Complexity

Volume 2018 (2018), Article ID 8546304, 13 pages

https://doi.org/10.1155/2018/8546304

## Finite-Time Nonfragile Synchronization of Stochastic Complex Dynamical Networks with Semi-Markov Switching Outer Coupling

Correspondence should be addressed to Yong-Ki Ma; rk.ca.ujgnok@amky

Received 13 July 2017; Accepted 14 December 2017; Published 16 January 2018

Academic Editor: Hiroki Sayama

Copyright © 2018 Rathinasamy Sakthivel 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.

#### Abstract

The problem of robust nonfragile synchronization is investigated in this paper for a class of complex dynamical networks subject to semi-Markov jumping outer coupling, time-varying coupling delay, randomly occurring gain variation, and stochastic noise over a desired finite-time interval. In particular, the network topology is assumed to follow a semi-Markov process such that it may switch from one to another at different instants. In this paper, the random gain variation is represented by a stochastic variable that is assumed to satisfy the Bernoulli distribution with white sequences. Based on these hypotheses and the Lyapunov-Krasovskii stability theory, a new finite-time stochastic synchronization criterion is established for the considered network in terms of linear matrix inequalities. Moreover, the control design parameters that guarantee the required criterion are computed by solving a set of linear matrix inequality constraints. An illustrative example is finally given to show the effectiveness and advantages of the developed analytical results.

#### 1. Introduction

During the past twenty years, the investigation of complex dynamical networks (CDNs) that consist of a huge number of interacting dynamical nodes has received a great deal of attention from various science and engineering areas, such as social networks, ecological prey-predator networks, protein networks, power grids, and ecosystems [1, 2]. It should be mentioned that the analysis of dynamical behaviors of CDNs has become a hot research topic in recent years. Among many dynamical behaviors, synchronization phenomenon is the most important behavior and several interesting and efficient methodologies have been developed in the literature to solve the synchronization problem of various kinds of CDNs; for instance, see [3–7]. In [4], the problem of inner synchronization of a CDN has been investigated by considering two different types of guaranteed cost dynamic feedback controllers, where the control gains corresponding to two feedback controllers have different dimensions subject to the topological structure of the CDN. In [5], the problem of outer synchronization between two hybrid-coupled delayed dynamical networks has been discussed by using the aperiodically adaptive intermittent pinning control, where a simple and elegant pinned-node selection scheme is proposed to achieve the required result. It is worth mentioning that the CDN representing real-time systems is generally affected by external noise factors or stochastic disturbances [8]. Thus, the consideration of external noise factors in the study of synchronization of CDNs is of great importance in the viewpoints of both theoretical and practical. By taking this fact into account, in recent years, research communities have eagerly investigated the problems of synchronization of CDNs with external stochastic disturbances, see for example [9–12]. Sakthivel et al. [10] presented some sufficient conditions that ensure the synchronization and solve the state estimation problem of discrete-time stochastic complex networks in the presence of uncertain inner couplings, where the interval matrix approach is employed to characterize the uncertainties encountered in the inner coupling terms. Li et al. [11] developed a new synchronization criterion for a class of discrete-time stochastic complex networks subject to partial mixed impulsive effects, where by using the Lyapunov stability theory and the variation of parameters formula, the required criterion is obtained.

It should be pointed out that the interconnection topology among the nodes of CDNs plays a significant role in the study of synchronization problem. In the existing literature, there have been two reported kinds of interconnection topologies, which are constant/fixed topology [13] and time-varying topology [14]. In [14], it has been illustrated that the time-varying interconnection topology is more general than the fixed one. Nevertheless, in most of the real networks, the connectivity of the network topology might be unfixed or randomly changing due to new creation or link failures. In order to tackle these types of issues, it is more appropriate to model the CDNs with randomly switching topologies that are governed by a Markov process. Based on this scenario and following the seminal works reported in [15, 16], some interesting and significant results about synchronization of CDNs with Markov jump topologies have been discussed; see [17–19]. Specifically, in [18], the problem of nonfragile synchronization for a class of discrete-time complex networks subject to Markov jumping switching topology has been investigated in a unified framework that includes the nonfragile synchronization control and nonfragile* l*_{2}–*l*_{∞} synchronization control problem as its special cases. However, it is worth pointing out that the aforementioned papers have considered constant transition rates in the Markov process. It should be mentioned that the Markov process might consist of time-varying transition rates when modeling practical systems. Such kind of process is known as semi-Markov process and few interesting results regarding semi-Markov jump systems have been addressed in recent literature [20–23]. Interestingly, in [21], by employing the supplementary variable technique and plant transformation, the state estimation and sliding mode control problems have been investigated for semi-Markovian jump systems in the presence of mismatched uncertainties. Apparently, semi-Markov jump systems are comparatively more general than the traditional Markov jump systems [24]. Following the aforementioned seminal works, the concept of semi-Markov process has further been employed in the network topology to obtain the synchronization criteria for CDNs (see [25, 26] and the references therein). However, only very few results about the synchronization of CDNs with semi-Markov jump topology have been reported in the literature, which stimulates us to do this present work.

