Inverse Projective Synchronization between Two Different Hyperchaotic Systems with Fractional Order

This paper mainly investigates a novel inverse projective synchronization between two di ﬀ erent fractional-order hyperchaotic systems, that is, the fractional-order hyperchaotic Lorenz system and the fractional-order hyperchaotic Chen system. By using the stability theory of fractional-order di ﬀ erential equations and Lyapunov equations for fractional-order systems, two kinds of suitable controllers for achieving inverse projective synchronization are designed, in which the generalized synchronization, antisynchronization, and projective synchronization of fractional-order hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system are also successfully achieved, respectively. Finally, simulations are presented to demonstrate the validity and feasibility of the proposed method.


Introduction
The history of fractional calculus is more than three centuries old. It has been found that the behavior of many physical systems can be properly described by fractional-order systems, for example, dielectric polarization, electrode-electrolyte polarization, electromagnetic waves, viscoelastic systems, quantitative finance, and diffusion waves 1-3 . Compared with the classical integer-order models, fractional-order derivatives provide an excellent instrument for the description of memory and hereditary properties of various materials and processes. Nowadays, fractional-order systems have attracted more and more people's attention. Many authors began to investigate the chaotic dynamics of fractional-order nonlinear systems with the help of fractional calculus. For examples, Podlubny proposed a generalization of the PID controller for the control of the fractional-order systems 4 , by using fractional calculus theory. Reference 5 investigated the chaos revealed by the pseudo inverse-based 2 Journal of Applied Mathematics trajectory planning algorithms. With the introduction of fractional derivatives, it was proved that many fractional-order differential systems behave chaotically, such as, Chua's circuit of order as low as 2.7 can produce a chaotic attractor 6 , chaos could exist in the fractionalorder Rössler equation with order as low as 2.4 7 . In 8 , the chaotic behavior and its control in the fractional-order Chen system is investigated. Sheu et al. studied the dynamics of the Newton-Leipnik system with fractional-order and found chaos existing in the fractionalorder system with order less than 3 9 . Lü numerically investigate the chaotic behaviors of the fractional-order Lü system, in which the lowest order to have chaos is 0. 3 10 . It was shown that fractional-order modified Duffing system of order less than 2.2 can still behave in a chaotic manner 11 . A hyperchaotic attractor is characterized as a chaotic attractor with more than one positive Lyapunov exponent which can increase the randomness and higher unpredictability of the corresponding system. It is believed that chaotic systems with higher-dimensional attractors have much wider applications. Recently, some authors studied the fractional-order hyperchaotic system by adding nonlinear terms in fractionalorder systems. Wu and Liu found that hyperchaotic behavior does exist in the fractionalorder hyperchaotic Chen system of order as low as 3.72 through numerical simulations 12 . Reference 13 discussed numerically the lowest orders for hyperchaos in fractional-order hyperchaotic Lorenz system which is 3.884. Hyperchaos can exist in the fractional-order Rössler hyperchaotic equation with order as low as 3.8 14 .
It has been well known that a chaotic system is a nonlinear deterministic system with unpredictable complexity. In the past decade, synchronization of chaotic systems has attracted considerable attention since the pioneering work of Pecora and Carroll 15 . The control and synchronization of chaotic systems have been developed and thoroughly studied over the past two decades for their applications in some engineering applications such as image processing, chemical and biological systems, information science, and secure communication 16, 17 . So far, many different types of synchronization have been introduced, such as complete synchronization 18 , lag synchronization 19 , generalized synchronization 20 , impulsive synchronization 21 , and phase synchronization 22 . In 2001, experimental investigations of antisynchronization of optically coupled semiconductor lasers with external cavities were presented by Wedekind and Parlitz 23 . Then Chil et al. 24 observed antisynchronization phenomena in coupled identical chaotic oscillators in 2003. Antisynchronization is a phenomenon that the state vectors of synchronized systems have