OO It is a matter of general consensus that in the last decade the H _
optimization for robust control has dominated the research effort in
control systems theory. Much attention has been paid equally to the
mathematical instrumentation and the computational aspects. There are
several excellent monographs that cover the standard topics in the area.
Among the recent issues we have to cite here Linear Robust Control
authored by Green and Limebeer (Prentice Hall 1995), Robust Controller
Design Using Normalized Coprime Factor Plant Descriptions - by McFarlane
and Glover (Springer Verlag 1989), Robust and Optimal Control - by Zhou,
Doyle and Glover (Prentice Hall 1996). Thus, when the authors of the
present monograph decided to start the work they were confronted with a
very rich literature on the subject. However two reasons motivated their
initiative. The first concerns the theory in which the whole development
of the book was embedded. As is well known, there are several ways of
approach- oo ing H and robust control theory. Here we mention three
relevant direc- tions chronologically ordered: a) the first makes use of
a generalization of the Beurling-Lax theorem to Krein spaces; b) the
second makes use of a generalization of Nevanlinna-Pick interpolation
theory and commutant lifting theorem; c) the third, and probably the
most attractive from an el- evate engineering viewpoint, is the two
Riccati equations based approach which offers a complete solution in
state space form.