On intrinsic ergodicity of factors of $\mathbb{Z}^d$ subshifts

TL;DR

This paper explores the intrinsic ergodicity of factors of $bZ^d$ subshifts, revealing that higher-dimensional shifts often have non-ergodic factors, contrasting with the one-dimensional case where all factors are intrinsically ergodic.

Contribution

It demonstrates that for $bZ^d$ with $d>1$, certain mixing properties imply the existence of non-intrinsically ergodic factors, and provides an example of a $bZ^2$ shift where all positive entropy factors are intrinsically ergodic.

Findings

Higher-dimensional shifts with mixing have non-ergodic factors.

All positive entropy factors of a specific $bZ^2$ shift are intrinsically ergodic.

Contrast between $bZ$ and $bZ^d$ subshifts regarding factor ergodicity.

Abstract

It is well-known that any $\mathbb{Z}$ subshift with the specification property has the property that every factor is intrinsically ergodic, i.e., every factor has a unique factor of maximal entropy. In recent work, other $\mathbb{Z}$ subshifts have been shown to possess this property as well, including $\beta$-shifts and a class of $S$-gap shifts. We give two results that show that the situation for $\mathbb{Z}^d$ subshifts with $d >1 $ is quite different. First, for any $d>1$, we show that any $\mathbb{Z}^d$ subshift possessing a certain mixing property must have a factor with positive entropy which is not intrinsically ergodic. In particular, this shows that for $d>1$, $\mathbb{Z}^d$ subshifts with specification cannot have all factors intrinsically ergodic. We also give an example of a $\mathbb{Z}^2$ shift of finite type, introduced by Hochman, which is not even topologically mixing, but for which every positive entropy factor is intrinsically ergodic.