**Memoirs of the American Mathematical Society**

1997;
62 pp;
Softcover

MSC: Primary 47; 15;

Print ISBN: 978-0-8218-0651-7

Product Code: MEMO/129/615

List Price: $42.00

Individual Member Price: $25.20

**Electronic ISBN: 978-1-4704-0200-6
Product Code: MEMO/129/615.E**

List Price: $42.00

Individual Member Price: $25.20

# Model Theory and Linear Extreme Points in the Numerical Radius Unit Ball

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*Michael A. Dritschel; Hugo J. Woerdeman*

This memoir initiates a model theory-based study of the numerical radius norm. Guided by the abstract model theory of Jim Agler, the authors propose a decomposition for operators that is particularly useful in understanding their properties with respect to the numerical radius norm. Of the topics amenable to investigation with these tools, the following are presented:

- A complete description of the linear extreme points of the \(n\times n\) matrix (numerical radius) unit ball
- Several equivalent characterizations of matricial extremals in the unit ball; that is, those members which do not allow a nontrivial extension remaining in the unit ball
- Applications to numerical ranges of matrices, including a complete parameterization of all matrices whose numerical ranges are closed disks

In addition, an explicit construction for unitary 2-dilations of unit ball members is given, Ando's characterization of the unit ball is further developed, and a study of operators satisfying \(|A| - \mathrm{Re} (e^{i\theta}A)\geq 0\) for all \(\theta\) is initiated.

#### Readership

Graduate students and research mathematicians interested in operator theory.

#### Table of Contents

# Table of Contents

## Model Theory and Linear Extreme Points in the Numerical Radius Unit Ball

- Contents vii8 free
- Abstract viii9 free
- Introduction 110 free
- 1. The Canonical Decomposition 514 free
- 2. The Extremals ∂[sup(e)] 1322
- 3. Extensions to the Extremals 3544
- 4. Linear Extreme points in C 3847
- 5. Numerical Ranges 4655
- 6. Unitary 2-Dilations 5463
- 7. Application to the inequality A „ Re (e[sup(iθ)]A) ≥ 0 5766
- Appendix 5968
- References 6069
- Index 6170 free