I. Introduction
Nonlinear systems have abundant dynamic behaviors, such as stability, bifurcation, chaos, and so on. The research on its stability and synchronization plays an important role in the fields of secure communication and automatic control [1], [2]. Synchronization of nonlinear systems has always been a hot issue. In 1953, Kanebkov was the first who proposed the concept of the finite-time stability [3], that is, under the action of the controller, the system can reach a stable state in a finite time, but the establishment time of finite-time stability is related to the initial state of the system. In vehicle control, robot tracking control, and other practical applications [4]–[6], accurate initial state is difficult be obtained. In 2012, Polyakov [7] proposed the fixed-time stability to solve this problem. The estimation of the upper bound of the establishment time is independent of the initial state. Therefore, more and more researchers began to invest in the study of fixed-time stability, and tried to find the less conservative upper bound estimate [8]–[11].