  The paper deals with output feedback stabilization of exponentially stable
systems by an integral controller. We propose appropriate Lyapunov functionals
to prove exponential stability of the closed-loop system. An example of
parabolic PDE (partial differential equation) systems and an example of
hyperbolic systems are worked out to show how exponentially stabilizing
integral controllers are designed. The proof is based on a novel Lyapunov
functional construction which employs the forwarding techniques.
