Operator Theory

This book is about numerical ranges and spectral properties of Hilbert space operators which have been of great interest to many mathematicians in the past decades. Nice properties with examples are explored. The properties of numerical range, for example, convexity and closedness are well known as proved in the classic Toeplitz - Hausdorff Theorem. In this book, we embark on the relationship between the spectrum and the numerical range of an operator, in particular, when the operator is normal. It is known that for a bounded linear operator on a Hilbert space, the spectrum is contained in the closure of its numerical range. For a normal operator, the numerical radius and the spectral radius coincides with the norm of the operator. These results are actually a contribution to the field of numerical ranges and spectra. For the reader to understand this book, it is paramount that a deep understanding of the theory of operators, especially on Hilbert spaces, General Topology, Functional Analysis and Abstract Algebra be put in place. This book is useful to both undergraduate students and postgraduate students.