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 Open Access
Modified function projective synchronization of complex dynamical networks with mixed timevarying and asymmetric coupling delays via new hybrid pinning adaptive control
 Piyapong Niamsup^{1},
 Thongchai Botmart^{2}Email author and
 Wajaree Weera^{3}
https://doi.org/10.1186/s1366201711835
© The Author(s) 2017
Received: 20 December 2016
Accepted: 15 April 2017
Published: 28 April 2017
Abstract
This paper investigates modified function projective synchronization (MFPS) for complex dynamical networks with mixed timevarying and hybrid asymmetric coupling delays, which is composed of state coupling, timevarying delay coupling and distributed timevarying delay coupling. In contrast to previous results, the coupling configuration matrix needs not be symmetric or irreducible. The MFPS of delayed complex dynamical networks is considered via either hybrid control or hybrid pinning control with nonlinear and adaptive linear feedback control, which contains error linear term, timevarying delay error linear term and distributed timevarying delay error linear term. Based on Lyapunov stability theory, adaptive control technique, the parameter update law and the technique of dealing with some integral terms, we will show that control may be used to manipulate the scaling functions matrix such that the drive system and response networks could be synchronized up to the desired scaling function matrix. Numerical examples are given to demonstrate the effectiveness of the proposed method. The results in this article generalize and improve the corresponding results of the recent works.
Keywords
 modified function projective synchronization
 complex dynamical network
 mixed timevarying delay
 mixed coupling delays
 hybrid adaptive pinning control
1 Introduction
Complex networks, as an interesting subject, have been thoroughly investigated for decades. These networks show very complicated behavior and can be used to model and explain many complex systems in nature such as computer networks, the world wide web, cellular and metabolic networks, transportation networks, communication networks, disease transmission networks, electrical power grids and so forth. Complex dynamical networks (CDNs) are prominent in describing the sophisticated collaborative dynamics in many sciences [1–5].
The time delay exists extensively in the real word networks. It is well known that the existence in a network may cause instability, poor performances and oscillations. Examples can be found in networks such as application engineering, electrical power networks, physical networks and many other. Thus, synchronization for CDNs with time delays in the dynamical nodes and coupling has become a key and significant topic. Synchronization of a class of general CDNs with coupling delays was investigated in [6–9]. Li et al. [8], introduced some general CDNs models with timevarying delays in network couplings and timevarying delays in dynamical nodes. Song et al. [10] investigated synchronization of general CDNs with mixed time, where mixed delay appeared in the hybrid coupling term, but not in the isolate systems. Furthermore, Li [11] considered synchronization for delayed CDNs with hybrid coupling, which is made up of constant coupling, discrete delay coupling and distributeddelay coupling, but the discrete and distributed delays are not different values. Up to now, unfortunately, there have only been few papers related to the topic of synchronization of CDNs with mixed timevarying delays in the dynamical nodes and timevarying delays in the hybrid coupling, which includes constant coupling, discrete timevarying delay coupling and distributed timevarying delay coupling, simultaneously. So, it is challenging to solve this synchronization problem for CDNs.
In the past few decades, control problems for synchronization have been widely studied in CDNs. Synchronization control methods have been developed for CDNs, for instance, feedback control [12, 13], active control [14], intermittent control [15, 16], sampleddata control [17], nonlinear feedback control [18], adaptive control [19–23], hybrid adaptive control [12], impulsive control [24], active sliding mode control [25] and other control methods. CDNs have a large number of nodes. It is often impossible to realize the control goal by controlling every node. It is possible to control a few nodes to realize the same goal. In engineering, it is usually difficult to control CDNs by adding controllers to all nodes. To reduce the number of controllers, a natural approach is to control CDNs by pinning part of nodes. Thus, a pinning control is a special control method of adding controllers to part of the nodes rather than all of the nodes [6, 10, 11, 21, 26–33]. Chen et al. [31] studied the pinning control problem of the coupled networks by controlling one single node. In [32], an adaptive controller was designed to synchronize delayed CDNs with timevarying coupling strength and timevarying delay. The work in [33] studied pinning adaptive synchronization of general CDNs via pinning adaptive controllers, where the pinning nodes can be randomly selected. In [21], with the aid of the nonlinear and adaptive feedback control and adaptive pinning feedback control method, the authors considered the FPS for CNDs with asymmetric coupling. However, the adaptive feedback control with mixed timevarying delays was not considered in feedback control. Thus, in this paper, we focus on the influences of hybrid pinning feedback control method with nonlinear and adaptive linear feedback control, which contains error linear term, timevarying delay error linear term and distributed timevarying delay error linear term.
