Triangular Tetrablock-contractions, factorization of contractions, dilation and subvarieties

TL;DR

This paper introduces triangular tetrablock-contractions, establishes their dilation theory, constructs a functional model, and connects these concepts to subvarieties and the $ ext{Berger-Coburn-Lebow}$ model theorem.

Contribution

It develops the theory of triangular $ ext{E}$-contractions, including dilation, functional models, and their relation to subvarieties, extending classical results in operator theory.

Findings

Every pure triangular $ ext{E}$-contraction dilates to a pure triangular $ ext{E}$-isometry.

Constructed a functional model for pure triangular $ ext{E}$-isometries.

Provided a new proof for the Berger-Coburn-Lebow Model Theorem and its generalizations.

Abstract

A commuting triple of Hilbert space operators $(A,B,P)$, for which the closed tetrablock $\bar{\mathbb E}$ is a spectral set, is called a \textit{tetrablock-contraction} or simply an $\mathbb E$-\textit{contraction}, where \[ \mathbb E=\{(a_{11},a_{22}, \det A):\, A=[a_{ij}]\in \mathcal M_2(\mathbb C), \; \|A\| <1 \} \subset \mathbb C^3 \] is a polynomially convex domain which is naturally associated with the $\mu$-synthesis problem. We introduce triangular $\mathbb E$-contractions and prove that every pure triangular $\mathbb E$-contraction dilates to a pure triangular $\mathbb E$-isometry. We construct a functional model for a pure triangular $\mathbb E$-isometry and apply that model to find a new proof for the famous Berger-Coburn-Lebow Model Theorem for commuting isometries. Next we give an alternative proof to the more generalized version of Berger-Coburn-Lebow Model, namely the factorization of a pure contraction due to Das, Sarkar and Sarkar (\textit{Adv. Math.} 322 (2017), 186 -- 200). We find a necessary and sufficient condition for the existence of $\mathbb E$-unitary dilation of an $\mathbb E$-contraction $(A,B,P)$ on the smallest dilation space and show that it is equivalent to the existence of a distinguished variety in $\mathbb E$ when the defect space $D_{P^*}$ is finite dimensional.