I. Introduction
The study of consensus problem of multi-agent systems has attracted enormous amounts of interest in the last few years by researchers in various fields due to their broad applications in flocking (see [1]-[3]), cooperative control [4], formation control [5], and many other areas. Consensus is a particular class of network control problem where the goal is for the individual nodes to reach an agreement on the states of all agents. It has been extensively studied by numerous researchers from various perspectives. Consensus algorithms of single integrator dynamic systems (see [6]- [12]) as well as double-integrator dynamic systems (see [13]-[16]) or even high order dynamic systems (see [17]-[19]) have been discussed. Fractional calculus is an old topic. The birth of fractional calculus can be dated back to the seventeenth century. Until the last few years, fractional calculus has become an active research field since many real systems are fractional-order rather than integer-order. Applications of fractional calculus have flourished in various science and engineering fields. It was found that many dynamical systems can be described with fractional calculous: electrochemistry, robotics, economics, electrical machines, electromagnetic waves, signal processing, nonlinear control and so on. Therefore, it is very meaningful to study the consensus problems of fractional-order systems. However, very few of the previous literature has dealt with the consensus problems of fractional-order systems. To the best of our knowledge, the consensus of fractional-order systems was first investigated in [20]–[22]. The convergence speed of fractional-order consensus algorithm was further discussed in [23]. A switching fractional order consensus protocol was proposed, which efficiently increased the convergence speed and ensured the exponential convergence as time tends to infinity. The consensus problem of fractional-order systems with input delays was investigated in [24]. Based on the generalized Nyquist stability criterion and Laplace transform method, a necessary and sufficient condition was derived to ensure the consensus of fractional-order systems with identical input delays over directed interaction topology. In the following work [25], they studied the fractional-order systems with nonuniform input and communication delays over directed static networks. Sufficient conditions were obtained to ensure the consensus of the fractional-order systems with simultaneously nonuniform input and communication delays.