A perspective on system theory

In broad terms "system theory" is concerned with the analytical and computational aspects of mathematical models of structures comprised of subsystems or components. This paper contains a section on the origins and history of system theory, and additional sections concerning: (i) nonlinear networks containing semiconductor devices, (ii) functional-analytic methods and the circle criterion, (iii) compartmental analysis and input-output models of a national economy, and (iv) functional expansions and Volterra series. The first section provides a broad view of system theory, with emphasis on its early development; the remaining sections illustrate in more detail the nature of some general problems and results of interest to this writer and others.