Complete Crouzeix equality without a scalar coefficient compression

We give an explicit irreducible matrix N of order six with unit-disk numerical range and a 2 by 2 matrix polynomial P satisfying max over the unit disk of ||P(z)|| = 1 and ||P[N]|| = 2. Every compression by constant complex unit coefficient vectors u and v nevertheless satisfies ||(v* P u)(N)|| <= sqrt(849)/16 < 2. The matrix is twice a member of the classical Li-Spitkovsky-Shukla family. A simple top singular pair provides a finite certificate of equality and of the obstruction to scalar coefficient compression. All assertions follow from explicit matrix calculations.

Authors

Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-10-13
DOI
https://doi.org/10.5281/zenodo.23350019
Primary Topic
Holomorphic and Operator Theory
Type
preprint
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preprint

Complete Crouzeix equality without a scalar coefficient compression

Myeongjin Kang
Zenodo (CERN European Organization for Nuclear Research)
Holomorphic and Operator Theory
preprint

Complete Crouzeix equality without a scalar coefficient compression

Myeongjin Kang
preprint en

Abstract

We give an explicit irreducible matrix N of order six with unit-disk numerical range and a 2 by 2 matrix polynomial P satisfying max over the unit disk of ||P(z)|| = 1 and ||P[N]|| = 2. Every compression by constant complex unit coefficient vectors u and v nevertheless satisfies ||(v* P u)(N)|| <= sqrt(849)/16 < 2. The matrix is twice a member of the classical Li-Spitkovsky-Shukla family. A simple top singular pair provides a finite certificate of equality and of the obstruction to scalar coefficient compression. All assertions follow from explicit matrix calculations.

Zenodo (CERN European Organization for Nuclear Research)
Holomorphic and Operator Theory
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