Mathematical Problems in Engineering

Volume 2015 (2015), Article ID 434651, 7 pages

http://dx.doi.org/10.1155/2015/434651

## Synchronization and Antisynchronization for a Class of Chaotic Systems by a Simple Adaptive Controller

School of Science, Qilu University of Technology, Jinan 250353, China

Received 21 August 2015; Accepted 20 October 2015

Academic Editor: Jonathan N. Blakely

Copyright © 2015 Ling Ren and Rongwei Guo. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper investigates the synchronization and antisynchronization for a class of chaotic system. Firstly, a necessary and sufficient condition is proposed to synchronize and antisynchronization simultaneously for the chaotic systems. Secondly, two methods are obtained to realize coexistence of synchronization and antisynchronization in the chaotic systems, and the corresponding adaptive controllers are also given. Finally, two numerical examples with simulation verify the correctness and effectiveness of the obtained results.

#### 1. Introduction

Since Lorenz firstly found the classical chaotic attractor in 1963 [1], as a most fascinating phenomenon in nonlinear dynamical system, chaos has been intensively studied over the past few decades; see [2, 3] and the references therein. It is well known that Pecora and Carroll firstly investigated the synchronization problem of chaotic systems in 1990 [4], and Ott et al. firstly presented a method to control chaotic systems successfully in 1990 in [5]. From then on, chaos control and chaos synchronization have received a great deal of attention in the area of nonlinear control as the significance of these two problems in both academic research and practical applications, and many important results were obtained; please refer to [6–11].

Up to date, several types of typical synchronization have been identified such as complete synchronization (CS), phase synchronization (PS), lag synchronization (LS), generalized synchronization (GS), anti-phase synchronization (AS), and projective synchronization (PS), and a variety of works have been done about the above problems; see [6, 7, 12–16] and the references therein. It is well known that the master system synchronizes the slave system which is equivalent to the error system that is asymptotically stable. That is to say, chaos synchronization is equivalent to the error system which is asymptotically stable. Similarly, the master system antisynchronizes the slave system which is equivalent to the sum system that is also asymptotically stable. From the view of control theory [17], in order to design a simple and physical controller, the following condition is necessary; that is, is an equilibrium point of the unforced nominal error system , , where , and is also an equilibrium point of the unforced nominal sum system , , where . Obviously, , that is, , is an equilibrium point of the error system . Whereas, , that is, , is an equilibrium point of the error system if and only if . Thus, the antisynchronization problem is more complex than the synchronization problem. However, this necessary condition is not considered in the most of the existing works on antisynchronization of chaotic systems [14, 15]. Although the authors have solved the antisynchronization of chaotic systems successfully, the controllers that have been obtained are complex; that is, some terms in those controllers are needed to counteract the redundant terms which make not the equilibrium point of the sum system . For example, in of (14) counteracts the redundant term in error system (13), and in of (14) also does; for details please see [15].

It should be pointed out that most of the existing works focus on investigating the same kind synchronization in a given chaotic system; that is, all the states of the slave system have the same kind synchronization to the corresponding states of the master system. For example, when we say that two systems are synchronized (or antisynchronized, or lagsynchronized, or something else) with each other, it means that each pair of the states between the interactive systems is complete synchronous (or antisynchronous, or something else). In [18], the authors firstly pointed out the coexistence and switching of anticipating synchronization and lag synchronization in an optical system. From then on, some important results have been obtained; see [19, 20]. However, there are no results which can give some conditions or algorithms to select what variables in the master chaotic system which can synchronize or antisynchronize the corresponding variables in the slave chaotic systems have been published so far. Therefore, the coexistence of synchronization and antisynchronization of a class of chaotic or hyperchaotic systems needs further research.

Motivated by the above two reasons, we investigate the synchronization and antisynchronization for a class of chaotic systems in this paper. Firstly, for a class of chaotic systems, we obtain a necessary and sufficient condition with which the master system can synchronize and antisynchronize the slave system simultaneously. Secondly, we give two methods to realize coexistence of synchronization and antisynchronization in the chaotic systems and design the corresponding adaptive controllers. Finally, two numerical examples with simulation verify the correctness and effectiveness of the obtained results.

#### 2. Preliminary Knowledge

This paper studies the synchronization and antisynchronization for a class of chaotic systems by adaptive control method. In order to develop this paper, some assumption and definitions are introduced firstly.

*Assumption 1 (see [17]). * is an equilibrium of the nonlinear system ; that is, .

*Remark 2. *Assumption 1 is a basic assumption of the system control theory. Without loss of generality, if , we can obtain a new system whose equilibrium is by making a coordinate transform .

*Definition 3 (see [12]). *Consider the following chaotic system: where is the state and is a smooth nonlinear vector function.

Let system (1) be the master system; then the corresponding slave system with controller is given as where is the state and is the controller to be designed.

Let , and the error system is described as We call master chaotic system (1) and slave system (2) reach complete synchronization if .

*Definition 4 (see [14]). *Consider master system (1) and slave system (2), let , and the sum system is described as We say master system (1) and slave (2) get antisynchronization if .

