Research Article | Open Access

# Compound Generalized Function Projective Synchronization for Fractional-Order Chaotic Systems

**Academic Editor:**Gian I. Bischi

#### Abstract

A modified function projective synchronization for fractional-order chaotic system, called compound generalized function projective synchronization (CGFPS), is proposed theoretically in this paper. There are one scaling-drive system, more than one base-drive system, and one response system in the scheme of CGFPS, and the scaling function matrices come from multidrive systems. The proposed CGFPS technique is based on the stability theory of fractional-order system. Moreover, we achieve the CGFPS between three-driver chaotic systems, that is, the fractional-order Arneodo chaotic system, the fractional-order Chen chaotic system, and the fractional-order Lu chaotic system, and one response chaotic system, that is, the fractional-order Lorenz chaotic system. Numerical experiments are demonstrated to verify the effectiveness of the CGFPS scheme.

#### 1. Introduction

Chaotic phenomenon in nonlinear system has attracted more and more attentions in recent years [1, 2]. In 1990,* chaos synchronization* was introduced by Pecora and Carroll for the first time [3], and after that, chaos synchronization has attracted a lot of attention over the last two decades. This is due to its potential applications in many areas of physics and engineering science [3], such as complex networks control [4], signal processing [5], information transmission [6], authenticated encryption [7], and secure communication [8, 9]. On the other hand, as physical interpretation of the fractional derivative becomes clear, many real-world physical systems can be more accurately described by fractional-order differential equations [10, 11]. Chaotic phenomenon has been also observed in many physical fractional-order systems, for example, microelectromechanical systems [12], gyroscopes [13], and electronic circuits [14, 15]. Meanwhile, more and more attentions [4, 7, 16, 17] have been paid on synchronization and control of fractional-order chaotic system in both theoretical and applied perspectives, since it is usually a prerequisite of many practical applications in chaos engineering, especially in authenticated encryption [7] and chaos communications [18].

Up to now, various types of chaos synchronization have been proposed, such as complete synchronization (CS) [3, 8], generalized synchronization (GS) [9, 19], projective synchronization (PS) [20], function projective synchronization (FPS) [21–23], combination synchronization [24], and compound synchronization [25]. Among all these schemes, PS and FPS have been extensively studied in recent years because the two schemes can not only obtain faster communication but also enhance the security of communications with its proportional feature. The PS scheme indicates that the drive system and response system could be arrived to synchronize with a scaling constant factor. The FPS scheme, generalized from the PS scheme, means that the driver system and response system could be synchronized with a scaling function matrix instead of the scaling factor. Since the scaling function matrix can be also used as additional secret codes, the FPS scheme can enhance the security in secure communication.

However, the response system could be synchronized to the drive system with a function matrix in FPS scheme, and there are only one drive system and one response system. Therefore, the FPS between multidrive systems and one response system, where the scaling function matrix comes from multidrive systems, is interesting and general topic. It is obvious that multidrive systems and one response system synchronization in FPS scheme can additionally enhance the security of communication; this is due to the fact that the transmitted signals can be split into several parts, and each part may be loaded in different drive systems; or the transmitted signals can be divided in time into different intervals, and the signals in different intervals may be loaded in different drive systems [24, 25]. Motivated by the abovementioned, a new function projective synchronization scheme, called compound generalized function projective synchronization (briefly denoted by CGFPS), is proposed for fractional-order chaotic systems in this paper. The CGFPS scheme means that there are multidrive systems and one response system, and the scaling function matrix comes from multidrive systems. This CGFPS technique is based on the stability theory of fractional-order systems. Some examples are demonstrated to verify the effectiveness of the CGFPS scheme.

The layout of this paper is organized as follows. In Section 2, the CGFPS scheme for fractional-order chaotic systems is presented. In Section 3, numerical experiments are used to verify the effectiveness of the CGFPS scheme. Finally, the conclusion ends the paper in Section 4.

#### 2. The Compound Generalized Function Projective Synchronization (CGFPS) for Fractional-Order Chaotic Systems

In this section, we will demonstrate the CGFPS scheme for fractional-order chaotic systems. As is well known, there are several definitions about fractional derivative. This paper will use the Caputo definition of the fractional derivative, described as follows [26]:where is called the Caputo operator, is the first integer which is not less than , and is the -order derivative for ; that is, .

