Although one can trace the heritage of system theory back through the centuries, the field did not truly come into its own until the post-World
War II era. By the mid-1960s, however, the field had expanded to the
point where communication between its various branches had begun to
break down, with some practitioners applying the new state space
concepts while others employed the traditional frequency domain techniques.
Moreover, the systems community was beginning to encounter new classes
of distributed, time-varying, multivariate, and discrete time systems, and
a multiplicity of new concepts associated therewith. As such, with the
goal of bringing some order into this mounting chaos, a number of
researchers began to search for a unified approach to linear system theory.
Although many approaches were tried with varying degrees of success, the
present work represents the culmination of one such search, in which
Hilbert space techniques are used to formulate a unified theory of linear
systems.