The book concisely presents the fundamental aspects of the theory of
operators on Hilbert spaces. The topics covered include functional
calculus and spectral theorems, compact operators, trace class and
Hilbert-Schmidt operators, self-adjoint extensions of symmetric
operators, and one-parameter groups of operators.
The exposition of the material on unbounded operators is based on a
novel tool, called the z-transform, which provides a way to encode full
information about unbounded operators in bounded ones, hence making many
technical aspects of the theory less involved.