Functional observability and the design of minimum order linear functional observers

The design of a minimum-order linear functional observer for linear time-invariant systems has been an open problem for over four decades. This technical note provides a solution to this problem. The technical note also introduces the concept of <i>Functional Observability/Detectability </i>and shows that the well-known concept of Observability/Detectability is a special case of <i>Functional Observability/Detectability</i>.