the same amplitude but opposite signs as those of the driving system, which can be characterized by the vanishing of the sum of relevant variables. In the past ten years, various synchronization control laws are obtained by using different control methods, see 25-27 . In 1996, projective synchronization phenomenon was first reported and discussed by González-Miranda 28 . In 1999, Mainieri and Rehacek first proposed the concept of projective synchronization which is characterized by the fact that the drive and response systems could be synchronized up to a scaling factor 29 . Then, Li 30 considered a new synchronization method called modified projective synchronization MPS , where the phases were locked and the amplitudes of the two coupled systems synchronized up to a scaling factor. The scaling factor is a constant transformation between the synchronized variables of the master and slave systems. In recent years, projective synchronization of chaotic systems has attracted increasing attention; some sufficient conditions for projective synchronization have been obtained, see 31-33 . Recently, synchronization of chaotic fractional differential systems becomes a challenging and interesting problem and starts to attract increasing attention due to its potential applications to secure communication and control processing. Most of the methods and results of chaos synchronization in the integer-order differential systems cannot be simply extended to the fractional-order systems because fractional differential dynamical systems are very complex. Some scholars did some research about projective synchronization of fractional systems. For instance, Shao et al. proposes a method to achieve projective synchronization of the fractional-order chaotic Rössler system 34 . Based on the idea of a nonlinear observer, Peng and Jiang proposed a new method and applied it to generalized projective synchronization for a class of fractional-order chaotic systems 35 ; Wu and Wang discussed a new fractional-order system and investigated its projective synchronization by designing the suitable nonlinear controller 36 . The hybrid projective synchronization of different dimensional fractional-order chaotic systems with different dimensions is investigated in 37 . In the meantime, some scholars have obtained some results about antisynchronization of fractional-order chaotic system; for example, based on the idea of active control, a novel active pinning control strategy is presented to achieve the antisynchronization of new uncertain fractional-order unified chaotic systems in paper 38 . In fact, adopting higher-dimensional chaotic systems has been proposed for secure communication, and the presence of more than one Lyapunov exponent clearly improves the security of communication schemes by generating more complex dynamics. Interest in synchronization of the fractional-order hyperchaotic systems is again motivated by secure communication applications. References 12, 13, 39 all discussed the synchronization of the fractional-order hyperchaotic systems. The antisynchronization of two identical and two nonidentical hyperchaotic fractional-order systems is investigated via sliding mode controller in 40 , but the results about the projective synchronization and inverse synchronization are relatively few.
However, to our best knowledge, there are few results about inverse projective synchronization of fractional-order hyperchaotic system. Wu and Lu investigated the generalized projective synchronization of fractional-order Chen hyperchaotic systems 12 . The issue of full state hybrid projective synchronization of fractional Chen-Lee hyperchaotic system is investigated in 41 . They all concentrated on projective synchronization of fractional hyperchaotic systems, but without considering inverse synchronization. References 38, 40 only discussed inverse synchronization without considering projective synchronization. Motivated by the above discussions, in this letter, we study the inverse projective synchronization between fractional-order hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system. Two novel control laws are derived for synchronizing fractionalorder hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system. The remainder of this paper is organized as follows. In Section 2, fractional-order hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system are described, and preliminary results are presented. In Section 3, two control schemes of inverse projective synchronization for fractional-order hyperchaotic systems of different structures are presented. In Section 4, numerical simulations are given to illustrate the effectiveness of the main results. Finally, conclusions are drawn in Section 5.