The problem of synchronization in CDNs has been extensively investigated over the past few decades. Synchronization of CDNs is one of the most important dynamical mechanisms for creating order in CDNs. Meanwhile, a number of methods developed for the synchronization of CDNs, including complete synchronization (CS) [24, 34], generalized synchronization (GS) [35], projection synchronization (PS) [36–38], outer and inner synchronization [39], modulephase synchronization [14, 23] etc., have been reported in the literature. Very recently, a new type of synchronization phenomenon in CDNs, called function projection synchronization (FPS), has emerged. FPS of CDNs was proposed in [12, 13, 21, 36], which means that the nodes of CDNs could be synchronized up to an equilibrium point or periodic orbit with a desired scaling function. Zhang et al. [40] presented the FPS in driveresponse dynamical networks (DRDNs) with coupled partially linear chaotic systems by assuming that the node dynamics are identical and by using a simple control law. Furthermore, Du et al. [12] investigated the problem of FPS of CDNs with or without external disturbances using error feedback control and adaptive error feedback control. In [13], a hybrid feedback control method was proposed for achieving FPS in CDNs with constant time delay and timevarying coupling delay. Shi et al. [21] proposed a control scheme to study FPS in a complex network with asymmetric coupling via adaptive feedback control and pinning feedback control, respectively. Moreover, a new type of synchronization, called modified function projective synchronization (MFPS), where the drive and response systems could be synchronized up to a desired scale function matrix was introduced in [41, 42]. Wang et al. [43] investigated modified function projective lag synchronization (MFPLS) of dynamical complex networks with disturbance and unknown parameters. The dynamical network is a complex network model containing uncertainty and coupling delay, where delay appears in a complex network but not in the isolate systems. From the above discussions, we can see that the problem of MFPS for CDNs with mixed timevarying delays in the network hybrid coupling and timevarying delays in the dynamical nodes via hybrid adaptive control and hybrid adaptive pinning control has not been fully investigated yet and remains open.
To the best of our knowledge, this is the first time that the MFPS of complex dynamical networks with mixed timevarying and asymmetric coupling delays via new hybrid adaptive control has been studied. We will give a comprehensive study on this topic, and the main contributions of this paper lie in the following aspects. (1) The mixed timevarying delays, with discrete and distributed timevarying delays, which are considered in the dynamical nodes and in hybrid asymmetric coupling simultaneously, are different from the timedelay case in [6, 7, 10, 11, 13, 30]. (2) For the coupling matrix, we do not assume that outer coupling configuration matrix is symmetric or irreducible, which is different from coupling in [8, 16]. (3) For the control method, MFPS is studied via either hybrid adaptive control or hybrid adaptive pinning control with nonlinear and adaptive linear feedback control, which contains error linear term, timevarying delay error linear term and distributed timevarying delay error linear term. The MFPS is different from the control method in [13, 21]. In addition, the pinning nodes can be randomly selected. From the above discussions, this work is one of the first reports of such investigation to further develop the MFPS of complex dynamical networks with mixed timevarying delays in the dynamical nodes and in asymmetric coupling via hybrid adaptive control or hybrid adaptive pinning control. Based on constructing a novel LyapunovKrasovskii functional, the adaptive control technique, the parameter update law and the technique of dealing with some integral terms, new sufficient conditions for guaranteeing the existence of the MFPS of delayed CDNs with asymmetric coupling delays are derived. Numerical examples are included to show the effectiveness of the proposed hybrid adaptive control and hybrid adaptive pinning control scheme.
The rest of the paper is organized as follows. Section 2 provides some mathematical preliminaries and the network model. Section 3 presents MFPS of the complex dynamical network with mixed timevarying delay and hybrid asymmetric coupling by hybrid adaptive control and hybrid adaptive pinning control, respectively. In Section 4 some numerical examples illustrate given theoretical results. The paper ends with conclusions in Section 5 and cited references.
