On commutants of composition operators embedded into $C_0$-semigroups

TL;DR

This paper explores the algebraic structure of commutants of composition operators within $C_0$-semigroups on Hardy spaces, revealing conditions for their isomorphism to function algebras and implications for operator properties.

Contribution

It provides new insights into the structure of commutants and bicommutants of composition operators embedded in $C_0$-semigroups, extending previous results to broader classes of operators.

Findings

Characterization of commutants as subalgebras of continuous functions

Conditions for the existence of non-trivial idempotents and projections

Results on minimality and double commutant property of the operators

Abstract

Let $C_\varphi$ be a composition operator acting on the Hardy space of the unit disc $H^p$ ($1\leq p < \infty$), which is embedded in a $C_0$-semigroup of composition operators $\mathcal{T}=(C_{\varphi_t})_{t\geq 0}.$ We investigate whether the commutant or the bicommutant of $C_\varphi$, or the commutant of the semigroup $\mathcal{T}$, are isomorphic to subalgebras of continuous functions defined on a connected set. In particular, it allows us to derive results about the existence of non-trivial idempotents (and non-trivial orthogonal projections if $p=2$) lying in such sets. Our methods also provide results concerning the minimality of the commutant and the double commutant property, in the sense that they coincide with the closure in the weak operator topology of the unital algebra generated by the operator. Moreover, some consequences regarding the extended eigenvalues and the strong compactness of such operators are derived. This extends previous results of Lacruz, Le\'on-Saavedra, Petrovic and Rodr\'iguez-Piazza, Fern\'andez-Valles and Lacruz and Shapiro on linear fractional composition operators acting on $H^2$.