Graphs, causality and stabilizability: linear, shift-invariant systems on L-2 [0,infinity)

T.T. Georgiou and M.C. Smith


This paper presents a number of basic elements for a system theory of linear, shift-invariant systems on L-2 [0,infinity). The framework is developed from first principles and considers a linear system to be a linear (possibly unbounded) operator on L-2 [0,infinity). The properties of causality and stabilizability are studied in detail and necessary and sufficient conditions for each are obtained. The idea of causal extendibility is discussed and related to operators defined on extended spaces. Conditions for w-stabilizability and w-stability are presented. The graph of the system (operator) will play a unifying role in the definitions and results. We will discuss the natural partial order on graphs (viewed as subspaces) and it