Combination Synchronization of Three Identical or Different Nonlinear Complex Hyperchaotic Systems

Xiaobing Zhou \*, Murong Jiang and Yaqun Huang

School of Information Science and Engineering, Yunnan University, Kunming 650091, China

\* Author to whom correspondence should be addressed; E-Mail: zhouxb.cn@gmail.com.

*Received: 5 August 2013; in revised form: 3 September 2013 / Accepted: 4 September 2013 / Published: 10 September 2013*

Abstract: In this paper, we investigate the combination synchronization of three nonlinear complex hyperchaotic systems: the complex hyperchaotic Lorenz system, the complex hyperchaotic Chen system and the complex hyperchaotic Lü system. Based on the Lyapunov stability theory, corresponding controllers to achieve combination synchronization among three identical or different nonlinear complex hyperchaotic systems are derived, respectively. Numerical simulations are presented to demonstrate the validity and feasibility of the theoretical analysis.

Keywords: combination synchronization; Lyapunov stability theory; complex hyperchaotic Lorenz system; complex hyperchaotic Chen system; complex hyperchaotic Lü system

#### 1. Introduction

Since Fowler *et al*. [1], introduced a complex Lorenz model to generalize the real Lorenz model in 1982, complex chaotic and hyperchaotic systems have attracted increasing attention, due to the fact that systems with complex variables can be used to describe the physics of a detuned laser, rotating fluids, disk dynamos, electronic circuits and particle beam dynamics in high energy accelerators [2]. When applying complex systems in communications, the complex variables will double the number of variables and can increase the content and security of the transmitted information. Many complex chaotic and hyperchaotic systems have been proposed ever since the 1980s. In [3], the authors studied

the chaotic unstable limit cycles of complex Van der Pol oscillators. The rich dynamics behaviors of the complex Chen and complex Lü systems were investigated in [4]. By adding state feedback controllers to their complex chaotic systems, complex hyperchaotic Chen, Lorenz and Lü systems were introduced and studied in [5–7], respectively. The authors [8] constructed a complex nonlinear hyperchaotic system by adding a cross-product nonlinear term to the complex Lorenz system. A complex modified hyperchaotic Lü system [9] was proposed by introducing complex variables to its real counterpart.

In 1990 [10], Pecora and Carroll proposed the drive-response concept for constructing the synchronization of coupled chaotic systems. Over the last two decades, synchronization in chaotic systems has been extensively investigated, due to its potential applications in various fields, such as chemical reactions, biological systems and secure communication. Mahmoud *et al*. [11] designed an adaptive control scheme to study the complete synchronization of chaotic complex nonlinear systems with uncertain parameters. The authors achieved phase synchronization and antiphase synchronization of two identical hyperchaotic complex nonlinear systems via an active control technique in [12]. Based on passive theory, the authors studied the projective synchronization of hyperchaotic complex nonlinear systems and its application in secure communications [13]. Liu *et al*. [14] investigated the modified function projective synchronization of general chaotic complex systems described by a unified mathematical expression.

The aforementioned synchronization schemes are based on the usual drive-response synchronization mode, which has one drive system and one response system. Recently, Luo [15] proposed a combination synchronization scheme, which has two drive systems and one response system. This synchronization scheme has advantages over the usual drive-response synchronization, such as being able to provide greater security in secure communication. In secure communication, the transmitted signals can be split into several parts, each part loaded in different drive systems, or can divide time into different intervals, the signals in different intervals being loaded in different drive systems. Thus, the transmitted signals can have stronger anti-attack ability and anti-translated capability than those transmitted by the usual transmission model.

Motivated by the above discussions, this paper aims to study the combination synchronization of three identical or different nonlinear complex hyperchaotic systems. The rest of this paper is organized as follows. Section 2 introduces the scheme of combination synchronization. In Section 3 and Section 4, we investigate combination synchronization among three identical and different complex nonlinear hyperchaotic systems, respectively. Finally, conclusions are given in Section 5.

#### 2. The Scheme of Combination Synchronization

Suppose that there are three nonlinear dynamical systems, two drive systems and one response system. The drive systems are given by:

$$
\dot{x} = f(x),
\tag{1}
$$

and

$$
\dot{y} = g(y). \tag{2}
$$

The response system is described by:

$$
\dot{z} = h(z) + U(x, y, z),
\tag{3}
$$

where x = (x1, x2, ..., xn)<sup>T</sup> , y = (y1, y2, ..., yn)<sup>T</sup> , z = (z1, z2, ..., zn)<sup>T</sup> are the state vectors of systems (1), (2) and (3), respectively, <sup>f</sup>(·), g(·), h(·) : <sup>R</sup><sup>n</sup> <sup>→</sup> <sup>R</sup><sup>n</sup> are three continuous vector functions and <sup>U</sup>(·) : <sup>R</sup><sup>n</sup> <sup>×</sup> <sup>R</sup><sup>n</sup> <sup>×</sup> <sup>R</sup><sup>n</sup> <sup>→</sup> <sup>R</sup><sup>n</sup> is a controller vector, which will be designed.

*Definition 1* [15]. For drive systems (1) and (2) and response system (3), they are said to be in combination synchronization if there exists three constant matrices, A, B, C <sup>∈</sup> <sup>R</sup><sup>n</sup> and <sup>C</sup> = 0, such that:

$$\lim\_{t \to +\infty} \parallel Ax + By - Cz \parallel = 0,\tag{4}$$

where · represents the matrix norm.

#### 3. Combination Synchronization among Identical Nonlinear Complex Hyperchaotic Systems

In this section, we take the complex hyperchaotic Lorenz system [6] as an example to investigate the combination synchronization among three identical systems.

