*Article* **Fractional-Order Interval Observer for Multiagent Nonlinear Systems**

**Haoran Zhang, Jun Huang \* and Siyuan He**

The School of Mechanical and Electrical Engineering, Soochow University, Suzhou 215131, China; zhrsudaedu@163.com (H.Z.); hesiyuan021108@163.com (S.H.)

**\*** Correspondence: cauchyhot@163.com

**Abstract:** A framework of distributed interval observers is introduced for fractional-order multiagent systems in the presence of nonlinearity. First, a frame was designed to construct the upper and lower bounds of the system state. By using monotone system theory, the positivity of the error dynamics could be ensured, which implies that the bounds could trap the original state. Second, a sufficient condition was applied to guarantee the boundedness of distributed interval observers. Then, an extension of Lyapunov function in the fractional calculus field was the basis of the sufficient condition. An algorithm associated with the procedure of the observer design is also provided. Lastly, a numerical simulation is used to demonstrate the effectiveness of the distributed interval observer.

**Keywords:** distributed interval observer; fractional-order multiagent systems; monotone system theory
