Cyclic nearly invariant subspaces for semigroups of isometries

TL;DR

This paper characterizes nearly invariant subspaces for semigroups of isometries, focusing on composition and Toeplitz operators, and explores their structure using Toeplitz kernels and model spaces.

Contribution

It provides a detailed description of nearly invariant subspaces for semigroups generated by automorphisms and multiplication operators, including their relation to model spaces.

Findings

Characterization of nearly invariant subspaces via model spaces.

Analysis of cyclic subspaces generated by Hardy functions.

Use of Toeplitz kernels and reproducing kernels techniques.

Abstract

In this paper, the structure of the nearly invariant subspaces for discrete semigroups generated by several (even infinitely many) automorphisms of the unit disc is described. As part of this work, the near $S^*$-invariance property of the image space $C_\varphi({\rm ker\, } T)$ is explored for composition operators $C_\varphi$, induced by inner functions $\varphi$, and Toeplitz operators $T$. After that, the analysis of nearly invariant subspaces for strongly continuous multiplication semigroups of isometries is developed with a study of cyclic subspaces generated by a single Hardy class function. These are characterised in terms of model spaces in all cases when the outer factor is a product of an invertible function and a rational (not necessarily invertible) function. Techniques used include the theory of Toeplitz kernels and reproducing kernels.