The first drive system is given by:

$$\begin{cases}
\dot{x}\_{11} = \alpha (x\_{21} - x\_{11}) + x\_{41}, \\
\dot{x}\_{21} = \gamma x\_{11} - x\_{21} - x\_{11} x\_{31}, \\
\dot{x}\_{31} = \frac{1}{2} (\bar{x}\_{11} x\_{21} + x\_{11} \bar{x}\_{21}) - \beta x\_{31} + x\_{41}, \\
\dot{x}\_{41} = \frac{1}{2} (\bar{x}\_{11} x\_{21} + x\_{11} \bar{x}\_{21}) - \sigma x\_{41},
\end{cases} \tag{5}$$

and the second drive system is described as follows:

$$\begin{cases}
\dot{x}\_{12} = \alpha (x\_{22} - x\_{12}) + x\_{42}, \\
\dot{x}\_{22} = \gamma x\_{12} - x\_{22} - x\_{12} x\_{32}, \\
\dot{x}\_{32} = \frac{1}{2} (\bar{x}\_{12} x\_{22} + x\_{12} \bar{x}\_{22}) - \beta x\_{32} + x\_{42}, \\
\dot{x}\_{42} = \frac{1}{2} (\bar{x}\_{12} x\_{22} + x\_{12} \bar{x}\_{22}) - \sigma x\_{42}.
\end{cases} \tag{6}$$

The response system takes the following form:

$$\begin{cases}
\dot{x}\_{13} = \alpha (x\_{23} - x\_{13}) + x\_{43} + U\_1 + iU\_2, \\
\dot{x}\_{23} = \gamma x\_{13} - x\_{23} - x\_{13}x\_{33} + U\_3 + iU\_4, \\
\dot{x}\_{33} = \frac{1}{2} (\bar{x}\_{13}x\_{23} + x\_{13}\bar{x}\_2) - \beta x\_{33} + x\_{43} + U\_5, \\
\dot{x}\_{43} = \frac{1}{2} (\bar{x}\_{13}x\_{23} + x\_{13}\bar{x}\_{23}) - \sigma x\_{43} + U\_6,
\end{cases} \tag{7}$$

where α, β, γ and σ are positive parameters, x<sup>11</sup> = u<sup>1</sup> + iu2, x<sup>21</sup> = u<sup>3</sup> + iu4, x<sup>12</sup> = v<sup>1</sup> + iv2, x<sup>22</sup> = <sup>v</sup><sup>3</sup> <sup>+</sup> iv4, x<sup>13</sup> <sup>=</sup> <sup>w</sup><sup>1</sup> <sup>+</sup> iw2, x<sup>23</sup> <sup>=</sup> <sup>w</sup><sup>3</sup> <sup>+</sup> iw<sup>4</sup> are complex variables and <sup>i</sup> <sup>=</sup> √−<sup>1</sup>; <sup>u</sup>i, vi, w<sup>i</sup> (<sup>i</sup> <sup>=</sup> 1, 2, 3, 4), x<sup>31</sup> = u5, x<sup>41</sup> = u6, x<sup>32</sup> = v5, x<sup>42</sup> = v6, x<sup>33</sup> = w5, x<sup>43</sup> = w<sup>6</sup> are real variables. The overbar represents a complex conjugate function. U1, U2, U3, U4, U<sup>5</sup> and U<sup>6</sup> are real controllers to be determined.

For the convenience of our discussions, we assume A = diag(l1, l2, l3, l4), B = diag(m1, m2, m3, m4), C = diag(k1, k2, k3, k4) in our synchronization scheme.

We define error states between systems (5), (6) and (7) as:

$$\begin{cases} e\_1 + ie\_2 = k\_1 x\_{13} - l\_1 x\_{11} - m\_1 x\_{12}, \\\\ e\_3 + ie\_4 = k\_2 x\_{23} - l\_2 x\_{21} - m\_2 x\_{22}, \\\\ e\_5 = k\_3 x\_{33} - l\_3 x\_{31} - m\_3 x\_{32}, \\\\ e\_6 = k\_4 x\_{43} - l\_4 x\_{41} - m\_4 x\_{42}, \end{cases} \tag{8}$$

such that:

$$\begin{cases} \lim\_{t \to \infty} \parallel k\_1 x\_{13} - l\_1 x\_{11} - m\_1 x\_{12} \parallel = 0, \\ \lim\_{t \to \infty} \parallel k\_2 x\_{23} - l\_2 x\_{21} - m\_2 x\_{22} \parallel = 0, \\ \lim\_{t \to \infty} \parallel k\_3 x\_{33} - l\_3 x\_{31} - m\_3 x\_{32} \parallel = 0, \\ \lim\_{t \to \infty} \parallel k\_4 x\_{43} - l\_4 x\_{41} - m\_4 x\_{42} \parallel = 0. \end{cases} \tag{9}$$

Thus, we have the following error dynamical system:

$$\begin{cases} \dot{e}\_1 + i\dot{e}\_2 = k\_1 \dot{x}\_{13} - l\_1 \dot{x}\_{11} - m\_1 \dot{x}\_{12}, \\ \dot{e}\_3 + i\dot{e}\_4 = k\_2 \dot{x}\_{23} - l\_2 \dot{x}\_{21} - m\_2 \dot{x}\_{22}, \\ \dot{e}\_5 = k\_3 \dot{x}\_{33} - l\_3 \dot{x}\_{31} - m\_3 \dot{x}\_{32}, \\ \dot{e}\_6 = k\_4 \dot{x}\_{43} - l\_4 \dot{x}\_{41} - m\_4 \dot{x}\_{42}. \end{cases} \tag{10}$$

Substituting Equations (5)–(7) in Equation (10) and separating the real and imaginary parts yields:

$$\begin{cases} \dot{e}\_1 = k\_1[\alpha(w\_3 - w\_1) + w\_6] - l\_1[\alpha(u\_3 - u\_1) + u\_6] - m\_1[\alpha(v\_3 - v\_1) + v\_6] + k\_1U\_1, \\ \dot{e}\_2 = k\_1\alpha(w\_4 - w\_2) - l\_1\alpha(u\_4 - u\_2) - m\_1\alpha(v\_4 - v\_2) + k\_1U\_2, \\ \dot{e}\_3 = k\_2(\gamma w\_1 - w\_3 - w\_1w\_5) - l\_2(\gamma u\_1 - u\_3 - u\_1u\_5) - m\_2(\gamma v\_1 - v\_3 - v\_1v\_5) + k\_2U\_3, \\ \dot{e}\_4 = k\_2(\gamma w\_2 - w\_4 - w\_2w\_5) - l\_2(\gamma u\_2 - u\_4 - u\_2u\_5) - m\_2(\gamma v\_2 - v\_4 - v\_2v\_5) + k\_2U\_4, \\ \dot{e}\_5 = k\_3(w\_1w\_3 + w\_2w\_4 - \beta w\_5 + w\_6) - l\_3(u\_1u\_3 + u\_2u\_4 - \beta u\_5 + u\_6) \\ \quad - m\_3(v\_1v\_3 + v\_2v\_4 - \beta v\_5 + v\_6) + k\_3U\_5, \\ \dot{e}\_6 = k\_4(w\_1w\_3 + w\_2w\_4 - \sigma w\_6) - l\_4(u\_1u\_3 + u\_2u\_4 - \sigma u\_6) - m\_4(v\_1v\_3 + v\_2v\_4 - \sigma v\_6) + k\_4U\_6. \end{cases} \tag{11}$$

Then, we obtain the following results.

