The failure of rational dilation on the tetrablock

TL;DR

This paper demonstrates the failure of rational dilation on the tetrablock domain in complex three-space by providing a counterexample, and explores the role of fundamental operators in the spectral set theory of operator tuples.

Contribution

It constructs a specific $ ext{E}$-contraction that violates necessary conditions for rational dilation, and develops a functional model for pure $ ext{E}$-isometries emphasizing fundamental operators.

Findings

Counterexample shows rational dilation fails on the tetrablock.

Fundamental operators must satisfy specific commutation and difference conditions.

A concrete functional model for pure $ ext{E}$-isometries is developed.

Abstract

We show by a counter example the failure of rational dilation on the tetrablock, a polynomially convex and non-convex domain in $\mathbb C^3$, defined as $$ \mathbb E = \{ (x_1,x_2,x_3)\in\mathbb C^3\,:\, 1-zx_1-wx_2+zwx_3\neq 0 \textup{ whenever } |z|\leq 1, |w|\leq 1 \}. $$ A commuting triple of operators $(T_1,T_2,T_3)$ for which the closed tetrablock $\overline{\mathbb E}$ is a spectral set, is called an $\mathbb E$-contraction. For an $\mathbb E$-contraction $(T_1,T_2,T_3)$, the two operator equations $$ T_1-T_2^*T_3=D_{T_3}X_1D_{T_3} \textup{ and } T_2-T_1^*T_3= D_{T_3}X_2D_{T_3}, \quad D_{T_3}=(I-T_3^*T_3)^{\frac{1}{2}},$$ have unique solutions $A_1,A_2$ on $\mathcal D_{T_3}=\overline{Ran} D_{T_3}$ and they are called the fundamental operators of $(T_1,T_2,T_3)$. For a particular class of $\mathbb E$-contractions, we prove it necessary for the existence of rational dilation that the corresponding fundamental operators $A_1,A_2$ satisfy the conditions \begin{equation}\label{abstract} A_1A_2=A_2A_1 \textup{ and } A_1^*A_1-A_1A_1^*=A_2^*A_2-A_2A_2^*. \end{equation} Then we construct an $\mathbb E$-contraction from that particular class which fails to satisfy (\ref{abstract}). We produce a concrete functional model for pure $\mathbb E$-isometries, a class of $E$-contractions analogous to the pure isometries in one variable. The fundamental operators play the main role in this model.