Exchangeable measures for subshifts

Abstract

Let $\Om$ be a Borel subset of $S^\Bbb N$ where $S$ is countable. A measure is called exchangeable on $\Om$, if it is supported on $\Om$ and is invariant under every Borel automorphism of $\Om$ which permutes at most finitely many coordinates. De-Finetti's theorem characterizes these measures when $\Om=S^\Bbb N$. We apply the ergodic theory of equivalence relations to study the case $\Om\neq S^\Bbb N$, and obtain versions of this theorem when $\Om$ is a countable state Markov shift, and when $\Om$ is the collection of beta expansions of real numbers in $[0,1]$ (a non-Markovian constraint).