Theorem 1. If the controllers are chosen as follows:

⎧ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩ <sup>U</sup><sup>1</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k1 {(k1w<sup>1</sup> − l1u<sup>1</sup> − m1v1)+[k1α(w<sup>3</sup> − w1) + k1w<sup>6</sup> − l1α(u<sup>3</sup> − u1) − l1u<sup>6</sup> − m1α(v<sup>3</sup> − v1) − m1v6] − α(k1w<sup>2</sup> − l1u<sup>2</sup> − m1v2)}, <sup>U</sup><sup>2</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k1 {(k1w<sup>2</sup> − l1u<sup>2</sup> − m1v2)+[k1α(w<sup>4</sup> − w2) − l1α(u<sup>4</sup> − u2) − m1α(v<sup>4</sup> − v2)] + α(k1w<sup>1</sup> − l1u<sup>1</sup> − m1v1) − γ(k2w<sup>3</sup> − l2u<sup>3</sup> − m2v3)}, <sup>U</sup><sup>3</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k2 {(k2w<sup>3</sup> − l2u<sup>3</sup> − m2v3)+[k2(γw<sup>1</sup> − w<sup>3</sup> − w1w5) − l2(γu<sup>1</sup> − u<sup>3</sup> − u1u5) − m2(γv<sup>1</sup> − v<sup>3</sup> − v1v5)] + γ(k1w<sup>2</sup> − l1u<sup>2</sup> − m1v2) − β(k2w<sup>4</sup> − l2u<sup>4</sup> − m2v4)}, <sup>U</sup><sup>4</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k2 {(k2w<sup>4</sup> − l2u<sup>4</sup> − m2v4)+[k2(γw<sup>2</sup> − w<sup>4</sup> − w2w5) − l2(γu<sup>2</sup> − u<sup>4</sup> − u2u5) − m2(γv<sup>2</sup> − v<sup>4</sup> − v2v5)] + β(k2w<sup>3</sup> − l2u<sup>3</sup> − m2v3) − σ(k3w<sup>5</sup> − l3u<sup>5</sup> − m3v5)}, <sup>U</sup><sup>5</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k3 {(k3w<sup>5</sup> − l3u<sup>5</sup> − m3v5)+[k3(w1w<sup>3</sup> + w2w<sup>4</sup> − βw<sup>5</sup> + w6) − l3(u1u<sup>3</sup> + u2u<sup>4</sup> − βu<sup>5</sup> + u6) − m3(v1v<sup>3</sup> + v2v<sup>4</sup> − βv<sup>5</sup> + v6)] + σ(k2w<sup>4</sup> − l2u<sup>4</sup> − m2v4) − α(k4w<sup>6</sup> − l4u<sup>6</sup> − m4v6)}, <sup>U</sup><sup>6</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k4 {(k4w<sup>6</sup> − l4u<sup>6</sup> − m4v6)+[k4(w1w<sup>3</sup> + w2w<sup>4</sup> − σw6) − l4(u1u<sup>3</sup> + u2u<sup>4</sup> − σu6) − m4(v1v<sup>3</sup> + v2v<sup>4</sup> − σv6)] + α(k3w<sup>5</sup> − l3u<sup>5</sup> − m3v5)}, (12)

then driven systems (5) and (6) will achieve combination synchronization with response system (7).

*Proof.* Construct the following Lyapunov function:

$$V = \frac{1}{2}(e\_1^2 + e\_2^2 + e\_3^2 + e\_4^2 + e\_5^2 + e\_6^2). \tag{13}$$

Taking the time derivative of V along the trajectory of error dynamical system (11) yields:

$$\begin{aligned} \dot{V} &= e\_1 \dot{e}\_1 + e\_2 \dot{e}\_2 + e\_3 \dot{e}\_3 + e\_4 \dot{e}\_4 + e\_5 \dot{e}\_5 + e\_6 \dot{e}\_6 \\ &= e\_1 \{k\_1 [\alpha (w\_3 - w\_1) + w\_6] - l\_1 [\alpha (u\_3 - u\_1) + u\_6] - m\_1 [\alpha (v\_3 - v\_1) + v\_6] + k\_1 U\_1 \\ &+ e\_2 [k\_1 \alpha (w\_4 - w\_2) - l\_1 \alpha (u\_4 - u\_2) - m\_1 \alpha (v\_4 - v\_2) + k\_1 U\_2] \\ &+ e\_3 [k\_2 (\gamma w\_1 - w\_3 - w\_1 w\_5) - l\_2 (\gamma u\_1 - u\_3 - u\_1 u\_5) - m\_2 (\gamma v\_1 - v\_3 - v\_1 v\_5) + k\_2 U\_3] \\ &+ e\_4 [k\_2 (\gamma w\_2 - w\_4 - w\_2 w\_5) - l\_2 (\gamma u\_2 - u\_4 - u\_2 u\_5) - m\_2 (\gamma v\_2 - v\_4 - v\_2 v\_5) + k\_2 U\_4] \\ &+ e\_5 [k\_3 (w\_1 w\_3 + w\_2 w\_4 - \beta w\_5 + w\_6) - l\_3 (u\_1 u\_3 + u\_2 u\_4 - \beta u\_5 + u\_6) - m\_3 (v\_1 v\_3 + v\_2 v\_4 - \beta v\_5 + v\_6) + k\_3 U\_5] \\ &+ e\_6 [k\_4 (w\_1 w\_3 + w\_2 w\_4 - \sigma w\_6) - l\_4 (u\_1 u\_3 + u\_2 u\_4 - \sigma u\_6) - m\_4 (v\_1 v\_3 + v\_2 v\_4 - \sigma v\_6) + k\_4 U\_6]. \end{aligned} \tag{14}$$

