I. Introduction
The transient and steady-state behavior of nonlinear systems is known as a significant performance indicator for the tracking control problem [1], [2], [3], [4]. A methodology called prescribed performance control (PPC) has been proposed in the pioneering works [5] and [6] where the comprehensive performance of the output tracking error is guaranteed upon using two decay boundary functions. Afterwards, much attention has been drawn to the tracking control problem with guaranteed performance, for instance [7], [8], [9], while the assumptions that, the involved system uncertain parameters are constant [7], [9] or the control coefficients are known [7], [8], are usually needed. It is imminently desired to develop an effective PPC algorithm for general nonlinear systems suffering from unknown time-varying parameters and control coefficients.