Smooth ergodic theory of $\mathbb{Z}^d$-actions

TL;DR

This paper develops a comprehensive ergodic theory framework for smooth $bZ^d$-actions, analyzing entropy, Lyapunov exponents, and geometric structures like unstable manifolds, extending classical formulas and establishing new relationships.

Contribution

It introduces a general setting for ergodic theory of smooth $bZ^d$-actions, constructs unstable manifolds, and generalizes entropy formulas to higher-dimensional actions.

Findings

Established controls on local geometry of unstable manifolds.

Generalized Ledrappier--Young entropy formula for $bZ^d$-actions.

Proved entropy product structure along coarse unstable manifolds.

Abstract

In the first part of this paper, we formulate a general setting in which to study the ergodic theory of differentiable $\mathbb{Z}^d$-actions preserving a Borel probability measure. This framework includes actions by $C^{1+\text{H\"older}}$ diffeomorphisms of compact manifolds. We construct intermediate and coarse unstable manifolds for the action and establish controls on their local geometry. In the second part we consider the relationship between entropy, Lyapunov exponents, and the geometry of conditional measures for rank-1 systems given by a number of generalizations of the Ledrappier--Young entropy formula. In the third part, for a smooth action of $\mathbb{Z}^d$ preserving a Borel probability measure, we show that entropy satisfies a certain "product structure" along coarse unstable manifolds. Moreover, given two smooth $\mathbb{Z}^d$-actions---one of which is a measurable factor of the other---we show that all coarse Lyapunov exponents contributing to the entropy of the factor system are coarse Lyapunov exponents of the total system. As a consequence, we derive an Abramov--Rohlin formula for entropy subordinated to coarse unstable manifolds.