Substituting Equation (12) into Equation (14), then:

$$\begin{aligned} \dot{V} &= \mathbb{E}\left[\left[\alpha\_{1}\left(\alpha\_{0}-w\_{1}\right)+w\_{0}\right]-\mathbb{I}\left[\alpha\_{0}\left(u\_{0}-u\_{1}\right)+w\_{0}\right]-\mathbb{I}\left[\alpha\_{0}v\_{1}-I\_{1}w\_{1}-w\_{1}v\_{1}\right] \\ &+ k\_{1}\left(\alpha\_{0}-w\_{1}\right)+k\_{1}v\_{0}-I\_{1}\left(u\_{0}-u\_{1}\right)-l\_{1}v\_{0}-m\_{1}\left(\alpha\_{0}-v\_{1}\right)-m\_{1}v\_{0}-\alpha\_{1}V\_{0}-\alpha\_{1}V\_{1}\omega\_{1}-l\_{1}\omega\_{1}V\_{1}\right] \\ &+ k\_{1}\left(\alpha\_{0}w\_{1}-w\_{1}\right)-l\_{1}v\_{0}\left(u\_{0}-u\_{1}\right)-m\_{1}\left(\alpha\_{0}-v\_{2}\right)-m\_{1}v\_{0}-m\_{1}v\_{2} \\ &+ k\_{1}\left(\alpha\_{1}w\_{1}-w\_{0}-m\_{1}\left(v\_{0}-u\_{1}\right)-m\_{1}\left(v\_{2}-v\_{3}\right)-m\_{2}\left(\gamma\_{1}-v\_{3}-v\_{4}\right)-\left[\beta\_{0}v\_{3}-l\_{2}v\_{2}-m\_{2}v\_{3}\right] \\ &+ k\_{2}\left(\gamma\_{1}w\_{1}-w\_{3}-w\_{0}v\_{3}\right)-l\_{2}\left(\gamma\_{1}u\_{1}-u\_{3}\operatorname{tr}\_{1}\right)-m\_{2}\left(\gamma\_{1}v\_{1}-v\_{3}-v\_{4}\right)-\left[k\_{3}v\_{3}-l\_{3}\operatorname{tr}\_{2}\right] \\ &+ k\_{2}\left(\gamma\_{1}w\_{2}-w\_{3}-m\_{1}v\_{2}\right)-l\_{2}\left(\gamma\_{$$

Since <sup>V</sup>˙ <sup>≤</sup> <sup>0</sup> as <sup>t</sup> → ∞, according to the Lyapunov stability theory, we know <sup>e</sup><sup>i</sup> <sup>→</sup> 0(<sup>i</sup> <sup>=</sup> 1, 2, 3, 4, 5, 6), *i.e*., lim<sup>t</sup>→∞ e = 0. Therefore, drive systems (5) and (6) will achieve combination synchronization with the response system (7).

This completes the proof.

The following corollaries can be easily obtained from Theorem 1.

Corollary 1. (i) Suppose that l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 0 and k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, and if the controllers are chosen as follows:

⎧ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩ U<sup>1</sup> = − {(w<sup>1</sup> − m1v1)+[α(w<sup>3</sup> − w1) + w<sup>6</sup> − m1α(v<sup>3</sup> − v1) − m1v6] − α(w<sup>2</sup> − m1v2)}, U<sup>2</sup> = − {(w<sup>2</sup> − m1v2)+[α(w<sup>4</sup> − w2) − m1α(v<sup>4</sup> − v2)] + α(w<sup>1</sup> − m1v1) − γ(w<sup>3</sup> − m2v3)}}, U<sup>3</sup> = − {(w<sup>3</sup> − m2v3) + [(γw<sup>1</sup> − w<sup>3</sup> − w1w5) − m2(γv<sup>1</sup> − v<sup>3</sup> − v1v5)] + γ(w<sup>2</sup> − m1v2) − β(w<sup>4</sup> − m2v4)}, U<sup>4</sup> = − {(w<sup>4</sup> − m2v4) + [(γw<sup>2</sup> − w<sup>4</sup> − w2w5) − m2(γv<sup>2</sup> − v<sup>4</sup> − v2v5)] + β(w<sup>3</sup> − m2v3) − σ(w<sup>5</sup> − m3v5)}, U<sup>5</sup> = − {(w<sup>5</sup> − m3v5) + [(w1w<sup>3</sup> + w2w<sup>4</sup> − βw<sup>5</sup> + w6) − m3(v1v<sup>3</sup> + v2v<sup>4</sup> − βv<sup>5</sup> + v6)] + σ(w<sup>4</sup> − m2v4) − α(w<sup>6</sup> − m4v6)}, U<sup>6</sup> = − {(w<sup>6</sup> − m4v6) + [(w1w<sup>3</sup> + w2w<sup>4</sup> − σw6) − m4(v1v<sup>3</sup> + v2v<sup>4</sup> − σv6)] + α(w<sup>5</sup> − m3v5)}, (16)

then drive system (6) will achieve projective synchronization with response system (7).

(ii) Suppose that m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 0 and k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, and if the controllers are chosen as follows:

$$\begin{cases} U\_1 = -\left\{ (w\_1 - l\_1 u\_1) + [\alpha(w\_3 - w\_1) + w\_6 - l\_1 \alpha(u\_3 - u\_1) - l\_1 u\_6] - \alpha(w\_2 - l\_1 u\_2) \right\}, \\ U\_2 = -\left\{ (w\_2 - l\_1 u\_2) + [\alpha(w\_4 - w\_2) - l\_1 \alpha(u\_4 - u\_2)] + \alpha(w\_1 - l\_1 u\_1) - \gamma(w\_3 - l\_2 u\_3) \right\}, \\ U\_3 = -\left\{ (w\_3 - l\_2 u\_3) + [(\gamma w\_1 - w\_3 - w\_1 w\_5) - l\_2(\gamma u\_1 - u\_3 - u\_1 u\_5)] + \gamma(w\_2 - l\_1 u\_2) - \beta(w\_4 - l\_2 u\_4) \right\}, \\ U\_4 = -\left\{ (w\_4 - l\_2 u\_4) + [(\gamma w\_2 - w\_4 - w\_2 w\_5) - l\_2(\gamma u\_2 - u\_4 - u\_2 u\_5)] + \beta(w\_3 - l\_2 u\_3) - \sigma(w\_5 - l\_3 u\_5) \right\}, \\ U\_5 = -\left\{ (w\_5 - l\_3 u\_5) + [(w\_1 w\_3 + w\_2 w\_4 - \beta w\_5 + w\_6) - l\_3(u\_1 u\_3 + u\_2 u\_4 - \beta u\_5 + u\_6)] \right\}, \\ \quad + \sigma(w\_4 - l\_2 u\_4) - \alpha(w\_6 - l\_4 u\_6)), \\ U\_6 = -\left\{ (w\_6 - l\_4 u\_6) + [(w\_1 w\_3 + w\_2 w\_4 - \sigma w\_6) - l\_4(u\_1 u\_3 + u\_2 u\_4 - \sigma u\_6)] + \alpha(w\_5 - l\_3 u\_5) \right\}, \end{cases} \tag{17}$$

then drive system (5) will achieve projective synchronization with response system (7).

