Stability and Collapse of the Lyapunov spectrum for Perron-Frobenius Operator cocycles

TL;DR

This paper analyzes the Lyapunov spectrum of Perron-Frobenius cocycles generated by random Blaschke products, revealing conditions for stability and demonstrating how small perturbations can cause spectrum collapse, with implications for dynamical systems.

Contribution

It provides a complete description of the Lyapunov spectrum for these cocycles, criteria for stability, and insights into spectrum collapse mechanisms under perturbations.

Findings

Complete Lyapunov spectrum characterization

Criteria for spectrum stability based on fixed point derivatives

Small perturbations can cause spectrum collapse to zero

Abstract

In this paper, we study random Blaschke products, acting on the unit circle, and consider the cocycle of Perron-Frobenius operators acting on Banach spaces of analytic functions on an annulus. We completely describe the Lyapunov spectrum of these cocycles. As a corollary, we obtain a simple random Blaschke product system where the Perron-Frobenius cocycle has infinitely many distinct Lyapunov exponents, but where arbitrarily small natural perturbations cause a complete collapse of the Lyapunov spectrum, except for the exponent 0 associated with the absolutely continuous invariant measure. That is, under perturbations, the Lyapunov exponents become 0 with multiplicity 1, and $-\infty$ with infinite multiplicity. This is superficially similar to the finite-dimensional phenomenon, discovered by Bochi \cite{Bochi-thesis}, that away from the uniformly hyperbolic setting, small perturbations can lead to a collapse of the Lyapunov spectrum to zero. In this paper, however, the cocycle and its perturbation are explicitly described; and further, the mechanism for collapse is quite different. We study stability of the Perron-Frobenius cocycles arising from general random Blaschke products. We give a necessary and sufficient criterion for stability of the Lyapunov spectrum in terms of the derivative of the random Blaschke product at its random fixed point, and use this to show that an open dense set of Blaschke product cocycles have hyperbolic Perron-Frobenius cocycles. In the final part, we prove a relationship between the Lyapunov spectrum of a single cocycle acting on two different Banach spaces, allowing us to draw conclusions for the same cocycles acting on $C^r$ functions spaces.