Upper frequent hypercyclicity and related notions

TL;DR

This paper advances the understanding of upper frequently hypercyclic operators by providing new characterizations, criteria, and simpler counterexamples, extending to a broader class of operators via upper Furstenberg families.

Contribution

It introduces a Birkhoff-type characterization and an Upper Frequent Hypercyclicity Criterion, and generalizes results to $ ext{A}$-hypercyclic operators using upper Furstenberg families.

Findings

Characterization of upper frequently hypercyclic weighted backward shifts.

Development of a simpler method for counterexamples in linear dynamics.

Extension of results to $ ext{A}$-hypercyclic operators with upper Furstenberg families.

Abstract

Enhancing a recent result of Bayart and Ruzsa we obtain a Birkhoff-type characterization of upper frequently hypercyclic operators and a corresponding Upper Frequent Hypercyclicity Criterion. As an application we characterize upper frequently hypercyclic weighted backward shifts on sequence spaces, which in turn allows us to come up with various counter-examples in linear dynamics that are substantially simpler than those previously obtained in the literature. More generally, we introduce the notion of upper Furstenberg families $\mathcal{A}$ and show that our main results hold for $\mathcal{A}$-hypercyclic operators with respect to such families.