Consider the fractional-order multidrive chaotic systems and one fractional-order response chaotic system described as follows:where system (2) is an -dimensional scaling-drive system [25], each system in (3) is an -dimensional base-drive system [25], and system (4) is an -dimensional response system. () and are fractional-order. () and are state vectors of fractional-order chaotic systems (2)–(4). () and are both differential nonlinear vector functions. is a controller which will be designed later. In the base-drive systems described in (3), each system is an individual chaotic system, which is not related to any other chaotic systems in (3), and the state variables in th chaotic system in (3) are .

Before we give a detailed definition of the CGFPS scheme, let us first recall the definitions of the FPS scheme and the compound synchronization scheme.

*Definition 1 (see [21–23]). *For the scaling-drive system (2) and the response system (4), in which the vector controller is changed as in response system (4), it is said to be function projective synchronization (FPS) if there exists scaling function matrix such thatwhere represents the Euclidean norm.

*Definition 2 (see [25]). *For the scaling-drive system (2), the base-drive systems (3), and the response system (4), it is said to be compound synchronization if there exists constant real matrix () such thatwhere represents the Euclidean norm.

Now, we establish the CGFPS scheme for fractional-order chaotic systems.

*Definition 3. *For the scaling-drive system (2), the base-drive systems (3), and the response system (4), it is said to be compound generalized function projective synchronization if there exist constant real matrix () and scaling function matrix () such thatwhere represents the Euclidean norm.

*Remark 4. *If , (), and is a nonzero function matrix, then the CGFPS scheme will be turned into FPS scheme. If , (), and () is a nonzero function matrix, then the CGFPS scheme will be turned into compound synchronization scheme. If () in the CGFPS scheme, then the CGFPS scheme will be turned into a chaos control problem.

*Remark 5. *The scaling function matrix () in the CGFPS comes from multidrive chaotic systems (2) and (3), and the scaling function matrix in the FPS comes from one drive system (2) (the scaling-drive system). On the other hand, the scaling matrix () in compound synchronization scheme is a real constant matrix. Therefore, the CGFPS scheme is more general than the FPS scheme and the compound synchronization scheme.

*Remark 6. *The scaling function matrix () in CGFPS comes from different chaotic systems ( chaotic systems), and the term in CGFPS may be nonlinear. On the other hand, the scaling function matrix in FPS comes from one drive system, and the term in compound synchronization is linear. Since additional secret codes can come from () and in the secure communication, the CGFPS scheme should be more secure than the FPS and compound synchronization schemes.

*Remark 7. *The scaling-drive system (2) and the base-drive systems (3) in CGFPS scheme are maybe integer order systems. Therefore, the CGFPS between integer order chaotic systems and fractional-order systems can be achieved. Moreover, the CGFPS can be applied for integer order chaotic systems; that is, the scaling-drive system (2), the base-drive systems (3), and response system (4) are integer order chaotic systems.

Next, we will discuss how to achieve CGFPS between the drive systems (2) and (3) and the response system (4). Now, let the CGFPS error vectors between the drive systems (2) and (3) and the response system (4) bewhere , (), () denotes the element of matrix , and () indicates the element of matrix .

Now, the controller in system (4) can be chosen aswhere is a feedback controller which will be designed later. () is a bounded nonzero constant matrix, and () is a bounded nonzero function matrix.

Using the above controller (9), system (4) can be rewritten asUsing , system (10) can be changed as

For response system (4), we assume that in this paper, where and . In fact, this assumption can be satisfied by many fractional-order chaotic/hyperchaotic systems, such as fractional-order Lorenz chaotic/hyperchaotic systems, fractional-order Chen chaotic/hyperchaotic systems, fractional-order Lü chaotic/hyperchaotic systems, and the fractional-order chaotic systems which will be described in Section 3.

Combining , system (11) can be rewritten aswhere .

Therefore, we can yield the following fractional-order system about errors:According to system (13), we know that the CGFPS between systems (2) and (3) and system (4) is turned into the following problem: choose suitable vectors such that fractional-order error system (13) is asymptotically converged to zero.

