The mathematical theory of "open" dynamical systems is a creation of the
twentieth century. Its humble beginnings focused on ideas of Laplace
transforms applied to linear problems of automatic control and to the
analysis and synthesis of electrical circuits. However during the second
half of the century, it flowered into a field based on an array of
sophisticated mathematical concepts and techniques from algebra,
nonlinear analysis and differential geometry. The central notion is that
of a dynamical system that exchanges matter, energy, or information with
its surroundings, i.e. an "open" dynamical system. The mathema- tization
of this notion evolved considerably over the years. The early
development centered around the input/output point of view and led to
important results, particularly in controller design. Thinking about
open systems as a "black box" that accepts stimuli and produces
responses has had a wide influence also in areas outside engineering,
for example in biology, psychology, and economics. In the early 1960's,
especially through the work of Kalman, input/state/output models came in
vogue. This model class accommodates very nicely the internal initial
conditions that are essentially always present in a dynamical system.
The introduction of input/state/output models led to a tempestuous
development that made systems and control into a mature discipline with
a wide range of concepts, results, algorithms, and applications.