Similarity problems in noncommutative polydomains

TL;DR

This paper extends classical operator similarity results to noncommutative polydomains, exploring joint similarity, noncommutative cones, and Berezin transforms, providing new analogues and characterizations in this advanced setting.

Contribution

It introduces noncommutative analogues of classical similarity theorems, linking similarity problems with noncommutative cones and Berezin transforms in operator theory.

Findings

Analogues of Rota's theorem for noncommutative polydomains

Characterization of similarity to isometries in noncommutative setting

Connection between similarity and positive invertible elements in noncommutative cones

Abstract

In this paper we consider several problems of joint similarity to tuples of bounded linear operators in noncommutative polydomains and varieties associated with sets of noncommutative polynomials. We obtain analogues of classical results such as Rota's model theorem for operators with spectral radius less than one, Sz.-Nagy characterization of operators similar to isometries (or unitary operators), and the refinement obtained by Foia\c s and by de Branges and Rovnyak for strongly stable contractions. We also provide analogues of these results in the context of joint similarity of commuting tuples of positive linear maps on the algebra of bounded linear operators on a separable Hilbert space. An important role in this paper is played by a class of noncommutative cones associated with positive linear maps, the Fourier type representation of their elements, and the constrained noncommutative Berezin transforms associated with these elements. It is shown that there is a intimate relation between the similarity problems and the existence of positive invertible elements in these noncommutative cones and the corresponding Berezin kernels.