Baras, John S.Bensoussan, AlainThe problem of observers has been introduced in control literature. Let us consider a dynamic system which is deterministic, but whose initial state is unknown. An observer is a model which mimics the behavior of the physical system, in particular its state becomes closer , closer as time evolves to the state of the physical system. There is a great deal of freedom in such a design , it is important to investigate various kinds of observers.en-USTechnical Report