*Definition 5 (see [21]). *Consider master system (1) and slave system (2). If error system (3) and sum system (4) can realize stabilization under the controllers and , respectively, where is a smooth function, we say master system (1) synchronizes and antisynchronizes slave system (2) simultaneously; that is, slave system (1) can synchronize and antisynchronize slave system (2) using a controller with the same form.

*Definition 6 (see [18]). *Consider master system (1) and slave system (2). If there exists a controller satisfying and , where , and , we say master system (1) and slave system (2) can realize the coexistence of synchronization and antisynchronization. That is to say, some variables () get synchronization, while some variables () realize antisynchronization.

With the development of this paper, we introduce our previous result which can make the error system or the sum system reach stabilization.

Lemma 7 (see [13]). *Consider error system (3). If and the remainder error system is asymptotically stable, then the controlled error system is given as , and , where , , and the feedback is updated according to the following update law: *

Similarly, for sum system (4), if and the remainder sum system is asymptotically stable, where , , then the controlled sum system is given as , and , and the feedback is updated according to the following update law: where is a positive constant.

#### 3. Main Results

In this section, we firstly give a necessary and sufficient condition for a class of chaotic systems, by which we can determine whether master system (1) and slave system (2) realize synchronization and antisynchronization simultaneously or not.

Theorem 8. *Master system (1) and slave system (2) realize synchronization and antisynchronization simultaneously if and only if ; that is, is the odd function. * *() Since , obviously, and are the equilibria of unforced nominal error system (3) and sum system (4), that is, , respectively. Then, if the controller can stabilize error system (3), sum system (4) can be stabilized by the controller .* *() If there exist the controllers and stabilizing error systems (3) and (4), respectively, we can obtain that and are the equilibria of unforced nominal error system (3) and sum system (4), respectively. Then, .*

*Remark 9. *Although, the problem of synchronization and antisynchronization simultaneously of 4-dimension hyperchaotic system has been investigated in [22], no sufficient or necessary and sufficient condition for the general chaotic systems was proposed. Theorem 8 gives a necessary and sufficient condition for a class of chaotic systems.

*If is not an odd function, master system (1) and slave system (2) cannot realize synchronization and antisynchronization simultaneously according to Theorem 8. Under this condition, they can reach the coexistence of synchronization and antisynchronization. Then, we give two methods to realize the coexistence of synchronization and antisynchronization for a class of chaotic systems.*

*Firstly, consider master system (1) and slave system (2), and let where , , and and , , . The error system is given as *

*Remark 10. *Obviously, if , , then master system (1) and slave system (2) reach complete synchronization. If some , while some , , we say master system (1) and slave system (2) realize the coexistence of synchronization and antisynchronization.

*According to Assumption 1, we give the following conclusion.*

*Theorem 11. Consider unforced nominal error system (8); that is, . If satisfies the condition and at least one and one , where , master system (1) and slave system (2) can reach coexistence of synchronization and antisynchronization.*

*Proof. *According to Assumption 1, should be the equilibrium of unforced nominal error system (8), and thus . And there exist at least one and one , where , and master system (1) and slave system (2) can reach coexistence of synchronization and antisynchronization according to Definition 6.

*Remark 12. *Theorem 11 gives a condition which can determine what variables in master system (1) can synchronize the corresponding variables in slave system (2), while other variables in master system (1) can antisynchronize the corresponding variables in slave system (2).

*Inspired by the results in [16], we give another method to realize coexistence of synchronization and antisynchronization for a class of chaotic systems. First of all, master system (1) is rewritten as where , , if ; that is, is an even function, and we can get the following conclusion.*

*Theorem 13. Consider master system (1) and slave system (2), let , , , and master system (1) can be written as The slave system is described as where , , , is the controller to be designed. If , master system (1) and slave system (2) realize the coexistence of synchronization and antisynchronization.*

*Proof. *Let , , and then Obviously, if , then , , are the equilibria of unforced nominal sum system (12) and error system (13), respectively. According to the nonlinear control theory, the controller designed can reach the coexistence of synchronization and antisynchronization of system (1) and (2).

*4. Illustrative Example*

*In this section, we give two numerical examples to illustrate how to use the results we obtained in this paper to realize the synchronization and antisynchronization simultaneously and the coexistence of synchronization and antisynchronization, respectively.*

*Example 14. *Consider the following chaotic system [23]: where and , and system (14) is chaotic.

*Obviously, system (14) satisfies Theorem 8; thus this system can realize the synchronization and antisynchronization simultaneously.*

*Let system (14) be the master system, and then the slave system is as follows: where is the controller to be designed.*

*Let , and if , the following remainder error system is asymptotically stable. According to Lemma 7, the controller is designed as , and .*

*By the similar procedure, the controller can make master system (14) antisynchronizes slave system (15), where .*

*Next, we give numerical simulations. The initial values of master system (14) and slave system (15) are , , and . The simulation results are given in Figures 1 and 2, respectively.*