Corollary 2. Suppose that l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 0, m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 0 and k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, and if the controllers are chosen as follows:

$$\begin{cases} U\_1 = -[w\_1 + \alpha(w\_3 - w\_1) + w\_6 - \alpha w\_2], \\ U\_2 = -[w\_2 + \alpha(w\_4 - w\_2) + \alpha w\_1 - \gamma w\_3], \\ U\_3 = -(w\_3 + \gamma w\_1 - w\_3 - w\_1 w\_5 + \gamma w\_2 - \beta w\_4), \\ U\_4 = -(w\_4 + \gamma w\_2 - w\_4 - w\_2 w\_5 + \beta w\_3 - \sigma w\_5), \\ U\_5 = -(w\_5 + w\_1 w\_3 + w\_2 w\_4 - \beta w\_5 + w\_6 + \sigma w\_4 - \alpha w\_6), \\ U\_6 = -(w\_6 + w\_1 w\_3 + w\_2 w\_4 - \sigma w\_6 + \alpha w\_5), \end{cases} \tag{18}$$

then system (7) is stabilized to the equilibrium, O(0, 0, 0, 0, 0, 0).

Remark 1: The proofs of Corollary 1 and Corollary 2 are similar to those of theorem 1, so we omitted them.

In the following, numerical experiments are given to demonstrate our results. The fourth-order Runge-Kutta method is used with a time step size of 0.001. The system parameters are given as α = 8, β = 5, γ = 50 and σ = 15, so that the complex Lorenz system exhibits hyperchaotic behavior. We assume k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 1 and m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 1, and the initial states for drive systems (5) and (6) and response system (7) are arbitrarily given by (x11(0), x21(0), x31(0), x41(0)) = (4 − 0.3i, 2.2 − 0.8i, 4.9, 1.1),(x12(0), x22(0), x32(0), x42(0)) = (4.4 − 0.6i, 3.3 − 1.4i, 5.3, 1.4) and (x13(0), x23(0), x33(0), x43(0)) = (4.6 − 1.8i, 1.6 − 1.9i, 2.5, 2), *i.e*., (u1(0), u2(0), u3(0), u4(0), u5(0), u6(0)) = (4, −0.3, 2.2, −0.8, 4.9, 1.1), (v1(0), v2(0), v3(0), v4(0), v5(0), v6(0)) = (4.4, −0.6, 3.3, −1.4, 5.3, 1.4) and (w1(0), w2(0), w3(0), w4(0), w5(0), w6(0)) = (4.6, −1.8, 1.6, −1.9, 2.5, 2), respectively. The corresponding numerical results are shown in Figures 1 and 2. Figure 1 displays the time response of the combination synchronization errors, e1, e2, e3, e4, e5ande6. The errors converge to zero, which implies that systems (5), (6) and (7) have achieved combination synchronization. Figures 2 depicts the time responses of the states, u<sup>1</sup> + v<sup>1</sup> *versus* w1, u<sup>2</sup> + v<sup>2</sup> *versus* w2, u<sup>3</sup> + v<sup>3</sup> *versus* w3, u<sup>4</sup> + v<sup>4</sup> *versus* w4, u<sup>5</sup> + v<sup>5</sup> *versus* w<sup>5</sup> and u<sup>6</sup> + v<sup>6</sup> *versus* w6, respectively. Next, suppose that k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 0 and m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 0. The time evolution of the states, w1, w2, w3, w4, w5, w6, of system (7) with controller (18) are displayed in Figure 3, which illustrates that system (7) is stabilized to the equilibrium, O(0, 0, 0, 0, 0, 0).

Figure 1. Combination synchronization errors, e1, e2, e3, e4, e<sup>5</sup> and e6, between drive systems (5) and (6) and response system (7).

Figure 2. Time responses for states u<sup>i</sup> + v<sup>i</sup> *versus* wi, i = 1, 2, ..., 6, respectively.

#### Figure 3. Time evolution of the states for system (7).

4. Combination Synchronization among Different Nonlinear Complex Hyperchaotic Systems

In this section, we investigate the combination synchronization among three different nonlinear complex hyperchaotic systems. The hyperchaotic complex Lorenz system [6] and the hyperchaotic complex Chen system [5], respectively, describe the drive systems:

$$\begin{cases}
\dot{x}\_1 = \alpha\_1 (x\_2 - x\_1) + x\_4, \\
\dot{x}\_2 = \gamma\_1 x\_1 - x\_2 - x\_1 x\_3, \\
\dot{x}\_3 = \frac{1}{2} (\bar{x}\_1 x\_2 + x\_1 \bar{x}\_2) - \beta\_1 x\_3 + x\_4, \\
\dot{x}\_4 = \frac{1}{2} (\bar{x}\_1 x\_2 + x\_1 \bar{x}\_2) - \sigma\_1 x\_4,
\end{cases} \tag{19}$$

and

$$\begin{cases} \dot{y}\_1 = \alpha\_2 (y\_2 - y\_1), \\ \dot{y}\_2 = (\gamma\_2 - \alpha\_2) y\_1 - y\_1 y\_3 + \gamma\_2 y\_2 + y\_4, \\ \dot{y}\_3 = \frac{1}{2} (\bar{y}\_1 y\_2 + y\_1 \bar{y}\_2) - \beta\_2 y\_3 + y\_4, \\ \dot{y}\_4 = \frac{1}{2} (\bar{y}\_1 y\_2 + y\_1 \bar{y}\_2) - d\_2 y\_4, \end{cases} \tag{20}$$

and the hyperchaotic complex Lü system system [7] is the response system given by:

$$\begin{cases} \dot{z}\_1 = \rho\_3(z\_2 - z\_1) + z\_4 + U\_1 + iU\_2, \\ \dot{z}\_2 = \nu\_3 z\_2 - z\_1 z\_3 + z\_4 + U\_3 + iU\_4, \\ \dot{z}\_3 = \frac{1}{2}(\bar{z}\_1 z\_2 + z\_1 \bar{z}\_2) - \mu\_3 z\_3 + U\_5, \\ \dot{z}\_4 = \frac{1}{2}(\bar{z}\_1 z\_2 + z\_1 \bar{z}\_2) - \sigma\_3 z\_4 + U\_6, \end{cases} \tag{21}$$

