I. Introduction
Over the past decade, fuzzy or neural adaptive control design problem has extensively been studied for nonlinear systems in strict-feedback form, and many important control methods have been developed, for example [1] –[17]. Among them, [1] –[3] are for single-input and single-output nonlinear systems, [4]–[6] are for multiple-input and multiple-output nonlinear systems, while [7], [8] for large-scale nonlinear systems. Since the aforementioned adaptive control schemes require that the state variables should be available for measurement, several observer-based adaptive fuzzy output-feedback control schemes have been proposed by [9]– [17], and the immeasurable states problem is thus solved.