Here, let us recall one result for the stability of nonlinear fractional-order system [23, 27, 28]. Given the following -dimension nonlinear fractional-order systems,where , , is a nonlinear continuous vector function. The equilibrium point (i.e., ) of system (14) is asymptotically stable if all the eigenvalues of the Jacobean matrix satisfy . Then, we have the main theorem of this paper as follows.

Theorem 8. *If one chooses the feedback controller and real symmetric positive definite matrix , such thatthen the compound generalized function projective synchronization (CGFPS) between drive systems (2) and (3) and response system (4) can be achieved, where denotes the conjugate transpose of a matrix.*

*Proof. *Use According to the results mentioned in [23, 29], we can yield the following:Here, are the eigenvalues of the matrix .

According to the stability for nonlinear fractional-order systems [23, 27, 28], inequality (17) implies that the equilibrium point of fractional-order nonlinear system (13) is asymptotically stable; that is, This result implies that the compound generalized function projective synchronization (CGFPS) between drive systems (2) and (3) and response system (4) can be achieved. The proof is completed.

#### 3. Illustrative Examples

In this section, numerical experiments are used to verify the effectiveness of the CGFPS scheme in our paper. Some examples are given and the numerical simulations are performed.

Now, choose the fractional-order Arneodo chaotic system as the scaling-drive system, which is given by [30]The chaotic attractor of system (19) with is shown in Figure 1.

**(a)**

**(b)**

Then, choose the fractional-order Chen chaotic system and the fractional-order Lu chaotic system as the base-drive systems. The fractional-order Chen chaotic system [22] is defined asand the chaotic attractor of system (20) with is shown as in Figure 2. The fractional-order Lü chaotic system [31] is defined asand the chaotic attractor of system (21) with is shown in Figure 3.

**(a)**

**(b)**

**(a)**

**(b)**

Next, we choose the fractional-order Loren chaotic system as response system, which is given by [22]The chaotic attractor of system (22) with is shown in Figure 4.

**(a)**

**(b)**

Next, we discuss how to realize the CGFPS between drive systems (19)–(21) and response system (22). Now, let () be a constant real matrix and let () be a scaling function matrix, where ().

According to Section 2, we can yield thatwhere , () are the elements of matrix , and .

If we choose the feedback controller and the real symmetric positive definite matrix as followsthen, it follows thatwhere , which is a real symmetric positive definite matrix. According to (17) in Section 2, we obtainThen, according to the stability for nonlinear fractional-order systems [23, 27, 28], we havewhich implieswhere () denotes the th element of matrix .

Equations (27)–(28) indicate that the compound generalized function projective synchronization (CGFPS) between drive systems (19)–(21) and response system (22) can be achieved.

Now, two cases with numerical simulations are given to verify the effectiveness of proposed scheme.

*Case 1. *Choose (), , , and . The numerical results are shown in Figure 5, where the initial conditions are , , , and , respectively.

**(a)**

**(b)**

**(c)**

*Case 2. *Choose , (), , and . The corresponding numerical results are shown in Figure 6, where the initial conditions are , , , and , respectively.

**(a)**

**(b)**

**(c)**

#### 4. Conclusions

In this paper, we present a new synchronization scheme for fractional-order chaotic system and called this type of synchronization compound generalized function projective synchronization, or briefly denoted it by CGFPS. In this scheme, there are one scaling-drive system, more than one base-drive system, and one response system, and the scaling function matrix comes from multidrive systems. So, the CGFPS in this paper is different from all the previous synchronization reported before. To verify its effectiveness, we achieve the CGFPS between three drive systems and one response system, where the scaling-drive system is the fractional-order Arneodo chaotic system, the base-drive systems are the fractional-order Chen chaotic system and the fractional-order Lu chaotic system, and the response system is the fractional-order Loren chaotic system. Numerical simulations suggest that the presented CGFPS scheme works well. It is worth mentioning that the CGFPS scheme in our paper can also be used for other fractional-order chaotic systems or integer order chaotic systems.

#### Conflict of Interests

The authors declare no conflict of interests.

#### Authors’ Contribution

Ping Zhou and Chunde Yang proposed and designed the research. Ping Zhou, Hao Cai, and Chunde Yang performed the simulations. Hao Cai and Ping Zhou analyzed the simulation results. Ping Zhou and Chunde Yang wrote the paper. All authors have read and approved the final paper.

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#### Copyright

Copyright © 2016 Chunde Yang et al. 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.