On the Lyapunov equation in Banach spaces and applications to control problems

By extending the Lyapunov equation A ∗ Q + QA =− P to an arbitrary infinite-dimensional Banach space, we give stability conditions for a class of linear differential systems. Relationship between stabilizability and exact null-controllability is established. The result is applied to obtain new sufficient conditions for the stabilizability of a class of nonlinear control systems in Banach spaces.