I. Introduction
For ordinary differential equations (ODEs), the concept of input-to-state stability (ISS) was introduced in [3]. The corresponding theory is now well developed and has a firm theoretical basis. Several powerful tools for the investigation of ISS are available and a multitude of applications have been developed in nonlinear control theory, in particular, to robust stabilization of nonlinear systems [4], design of nonlinear observers [5], analysis of large-scale networks [6]– [8], etc.