2 Network model and mathematic preliminaries
Notations
The following notation will be used in this paper. \(\mathbb{R}^{n}\) denotes the ndimensional space and \(\\cdot\\) denotes the Euclidean vector norm; \(A^{T}\) denotes the transpose of matrix A; A is symmetric if \(A=A^{T}\); \(I_{N}\) denotes an Ndimensional identity matrix. For the matrix \(A\in\mathbb{R}^{N}\times\mathbb{R}^{N}\), the ith row and the ith column of A is called the ith rowcolumn pair of A. \(A_{l}\in\mathbb{R}^{(Nl)\times(Nl)}\) is the minor matrix of \(A\in\mathbb{R}^{N\times N}\) by removing arbitrary l (\(1\leq l\leq N\)) rowcolumn pairs of A. The symbol ⊗ denotes the Kronecker product.
Definition 2.1
Remark 1
If the scaling function matrix \(\alpha(t)= \operatorname{diag}(\alpha_{1}(t),\alpha _{2}(t),\ldots,\alpha_{n}(t))\) (\(i=1,2,\ldots,n\)) is the function of the time t, then the CDNs would realize modified function projective synchronization. If the scaling function matrix \(\alpha_{1}(t)=\alpha _{2}(t)=\cdots=\alpha_{n}(t)\), then the synchronization problem will be reduced to the function projective synchronization [12, 13, 21, 36]. If the scaling function matrix \(\alpha_{1}(t)=\alpha _{1}, \alpha_{2}(t)=\alpha_{2},\ldots,\alpha_{n}(t)=\alpha_{n}\), then the synchronization problem will be reduced to the projective synchronization [36, 37]. If the scaling function matrix \(\alpha _{1}(t)=1, \alpha_{2}(t)=1,\ldots,\alpha_{n}(t)=1\), then the synchronization problem will be reduced to the common synchronization [8, 16]. If the scaling function matrix \(\alpha_{1}(t)=0, \alpha_{2}(t)=0,\ldots,\alpha_{n}(t)=0\), then the synchronization problem turns into a chaos control problem [34]. Therefore, MFPS is a more general form that includes many kinds of synchronization as its special cases.
Remark 2
Remark 3
If \(h(t)=k(t)\), the network model (1) turns into the complex dynamical network proposed by [11], where discrete and distributed timevarying delays appeared in a driverespond network. If \(h(t)\neq k(t)\), the result in [11] cannot be used to decide whether the synchronization of network model (1) can be achieved.
In the rest of this paper, we need the following assumption and some lemmas.
Assumption 1
The timevarying delay functions \(h(t)\) and \(k(t)\) satisfy conditions that \(h(t)\) is differential, \(0\leq h(t)\leq h\), \(0\leq k(t)\leq k\) and \(0\leq \dot{h}(t)\leq\beta<1\).
Lemma 2.2
Cauchy inequality [44]
Lemma 2.3
[44]
Lemma 2.4
[45]
 (i)
\(c(A\otimes B)=(cA)\otimes B=A\otimes(cB)\),
 (ii)
\((A\otimes B)^{T}=A^{T}\otimes B^{T}\),
 (iii)
\((A\otimes B)(C\otimes D)=(AC)\otimes(BD)\),
 (iv)
\(A\otimes B\otimes C=(A\otimes B)\otimes C=A\otimes(B\otimes C)\).
Lemma 2.5
[46]
Assume that A and B are the \(N\times N\) Hermitian matrices. Suppose that \(\alpha_{1}\geq\alpha _{2}\geq\cdots\geq\alpha_{N}\), \(\beta_{1}\geq\beta_{2}\geq\cdots\geq \beta_{N}\) and \(\gamma_{1}\geq\gamma_{2}\geq\cdots\geq\gamma_{N}\) are eigenvalues of matrices A, B and \(A+B\), respectively. Then one has \(\alpha_{i}+\beta_{N}\leq\gamma_{i}\leq\alpha_{i}+\beta_{1}\), \(i=1,2,\ldots, N\).