where α1, β1, γ1, σ1, α2, β2, γ2, d2, ρ3, ν3, μ<sup>3</sup> and σ<sup>3</sup> are positive parameters, x<sup>1</sup> = u<sup>1</sup> + iu2, x<sup>2</sup> = u<sup>3</sup> + iu4, y<sup>1</sup> = v<sup>1</sup> + iv2, y<sup>2</sup> = v<sup>3</sup> + iv4, z<sup>1</sup> = w<sup>1</sup> + iw2, z<sup>2</sup> = w<sup>3</sup> + iw<sup>4</sup> are complex variables and <sup>i</sup> <sup>=</sup> √−<sup>1</sup>; <sup>u</sup>i, vi, wi(<sup>i</sup> = 1, <sup>2</sup>, <sup>3</sup>, 4), <sup>x</sup><sup>3</sup> <sup>=</sup> <sup>u</sup>5, x<sup>4</sup> <sup>=</sup> <sup>u</sup>6, y<sup>3</sup> <sup>=</sup> <sup>v</sup>5, y<sup>4</sup> <sup>=</sup> <sup>v</sup>6, z<sup>3</sup> <sup>=</sup> <sup>w</sup><sup>5</sup> and <sup>z</sup><sup>4</sup> <sup>=</sup> <sup>w</sup><sup>6</sup> are real variables. The overbar represents the complex conjugate function. U1, U2, U3, U4, U<sup>5</sup> and U<sup>6</sup> are real control functions to be determined.

For the convenience of the following discussions, we assume A = diag(l1, l2, l3, l4), B = diag(m1, m2, m3, m4) and C = diag(k1, k2, k3, k4) in our synchronization scheme.

We define error states between drive systems (19)–(20) and response system (21) as:

$$\begin{cases} e\_1 + ie\_2 = k\_1 z\_1 - l\_1 x\_1 - m\_1 y\_1, \\ e\_3 + ie\_4 = k\_2 z\_2 - l\_2 x\_2 - m\_2 y\_2, \\ e\_4 = k\_3 z\_3 - l\_3 x\_3 - m\_3 y\_3, \\ e\_5 = k\_4 z\_4 - l\_4 x\_4 - m\_4 y\_4, \end{cases} \tag{22}$$

such that:

$$\begin{cases} \lim\_{t \to \infty} \parallel k\_1 z\_1 - l\_1 x\_1 - m\_1 y\_1 \parallel = 0, \\ \lim\_{t \to \infty} \parallel k\_2 z\_2 - l\_2 x\_2 - m\_2 y\_2 \parallel = 0, \\ \lim\_{t \to \infty} \parallel k\_3 z\_3 - l\_3 x\_3 - m\_3 y\_3 \parallel = 0, \\ \lim\_{t \to \infty} \parallel k\_4 z\_4 - l\_4 x\_4 - m\_4 y\_4 \parallel = 0. \end{cases} \tag{23}$$

Separating the real and imagery parts of Equation (22) gets the following:

$$\begin{cases} e\_1 = (k\_1 w\_1 - l\_1 u\_1 - m\_1 v\_1), \\ e\_2 = (k\_1 w\_2 - l\_1 u\_2 - m\_1 v\_2), \\ e\_3 = (k\_2 w\_3 - l\_2 u\_3 - m\_2 v\_3), \\ e\_4 = (k\_2 w\_4 - l\_2 u\_4 - m\_2 v\_4), \\ e\_5 = (k\_3 w\_5 - l\_3 u\_5 - m\_3 v\_5), \\ e\_6 = (k\_4 w\_6 - l\_4 u\_6 - m\_4 v\_6). \end{cases} \tag{24}$$

Thus, we have the following error dynamical system:

⎧ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩ e˙<sup>1</sup> = k1[ρ3(w<sup>3</sup> − w1) + w6] − l1[α1(u<sup>3</sup> − u1) + u6] − m1α2(v<sup>3</sup> − v1) + k1U1, e˙<sup>2</sup> = k1ρ3(w<sup>4</sup> − w2) − l1α1(u<sup>4</sup> − u2) − m1α2(v<sup>4</sup> − v2) + k1U2, e˙<sup>3</sup> = k2(−w1w<sup>5</sup> + ν3w<sup>3</sup> + w6) − l2(γ1u<sup>1</sup> − u<sup>3</sup> − u1u5) − m2[(γ<sup>2</sup> − α2)v<sup>1</sup> − v1v<sup>5</sup> + γ2v<sup>3</sup> + v6] + k2U3, e˙<sup>4</sup> = k2(−w2w<sup>5</sup> + ν3w4) − l2(γ1u<sup>2</sup> − u<sup>4</sup> − u2u5) − m2[(γ<sup>2</sup> − α2)v<sup>2</sup> − v2v<sup>5</sup> + γ2v4] + k2U4, e˙<sup>5</sup> = k3(w1w<sup>3</sup> + w2w<sup>4</sup> − μ3w5) − l3(u1u<sup>3</sup> + u2u<sup>4</sup> − β1u<sup>5</sup> + u6) − m3(v1v<sup>3</sup> + v2v<sup>4</sup> − β2v<sup>5</sup> + v6) + k3U5, e˙<sup>6</sup> = k4(w1w<sup>3</sup> + w2w<sup>4</sup> − σ3w6) − l4(u1u<sup>3</sup> + u2u<sup>4</sup> − σ1u6) − m4(v1v<sup>3</sup> + v2v<sup>4</sup> − d2v6) + k4U6. (25)

Similar to Section 3, we have the following results.

