2016/10/31 by Brown, Aaron, Hertz, Federico Rodriguez, Wang, Zhiren
#Dynamical Systems (math.DS) #FOS: Mathematics
paper · doi:10.48550/arxiv.1610.09997
In the first part of this paper, we formulate a general setting in which to study the ergodic theory of differentiable ℤd-actions preserving a Borel probability measure. This framework includes actions by C1+Hölder 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 ℤd preserving a Borel probability measure, we show that entropy satisfies a certain "product structure" along coarse unstable manifolds. Moreover, given two smooth ℤd-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.