On the Lyapunov spectrum of the twisted cocycle for substitutions

TL;DR

This paper investigates the Lyapunov spectrum of a complex twisted cocycle in substitution dynamical systems, revealing a zero exponent and implications for spectral singularity, while also analyzing local spectral measure dimensions.

Contribution

It extends understanding of the spectral properties of substitution systems by linking invariant sections to zero Lyapunov exponents and simplifies proofs of spectral singularity.

Findings

Presence of a zero Lyapunov exponent due to invariant sections.

Extension of singular spectrum results for a broad class of substitutions.

Computed local dimensions of spectral measures for irrational rotations.

Abstract

The paper is devoted to the properties of a complex matrix ``twisted,'' otherwise called ``spectral,'' cocycle, associated with substitution dynamical systems. Following a recent finding of Rajabzadeh and Safaee [arXiv:2501.16824] of an invariant section for the twisted cocycle, we indicate that this implies presence of a zero Lyapunov exponent. This has consequences for the spectral properties of substitution dynamical systems; in particular, this extends the scope and simplifies the proof of singular spectrum for a large class of substitutions on two symbols. We also obtain some results on positivity of the top exponent. In the appendix we compute the Lebesgue almost everywhere local dimension of spectral measures of some ``simple'' test functions, for almost every irrational rotation. This sheds some light on the earlier work of Bufetov and the author, relating the local dimension of spectral measures to pointwise Lyapunov exponents of the twisted cocycle. It should be noted that the paper has some (mutually acknowledged) overlap with [arXiv:2501.16824, Appendix].