Theorem 2. If the controllers are chosen as:

⎧ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩ <sup>U</sup><sup>1</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k1 {(k1w<sup>1</sup> − l1u<sup>1</sup> − m1v1)+[k1ρ3(w<sup>3</sup> − w1) + k1w<sup>6</sup> − l1α1(u<sup>3</sup> − u1) − l1u<sup>6</sup> − m1α2(v<sup>3</sup> − v1)] − α1(k1w<sup>2</sup> − l1u<sup>2</sup> − m1v2)}, <sup>U</sup><sup>2</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k1 {(k1w<sup>2</sup> − l1u<sup>2</sup> − m1v2)+[k1ρ3(w<sup>4</sup> − w2) − l1α1(u<sup>4</sup> − u2) − m1α2(v<sup>4</sup> − v2)] + α1(k1w<sup>1</sup> − l1u<sup>1</sup> − m1v1) − α2(k2w<sup>3</sup> − l2u<sup>3</sup> − m2v3)}, <sup>U</sup><sup>3</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k2 {(k2w<sup>3</sup> − l2u<sup>3</sup> − m2v3)+[k2(−w1w<sup>5</sup> + ν3w<sup>3</sup> + w6) − l2(γ1u<sup>1</sup> − u<sup>3</sup> − u1u5) − m2(γ<sup>2</sup> − α2)v<sup>1</sup> + m2v1v<sup>5</sup> − m2γ2v<sup>3</sup> − m2v6] + α2(k1w<sup>2</sup> − l1u<sup>2</sup> − m1v2) − β1(k2w<sup>4</sup> − l2u<sup>4</sup> − m2v4)}, <sup>U</sup><sup>4</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k2 {(k2w<sup>4</sup> − l2u<sup>4</sup> − m2v4)+[k2(−w2w<sup>5</sup> + ν3w4) − l2(γ1u<sup>2</sup> − u<sup>4</sup> − u2u5) − m2(γ<sup>2</sup> − α2)v<sup>2</sup> + m2v2v<sup>5</sup> − m2γ2v4] + β1(k2w<sup>3</sup> − l2u<sup>3</sup> − m2v3) − β2(k3w<sup>5</sup> − l3u<sup>5</sup> − m3v5)}, <sup>U</sup><sup>5</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k3 {(k3w<sup>5</sup> − l3u<sup>5</sup> − m3v5)+[k3(w1w<sup>3</sup> + w2w<sup>4</sup> − μ3w5) − l3(u1u<sup>3</sup> + u2u<sup>4</sup> − β1u<sup>5</sup> + u6) − m3(v1v<sup>3</sup> + v2v<sup>4</sup> − β2v<sup>5</sup> + v6)] + β2(k2w<sup>4</sup> − l2u<sup>4</sup> − m2v4) − γ1(k4w<sup>6</sup> − l4u<sup>6</sup> − m4v6)}, <sup>U</sup><sup>6</sup> <sup>=</sup> <sup>−</sup> <sup>1</sup> k4 {(k4w<sup>6</sup> − l4u<sup>6</sup> − m4v6)+[k4(w1w<sup>3</sup> + w2w<sup>4</sup> − σ3w6) − l4(u1u<sup>3</sup> + u2u<sup>4</sup> − σ1u6) − m4(v1v<sup>3</sup> + v2v<sup>4</sup> − d2v6)] + γ1(k3w<sup>5</sup> − l3u<sup>5</sup> − m3v5)}, (26)

then drive systems (19) and (20) will achieve combination synchronization with response system (21).

Corollary 3. (i) Suppose that l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 0 and k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, and if the controllers are chosen as follows:

⎧ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩ U<sup>1</sup> = − {(w<sup>1</sup> − m1v1)+[ρ3(w<sup>3</sup> − w1) + w<sup>6</sup> − m1α2(v<sup>3</sup> − v1)] − α1(w<sup>2</sup> − m1v2)}, U<sup>2</sup> = − {(w<sup>2</sup> − m1v2)+[ρ3(w<sup>4</sup> − w2) − m1α2(v<sup>4</sup> − v2)] + α1(w<sup>1</sup> − m1v1) − α2(w<sup>3</sup> − m2v3)}, U<sup>3</sup> = − {(w<sup>3</sup> − m2v3) + [(−w1w<sup>5</sup> + ν3w<sup>3</sup> + w6) − m2(γ<sup>2</sup> − α2)v<sup>1</sup> + m2v1v<sup>5</sup> − m2γ2v<sup>3</sup> − m2v6] + α2(w<sup>2</sup> − m1v2) − β1(w<sup>4</sup> − m2v4)}, U<sup>4</sup> = − {(w<sup>4</sup> − m2v4) + [(−w2w<sup>5</sup> + ν3w4) − m2(γ<sup>2</sup> − α2)v<sup>2</sup> + m2v2v<sup>5</sup> − m2γ2v4] + β1(w<sup>3</sup> − m2v3) − β2(w<sup>5</sup> − m3v5)}, U<sup>5</sup> = − {(w<sup>5</sup> − m3v5) + [(w1w<sup>3</sup> + w2w<sup>4</sup> − μ3w5) − m3(v1v<sup>3</sup> + v2v<sup>4</sup> − β2v<sup>5</sup> + v6)] + β2(w<sup>4</sup> − m2v4) − γ1(w<sup>6</sup> − m4v6)}, U<sup>6</sup> = − {(w<sup>6</sup> − m4v6) + [(w1w<sup>3</sup> + w2w<sup>4</sup> − σ3w6) − m4(v1v<sup>3</sup> + v2v<sup>4</sup> − d2v6)] + γ1(w<sup>5</sup> − m3v5)}, (27)

then drive system (20) will achieve projective synchronization with response system (21).

(ii) Suppose that m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 0 and k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, and if the controllers are chosen as follows:

⎧ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩ U<sup>1</sup> = − {(w<sup>1</sup> − l1u1)+[ρ3(w<sup>3</sup> − w1) + w<sup>6</sup> − l1α1(u<sup>3</sup> − u1) − l1u6] − α1(w<sup>2</sup> − l1u2)}, U<sup>2</sup> = − {(w<sup>2</sup> − l1u2)+[ρ3(w<sup>4</sup> − w2) − l1α1(u<sup>4</sup> − u2)] + α1(w<sup>1</sup> − l1u1) − α2(w<sup>3</sup> − l2u3)}, U<sup>3</sup> = − {(w<sup>3</sup> − l2u3) + [(−w1w<sup>5</sup> + ν3w<sup>3</sup> + w6) − l2(γ1u<sup>1</sup> − u<sup>3</sup> − u1u5)] + α2(w<sup>2</sup> − l1u2) − β1(w<sup>4</sup> − l2u4)}, U<sup>4</sup> = − {(w<sup>4</sup> − l2u4) + [(−w2w<sup>5</sup> + ν3w4) − l2(γ1u<sup>2</sup> − u<sup>4</sup> − u2u5)] + β1(w<sup>3</sup> − l2u3) − β2(w<sup>5</sup> − l3u5)}, U<sup>5</sup> = − {(w<sup>5</sup> − l3u5) + [(w1w<sup>3</sup> + w2w<sup>4</sup> − μ3w5) − l3(u1u<sup>3</sup> + u2u<sup>4</sup> − β1u<sup>5</sup> + u6)] + β2(w<sup>4</sup> − l2u4) − γ1(w<sup>6</sup> − l4u6)}, U<sup>6</sup> = − {(w<sup>6</sup> − l4u6) + [(w1w<sup>3</sup> + w2w<sup>4</sup> − σ3w6) − l4(u1u<sup>3</sup> + u2u<sup>4</sup> − σ1u6)] + γ1(w<sup>5</sup> − l3u5)}, (28)

then drive system (19) will achieve projective synchronization with response system (21).

