“…A powerful tool in this context is the averaging method, see, for instance, [1], [3], [5], [6], [7], [10], [11], and references therein. The key idea of the averaging method is to convert the stability analysis for a time varying system to the analysis of a time invariant system that can be approximated in certain manners by the original system as the small parameter approaches 0 (see Definitions 3.1 and 3.4).…”