I. Introduction
The analysis of intrinsic robustness of stability properties with respect to perturbations and input disturbances constitutes a widely recurrent theme in nonlinear system theory. The preservation of stability in regular perturbations, i.e., small changes of the vector field continuously depending on a given parameter, has been investigated since the 1940s and summarized in [35, Chapter VI] and [12, Section 56]. Singular perturbation theory is typically referenced whenever time-scale separation plays a major role in control engineering problems [15], [16]. Robustness of asymptotic stability in differential inclusions and complete hybrid systems under admissible perturbation radiuses are addressed in [3] and [5], respectively. Averaging techniques are largely employed to yield practical stability results under rapidly varying input disturbances [24]. In particular, practical stabilization techniques by means of high oscillatory controls have been developed in [17]–[19], [21], and [30]. More recently, Moreau and Aeyels [19] and Teel et al. [32] used a Lyapunov and a trajectory-based approach to prove the robustness of time-varying nonlinear systems whose solutions continuously depend on a small parameter, thus enclosing most of the hitherto-presented robustness problems in a semiglobal practical framework.