Nonsmooth Control-Lyapunov Functions

It is shown that the existence of a continuous controlLyapunov function (CLF) is necessary and sufficient for null asymptotic controllability of nonlinear finitedimensional control systems. The CLF condition is expressed in terms of a concept of generalized derivative that has been studied in set-valued analysis and the theory of differential inclusions with various names such as “upper contingent derivative.” This result generalizes to the non-smooth case the theorem of Artstein relating closed-loop feedback stabilization to smooth CLF’s. It relies on viability theory as well as optimal control techniques. A “non-strict” version of the results, analogous to the LaSalle Invariance Principle, is also provided.