mu$ on $X$ that projects to $\nu$, and show that the number of ergodic pre-images of $\nu$ is $d_{\pi,\nu}$ counting multiplicity. In other words, the degree $d_{\pi,\nu}$ is the sum of the multiplicity of $\mu$ where $\mu$ runs over the ergodic measures in the measure fiber $\pi^{-1}_*(\nu)$. This generalizes the following folklore result in symbolic dynamics for lifting fully supported invariant measures: Given a finite-to-one factor code $\pi: X \to Y$ between irreducible sofic shifts and an ergodic measure $\nu$ on $Y$ with full support, $\pi^{-1}_*(\nu)$ has at most $d_\pi$ ergodic measures in it, where $d_\pi$ is the degree of $\pi$. We apply our theory of structure of measure fibers to the special case of symbolic dynamical systems. In this case, we demonstrate that one can list all (finitely many) ergodic measures in the measure fiber $\pi^{-1}_*(\nu)$.