In this paper, the problem of the stabilization in the sample-and-hold sense for fully nonlinear systems with an arbitrary number of arbitrary discrete as well as of distributed time delays is studied. It is shown that steepest descent feedbacks, continuous or not, induced by Lyapunov-Krasovskii functionals in a suitable (large) class, are stabilizers in the sample-and-hold sense. The fact that discontinuities are overcome by the sampling and holding process enlarges greatly the possibility of finding successful controllers for retarded nonlinear systems, by means of control Lyapunov-Krasovskii functionals.
Introduction.The notion of stabilization in the sample-and-hold sense has been introduced in 1997 by Clarke et al. in [9], and widely studied for systems described by ordinary differential equations. The following result holds for nonlinear systems described by ordinary differential equations (see [9], [7]): any steepest descent feedback, induced by a control Lyapunov function, is a stabilizer in the sample-andhold sense. The main advantage of this result is the possibility of using discontinuous steepest descent feedbacks. In this paper we prove an analogous result for nonlinear retarded systems.The stabilization problem for nonlinear systems with delays in the state and/or in the input has involved many researchers, at least in the last ten years (see, for instance, [2], [3], [4], [16], [19], [34], [38], [40], [42], [43], [44], [56], [60]). Though many methods are available in the literature, the problem of global stabilization for fully nonlinear systems with an arbitrary number of discrete delays, of any size, and of distributed delays, is far from being completely solved. This paper concerns the study of a stabilization method for retarded systems, in the path proposed by Artstein in [1] for systems described by ordinary differential equations, that is, by control Lyapunov functions (see [1], [55]). A known problem when dealing with control Lyapunov functions is that in some cases the candidate feedback control law is discontinuous (see [7]). For instance, when control Lyapunov-Razumikhin functions for retarded systems (see [22]) are used, the problem of the feedback discontinuity can arise if one makes use of universal formulas such as Sontag's one (see [55]). By suitable control Lyapunov-Krasovskii functionals, continuous feedback control laws can be provided by universal formulas (see [23]), when the small control property is satisfied (see [55] and references therein). In [26], [27], the authors prove the equivalence of the existence of a completely locally Lipschitz control Lyapunov-Krasovskii functional satisfying the small control property, and the stabilizability property by means of