Lemma 2.6
[47]
If \(A=(a_{ij})_{(N\times N)}\) is irreducible and satisfies \(a_{ij}=a_{ji}\geq0\), \(i\neq j\); \(a_{ii}=\sum_{j=1,i\neq j}^{N}\), \(i,j=1,2,\ldots, N\), then, for any constant \(\xi>0\), all eigenvalues of the matrix \(A\Xi\) are negative definite, where \(\Xi=\operatorname{diag}( \xi ,0,\ldots,0)\).
Lemma 2.7
[48]
3 MFPS of delayed complex dynamical networks via hybrid adaptive control and hybrid adaptive pinning control
In this section, we give some sufficient conditions for MFPS of complex dynamical networks with discrete and distributed timevarying delays and hybrid asymmetric coupling delays (1) via hybrid adaptive control and hybrid adaptive pinning control.
3.1 MFPS under hybrid adaptive control
 1.
\(J(t)=f'(s(t),s(th(t)),\int_{tk(t)}^{t}s(\theta) \,d\theta) \in R^{n\times n}\) is the Jacobian of \(f(x(t), x(th(t)), \int _{tk(t)}^{t}x(s) \,ds)\) at \(s(t)\) with the derivative of \(f(x(t),x(th(t)),\int_{tk(t)}^{t}x(s) \,ds)\) with respect to \(x(t)\),
 2.
\(J_{h}(t)=f'(s(t),s(th(t)),\int_{tk(t)}^{t}s(\theta) \,d\theta) \in R^{n\times n}\) is the Jacobian of \(f(x(t),x(th(t)), \int _{tk(t)}^{t}x(s) \,ds)\) at \(s(th(t))\) with the derivative of \(f(x(t),x(th(t)),\int_{tk(t)}^{t}x(s) \,ds)\) with respect to \(x(th(t))\),
 3.
\(J_{k}(t)=f'(s(t),s(th(t)),\int_{tk(t)}^{t}s(\theta) \,d\theta )\in R^{n\times n}\) is the Jacobian of \(f(x(t),x(th(t)), \int _{tk(t)}^{t}x(s) \,ds)\) at \(\int_{tk(t)}^{t}s(\theta) \,d\theta\) with the derivative of \(f(x(t),x(th(t)),\int_{tk(t)}^{t}x(s) \,ds)\) with respect to \(\int_{tk(t)}^{t}x(s) \,ds\),
Theorem 3.1
Proof
Remark 4
If \(f(x_{i}(t),x_{i}(th(t)),\int _{tk(t)}^{t}x_{i}(s) \,ds)=f(x(t))\), \(h(t)=h\), \(k(t)=0\) and \(c_{3}\)=0, then system (1) reduces to the following network (4) presented in [7]. According to Theorem 3.1, we obtain the following corollary for the synchronization of network (4).
Corollary 3.2
Proof
The proof is similar to that of Theorem 3.1. Indeed, by setting \(J_{h}(t)=0\), \(J_{k}(t)=0\), \(k=0\), \(\beta=0\) and \(c_{3}=0\), one may easily derive the result, and hence the proof is omitted. □
Remark 5
The authors in [13, 21] presented the synchronization of complex dynamical networks via hybrid control, which is dependent on a nonlinear function \(f(\cdot)\). But in this paper, the controller (5) is independent of the nonlinear function \(f(\cdot)\). Therefore, for removing the nonlinear function \(f(\cdot)\), we employ some new techniques that make the implementation of controller easier with practice. This theorem can be applied to a great many complex dynamical networks in the real world.
3.2 MFPS under hybrid adaptive pinning control
Theorem 3.3
Proof
Remark 6
 Step I::

Choose some appropriate parameters \(\varepsilon_{i}\), \(i=1,2,\ldots,6\), and by taking appropriate \(\bar{d}_{1i}^{\ast}\), \(i=1,2,\ldots,l\), \(\bar{d}_{2}^{\ast}\) and \(\bar{d}_{3}^{\ast}\) such that the conditions in Theorem 3.3 are feasible.
 Step II::

The l pinned nodes are sorted according to the pinnednode selection scheme studied in [10] for the pinning controlled error dynamical network (31); so, the nodes to be pinned are chosen in the particular order. Let \(l=1\), if the first inequalities of Theorem 3.3 are satisfied, then the least number is 1; otherwise, go to next step.
