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
- Myeongjin Kang
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