Dynamic Lyapunov functions

Lyapunov functions are a fundamental tool to investigate stability properties of equilibrium points of linear and nonlinear systems. The existence of Lyapunov functions for asymptotically stable equilibrium points is guaranteed by converse Lyapunov theorems. Nevertheless the actual computation (of the analytic expression) of the function may be difficult. Herein we propose an approach to avoid the computation of an explicit solution of the Lyapunov partial differential inequality, introducing the concept of Dynamic Lyapunov function. These functions allow to study stability properties of equilibrium points, similarly to standard Lyapunov functions. In the former, however, a positive definite function is combined with a dynamical system that render Dynamic Lyapunov functions easier to construct than Lyapunov functions. Moreover families of standard Lyapunov functions can be obtained from the knowledge of a Dynamic Lyapunov function by rendering invariant a desired submanifold of the extended state-space. The invariance condition is given in terms of a system of partial differential equations similar to the Lyapunov pde. Differently from the latter, however, in the former no constraint is imposed on the sign of the solution or on the sign of the term on the right-hand side of the equation. Several applications and examples conclude the paper.