System Description and Preliminaries
Fractional calculus is a generalization of integration and differentiation to a noninteger-order integrodifferential operator D α t defined by

Journal of Applied Mathematics
There are some definitions for fractional derivatives. The commonly used definitions are Grunwald-Letnikov, Riemann-Liouville, and Caputo definitions. The Riemann-Liouvill fractional derivatives, defined by where n α , that is, n is the first integer which is not less than α, J β is the β-order Riemann-Liuvile integral operator which is described as follows: where Γ · is the gamma function, 0 < β ≤ 1.
In this paper, the following definition is used: where n α , it is a common practice to call the operator D α * the Caputo differential operator of order α because it has apparently first been used for the solution of practical problems by Caputo.
To obtain our results, the following lemma is presented. For a given autonomous linear system of fractional order ii stable if and only if either it is asymptotically stable or those critical eigenvalues which satisfy |arg λ i A | απ/2 have geometric multiplicity one. Figure 1 illustrates the stability region of the fractional-order system. The hyperchaotic Lorenz system with fractional order 13 is described as follows: where q is the fractional order, 0 < q < 1,  Figure 2. In 2009, Wu and Lu investigated the hyperchaotic behaviors in fractional-order Chen hyperchaotic system, which is described by

Inverse Projective Synchronization
In the section, we will discuss inverse projective synchronization behavior between fractional-order hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system. We assume that fractional-order hyperchaotic Lorenz system is the drive system and the response system is fractional-order hyperchaotic Chen system, which are governed by systems 3.1 and 3.2 , respectively, where x i and y i stand for state variables of the master system and the slave one, respectively. a, b, c, d, α, β, θ, γ, and η are parameters. u 1 , u 2 , u 3 , and u 4 are four nonlinear controllers will be designed later. where · is the Euclidean norm and σ diag σ 1 , σ 2 , σ 3 , σ 4 . It follows from 3.1 -3.3 that we have the following error dynamical system: Our aim is to find suitable control laws u i i 1, 2, 3, 4 for stabilizing the error variables of the system at the origin. For this goal, we propose the following control laws for the error dynamical system
Proof. Substituting 3.5 into 3.4 leads to the following error system: where Suppose λ is one of the eigenvalues of matrix A and the corresponding eigenvector is ε ε 1 , ε 2 , ε 3 , ε 4 T , that is, Aε λε.

3.14
According to the stability theory of fractional-order systems Lemma 2.1 , error system 3.6 is asymptotically stable, which implies that the inverse projective synchronization between systems 3.1 and 3.2 is achieved.
It follows from the control law 3.5 that the control law concerns the derive state variables and error variables, but is not related to the response ones. Now, we will propose another control scheme, which is associated with the response state variables and error variables. Theorem 3.3. For a given constant scaling matrix σ diag σ 1 , σ 2 , σ 3 , σ 4 and any initial conditions, if parameters a, b > 0 and d < 0, then fractional-order hyperchaotic Lorenz system 3.1 and Chen system 3.2 are inverse projective synchronized by the following control law:

3.15
Proof. Combining 3.15 with 3.4 , the error dynamical system can be described as

3.16
System 3.16 can be rewritten in the following matrix form:

3.18
Journal of Applied Mathematics 11 The following proof process is similar to that of Theorem 3.2, so is omitted here. Finally, we have Parameters a, b > 0 and d < 0, therefore λ λ < 0, which means any eigenvalue of matrix A satisfies

3.20
Based on Lemma 2.1, error system 3.16 is asymptotically stable, which implies the inverse projective synchronization between the drive system 3.1 and the response system 3.2 is achieved under the nonlinear controller 3.15 .
If we set the scaling matrix σ diag σ, σ, σ, σ σ 1 σ 2 σ 3 σ 4 σ , then inverse projective synchronization will be degraded to inverse generalized synchronization. So we have the following Corollaries 3.4 and 3.5 to realize inverse generalized synchronization between the drive system 3.1 and the response system 3.2 .

Corollary 3.4.
For a given constant scaling matrix σ diag σ, σ, σ, σ and any initial conditions, if parameters α, θ, γ and η > 0, then the inverse generalized synchronization between fractional-order hyperchaotic Lorenz system 3.1 and Chen system 3.2 will occur under the following control law:

3.21
Corollary 3.5. For a given constant scaling matrix σ diag σ, σ, σ, σ and any initial conditions, if parameters a, b > 0 and d < 0, then inverse generalized synchronization between fractional-order hyperchaotic Lorenz system 3.1 and Chen system 3.2 are achieved under the following control law:

Numerical Simulations
In this section, to verify and demonstrate the effectiveness of the proposed controller laws, we discuss the simulation results of inverse projective synchronization between fractional-order

Conclusions
In this paper, based on the stability theory of fractional-order differential equations and Lyapunov equations for fractional-order systems, two criteria are provided to ensure the inverse projective synchronization between fractional-order hyperchaotic Lorenz system and fractional-order hyperchaotic Chen system. There are few papers about the inverse projective synchronization of fractional-order hyperchaotic system, which often exists in real systems. The feasibility and effectiveness of proposed schemes have been validated by computer simulation. It should be noted that complete synchronization, antisynchronization, and general projective synchronization are the special cases of inverse projective synchronization. Therefore, the results of this paper are more applicable and representative. Meanwhile, results could be extended to other fractional-order hyperchaotic system, such as fractional-order hyperchaotic Lü system, and fractional-order hyperchaotic Rössler system.