Remark 7
In Theorem 3.3, we investigated the MFPS of complex dynamical networks via hybrid control, where the control \(u_{i1}(t)\) is a nonlinear control (not pinning control) to apply for every node. By using the principle of function projective synchronization, this control needs to be applied for every node. And \(\bar{u}_{i2}(t)\) is an adaptive pinning control to apply for the first l nodes \(1\leq i\leq l\) by using the principle of pinning control approach. This technique for applying both of controls has been considered in [21].
Remark 8
If we investigate the dynamical nodes without delays and ignore the adaptive linear feedback control, which contains timevarying delay error linear term and distributed timevarying delay error linear term, we can see the general model of the complex dynamical networks in [13, 21]. By comparison, this paper contains discrete and distributed timevarying delays in dynamical nodes and adaptive linear feedback control simultaneously. Furthermore, it also develops the preexisting research.
Remark 9
However, there is room for improvement. First, the timevarying delays are still necessarily differentiable. So, we should remove them, which means that fast timevarying delays are allowed. Second, even though the hybrid pinning adaptive control can reduce the number of controllers, it cannot reduce the control cost. Hence, combining the intermittent control technique and the pinning control strategy should be considered together.
4 Numerical examples
In this section, we present three examples to illustrate the effectiveness and the reduced conservatism of our result.
Example 4.1
Solution: From conditions (8)(10) of Theorem 3.1 and with positive constants \(\varepsilon _{1}=9.86\), \(\varepsilon_{2}=8.75\), \(\varepsilon_{3}=12.9\), \(\varepsilon _{4}=10.2\), \(\varepsilon_{5}=13.10\), \(\varepsilon_{6}=10.70\), one can check that the last three conditions in Theorem 3.1 are satisfied. From the conditions of Theorem 3.1, we obtain \(d_{1}^{\ast}>5.7340\), \(d_{2}^{\ast}>4.2786 \) and \(d_{3}^{\ast}> 2.9042\).
Remark 10
The advantage of Example 4.1 is that the discrete and distributed timevarying delays are different values, i.e., \(h(t)=0.1+0.1\sin^{2} t\), \(k(t)=0.1\cos^{2} t\). Moreover, in these examples we still consider discrete and distributed timevarying delays in the dynamical nodes and the hybrid coupling term simultaneously. Hence the synchronization conditions in [11] cannot be applied to these examples.
Example 4.2
Remark 11
In Examples 4.1 and 4.2, we see that every state variable of the error networks of (39) and (41) is unstable without control. After applying controllers (5) and (27), all the state variables of the error networks of (39) and (41) quickly converge to 0. That shows the effectiveness of the controllers.
Example 4.3
5 Conclusions
In this paper, modified function projective synchronization (MFPS) for complex dynamical networks with mixed timevarying and hybrid coupling delays was investigated. It is assumed that the coupling configuration matrix need not be symmetric or irreducible and it contains state coupling, timevarying delay coupling and distributed timevarying delay coupling. Firstly, we considered MFPS via either hybrid control or hybrid pinning control with nonlinear and adaptive linear feedback control, which contains error linear term, timevarying delay error linear term and distributed timevarying delay error linear term. Secondly, by using a novel LyapunovKrasovskii functional, a new adaptive control technique, the parameter update law and the technique of dealing with some integral terms, improved MFPS criteria of delayed CDNs with asymmetric coupling delays are obtained. In addition, the pinning nodes can be randomly selected. Finally, numerical examples are included to show the effectiveness of the proposed hybrid adaptive control and hybrid adaptive pinning control scheme. The results in this paper generalize and improve the corresponding results of the recent works.
Declarations
Acknowledgements
The authors would like to thank the editor and the reviewers for their helpful advice. The first author was financially supported by Chiang Mai University, Chiang Mai, Thailand. The second author was financially supported by the Thailand Research Fund (TRF), the Office of the Higher Education Commission (OHEC), Khon Kaen University (grant number: MRG5880009) and the National Research Council of Thailand and Khon Kaen University 2017 (grant number: 600061). The third author was financially supported by University of Pha Yao, Pha Yao, Thailand.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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