A Note on Functional Observability

In this note, we propose an alternative to characterize the functional observability for linear systems. The main feature is that we obtain a necessary and sufficient condition for the existence of a stable multi-functional observer of a time-invariant linear system. The proof of this condition is constructive and it leads to design a stable observer via a new procedure, neither based on the solution of a Sylvester equation nor on the use of canonical state space forms.