Lyapunov–Krasovskii characterization of the input-to-state stability for neutral systems in Hale's form

The primary contribution of the paper is to show that the Lyapunov–Krasovskii conditions available in the literature, for checking the input-to-state stability property of systems described by neutral functional differential equations in Hale’s form, are also necessary. It is shown that the Lyapunov–Krasovskii conditions available in the literature are equivalent to the input-to-state stability property, for an enlarged class of neutral systems. Finally, a novel necessary and sufficient condition for the input-to-state stability property is provided.