Isometric dilations and von Neumann inequality for finite rank commuting contractions

TL;DR

This paper introduces a new class of commuting contractions on Hilbert spaces for n ≥ 3, demonstrating that under certain conditions, these tuples always have explicit isometric dilations and satisfy a refined von Neumann inequality.

Contribution

It defines the class $ ext{P}_n( ext{H})$ and proves that tuples in this class admit explicit isometric dilations and satisfy a refined von Neumann inequality under rank-finiteness assumptions.

Findings

Tuples in $ ext{P}_n( ext{H})$ always admit explicit isometric dilations.

They satisfy a refined von Neumann inequality involving algebraic varieties.

The results hold even in finite-dimensional Hilbert spaces under certain conditions.

Abstract

Motivated by Ball, Li, Timotin and Trent's Schur-Agler class version of commutant lifting theorem, we introduce a class, denoted by $\mathcal{P}_n(\mathcal{H})$, of $n$-tuples of commuting contractions on a Hilbert space $\mathcal{H}$. We always assume that $n \geq 3$. The importance of this class of $n$-tuples stems from the fact that the von Neumann inequality or the existence of isometric dilation does not hold in general for $n$-tuples, $n \geq 3$, of commuting contractions on Hilbert spaces (even in the level of finite dimensional Hilbert spaces). Under some rank-finiteness assumptions, we prove that tuples in $\mathcal{P}_n(\mathcal{H})$ always admit explicit isometric dilations and satisfy a refined von Neumann inequality in terms of algebraic varieties in the closure of the unit polydisc in $\mathbb{C}^n$.