Corollary 4. Suppose that l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 0, m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 0 and k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, and if the controllers are chosen as follows:

$$\begin{cases} U\_1 = -\{w\_1 + [\rho\_3(w\_3 - w\_1) + w\_6] - \alpha\_1 w\_2\}, \\ U\_2 = -[w\_2 + \rho\_3(w\_4 - w\_2) + \alpha\_1 w\_1 - \alpha\_2 w\_3], \\ U\_3 = -[w\_3 + (-w\_1 w\_5 + \nu\_3 w\_3 + w\_6) + \alpha\_2 w\_2 - \beta\_1 w\_4], \\ U\_4 = -[w\_4 + (-w\_2 w\_5 + \nu\_3 w\_4) + \beta\_1 w\_3 - \beta\_2 w\_5], \\ U\_5 = -[w\_5 + (w\_1 w\_3 + w\_2 w\_4 - \mu\_3 w\_5) + \beta\_2 w\_4 - \gamma\_1 w\_6], \\ U\_6 = -[w\_6 + (w\_1 w\_3 + w\_2 w\_4 - \sigma\_3 w\_6) + \gamma\_1 w\_5], \end{cases} \tag{29}$$

then system (21) is stabilized to the equilibrium, O(0, 0, 0, 0, 0, 0).

In what follows, numerical experiments are given to demonstrate our results. The fourth-order Runge-Kutta method is used with a time step size of 0.001. The system parameters are given as α<sup>1</sup> = 8, β<sup>1</sup> = 5, γ<sup>1</sup> = 50, σ<sup>1</sup> = 15, α<sup>2</sup> = 36, β<sup>2</sup> = 4, γ<sup>2</sup> = 25, d<sup>2</sup> = 5, ρ<sup>3</sup> = 42, ν<sup>3</sup> = 25, μ<sup>3</sup> = 6 and σ<sup>3</sup> = 5, so that the three complex nonlinear hyperchaotic systems exhibit hyperchaotic behaviors, respectively.

First, we assume k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 1 and m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 1, and the initial states for the drive systems and response systems are arbitrarily given by (x1(0), x2(0), x3(0), x4(0)) = (2.0 − i, 5.8 − 2i − 12, −16),(y1(0), y2(0), y3(0), y4(0)) = (1.7+2.3i, 0.1 − 14i, −16, −18) and (z1(0), z2(0), z3(0), z4(0)) = (3.6 − 0.6i, 0.9 − i, 13, 15), *i.e*., (u1(0), u2(0), u3(0), u4(0), u5(0), u6(0)) = (2.0, −1, 5.8, −2, −12, −16), (v1(0), v2(0), v3(0), v4(0), v5(0), v6(0)) = (1.7, 2.3, 0.1, −14, −16, −18) and (w1(0), w2(0), w3(0), w4(0), w5(0), w6(0)) = (3.6, −0.6, 0.9, −1, 13, 15), respectively. The corresponding numerical results are shown in Figures 4 and 5. Figure 4 displays the time response of the combination synchronization errors, e1, e2, e3, e4, e5ande6. The errors converge to zero, which implies that systems (19), (20) and (21) have achieved combination synchronization. Figure 5 depicts the time responses of the states, u1+v<sup>1</sup> *versus* w1, u2+v<sup>2</sup> *versus* w2,u3+v<sup>3</sup> *versus* w3,u4+v<sup>4</sup> *versus* w4,u<sup>5</sup> + v<sup>5</sup> *versus* w<sup>5</sup> and u<sup>6</sup> + v<sup>6</sup> *versus* w6, respectively. When k<sup>1</sup> = k<sup>2</sup> = k<sup>3</sup> = k<sup>4</sup> = 1, l<sup>1</sup> = l<sup>2</sup> = l<sup>3</sup> = l<sup>4</sup> = 0 and m<sup>1</sup> = m<sup>2</sup> = m<sup>3</sup> = m<sup>4</sup> = 0, the time evolution of the states, w1, w2, w3, w4, w5, w6, of system (21) with controller (29) are displayed in Figure 6, which means that system (21) is stabilized to the equilibrium, O(0, 0, 0, 0, 0, 0).

Figure 4. Combination synchronization errors, e1, e2, e3, e4, e<sup>5</sup> and e6, between drive systems (19), (20) and response system (21).

Figure 5. Time responses for states u<sup>i</sup> + v<sup>i</sup> *versus* wi, i = 1, 2, ..., 6, respectively.

Figure 6. Time evolution of the states for system (21).

#### 5. Conclusions

This paper investigates the combination synchronization of three nonlinear complex hyperchaotic systems: the complex hyperchaotic Lorenz system, the complex hyperchaotic Chen system and the complex hyperchaotic Lü system. Based on the Lyapunov stability theory, corresponding controllers to achieve combination synchronization among three identical or different nonlinear complex hyperchaotic systems are derived, respectively. Numerical simulations are conducted to illustrate the validity and feasibility of the theoretical analysis. When applying the complex systems in communications, the complex variables will double the number of variables and can increase the content and security of the transmitted information. Furthermore, combination synchronization between two drive systems and one response system has obvious advantages over synchronization between one drive system and one response system. Thus combination synchronization of complex nonlinear systems can find better applications in security communication. However, in practical chaotic synchronization, mismatched parameters exist, and the external disturbances are always unavoidable [17]. In our future work, we will investigate robust combination synchronization in the existence of mismatched parameters and external disturbances.

#### Acknowledgments

The authors sincerely thank the referees for their helpful comments. This work was supported by the the Natural Science Foundation of Yunnan Province under grant No. 2009CD019 and the Natural Science Foundation of China under grants No. 61065008 and No. 11161055.

#### Conflicts of Interest

The authors declare no conflict of interest.

#### References


Reprinted from *Entropy*. Cite as: Tian, X.; Fei, S. Robust Control of a Class of Uncertain Fractional-Order Chaotic Systems with Input Nonlinearity via an Adaptive Sliding Mode Technique. *Entropy* **2014**, *16*, 729-746.

## *Article*
