Similarity to an isometry of composition operators on the half-plane

TL;DR

This paper characterizes when weighted composition operators on the Hardy space of the half-plane are similar to isometries, extending known conditions for isometries to the broader class of similar operators.

Contribution

It provides necessary conditions and a complete characterization of rational composition operators on H^p(C+) that are similar to isometries.

Findings

Necessary conditions for similarity to isometries.

Complete characterization of rational composition operators.

Extension of isometry conditions to similarity cases.

Abstract

Necessary and sufficient conditions are already known in the Hardy spaces of both the disc and the half plane for a composition operator to be an isometry, by Nordgren in the disc and by Chalendar and Partington in the half plane. All the same, conditions for such an operator to be similar to an isometry have taken much longer to find. We present some necessary conditions for general weighted composition operators to be similar to an isometry, and use them to produce a complete characterisation of the rational composition on $H^p(\C^+)$ which have this property.