It is worth mentioning that most of the available controller design approaches have a predominant assumption that the designed controller can be implemented accurately. But in some real situations, such an assumption is not always true as the controllers are often very sensitive or fragile to their parameters’ variations. Furthermore, it should be noted that a small perturbation in controllers may lead to undesirable oscillatory behavior or even instability [27]. Hence, it is desirable as well as necessary to ensure the insensitivity of the controller to certain parameter perturbations without loss of the robust stability and thus, the investigation of nonfragile or resilient controller design that has been capable of tolerating some level of controller parameter gain variations has been enormously increased in recent years [28–31]. To mention a few, in [28], a robust resilient control problem of discrete-time Markov jump nonlinear systems has been solved by employing the linear matrix inequality and stochastic analysis techniques; in [29], based on the dissipative theory and the event-triggered sampling scheme, the nonfragile control design problem for a class of network-based singular systems with input time-varying delay and external disturbances has been addressed. Therefore, it is reasonable to consider the nonfragile control design in the study of synchronization of CDNs. It is noteworthy that only few research papers regarding the nonfragile control design for achieving the synchronization of CDNs have been published; see [32, 33]. On the other hand, it is worth mentioning that most of the existing results based on the classical control theory dealt with the asymptotic property of control system trajectories over an infinite-time interval and did not possess any restriction to the system states. But in many practical problems, it is required that the described system state does not exceed a certain bound during a fixed finite-time interval [34]. According to this fact, a great number of interesting results on finite-time control design have been proposed for the synchronization of various CDNs; for instance, see [35–38]. To the best of our knowledge, however, the problem of robust nonfragile synchronization has not been fully investigated for a class of CDNs over a prescribed finite-time interval.

Motivated by the above analysis, in this paper, we focus on the finite-time nonfragile synchronization problem is investigated for a class of CDNs subject to semi-Markov jump topology and stochastic noises. More precisely, a new delay-dependent sufficient condition under which the considered CDNs are synchronized to the target network within a given finite-time interval is developed in terms of linear matrix inequalities by utilizing the Lyapunov stability theory and the stochastic analysis techniques. Subsequently, based on the developed condition, a design algorithm of the proposed nonfragile state feedback controller that can ensure the finite-time stochastic synchronization of the addressed network is presented. Eventually, a numerical example is shown to illustrate the effectiveness of the proposed theoretical results.

The rest of this paper is organized as follows: in Section 2, the problem formulation of the network model under study and the preliminaries required to obtain the main results are given. The finite-time stochastic synchronization criterion for the considered network model is presented in Section 3. A numerical example and its simulations are provided in Section 4. Conclusion of this paper is given in Section 5.

#### 2. Problem Formulation and Preliminaries

In this paper, we consider a class of complex dynamical networks (CDNs) with semi-Markov jump outer coupling and stochastic noise, which consists of identical nodes and is defined over the Wiener process probability space , where is the sample space, is the algebra of events and is the probability measure defined on . Such a network model can be described in the following form:where denotes the state vector of the th node; is a known real constant matrix with suitable dimension; represents a nonlinear vector-valued function; the constant is the coupling strength of network; are the elements of the outer coupling matrix which describes the network topological structure and is assumed to follow a semi-Markov process which to be defined later. In particular, is defined as follows: if there exists a connection between node and node , then ; otherwise, . Further, the diagonal elements of the outer coupling matrix are given as follows: ; represents the inner coupling matrix and is a positive diagonal matrix with appropriate dimension; is the control input of the th node which to be defined later; the function is the noise intensity vector-valued function; is a -dimensional Brownian motion defined on the probability space with , and for , where is the mathematical expectation; is the time-varying delay function satisfying and , where , , and are known scalars; and denotes the initial value of the th node’s state and is assumed to be a continuous vector-valued function.

Now, let us define the semi-Markov jump process of the outer coupling matrix. The process is a continuous-time homogeneous semi-Markov process with right continuous trajectories and takes values in a finite set . More precisely, is associated with the transition probability matrix which is given by the following transition rates:

where is the sojourn time, and for is the transition rate from mode at time to mode at time and . For notational simplicity, we hereafter denote the semi-Markov process parameter by . For example, is denoted by .

To synchronize all the identical nodes in the network (1) to a common value, let us define the synchronization error vector as , where is the state vector of the unforced isolated node that can be expressed as and is assumed to be noise-free, that is, . Based on this error vector, we now choose a robust state feedback controller to achieve the synchronization of network (1), which is insensitive to the uncertain perturbations or gain fluctuations and of the form:where is the feedback controller gain matrix that is to be determined in the forthcoming section, is a time-varying matrix representing the controller gain fluctuations, and is a stochastic variable describing the randomly occurring controller gain fluctuations. It is here assumed that takes the form , where and are known real constant matrices and is an unknown time-varying matrix satisfying . Further, it is assumed that the stochastic variable obeys the Bernoulli distribution with the following probability rules: (i) and (ii) , where .

Then, by using (1) and (3), the closed-loop form of the error system can be obtained as follows:where and . By using the Kronecker product properties and mathematical manipulations, the error system (4) can be written in the following compact form: where , , and .

In order to develop the main results, the following assumptions and definition are required.

*Assumption 1. *For the nonlinear function , there exists a known real constant matrix such that for any .

*Assumption 2. *The noise intensity function is uniformly Lipschitz continuous in terms of the following inequality of trace inner product: , where and are known nonnegative constants.

*Assumption 3. *For each , all the real parts of eigenvalues of are negative except an eigenvalue with multiplicity , which means that the reverse of the graph generated by the matrix contains a rooted spanning directed tree for every .

*Definition 4 (see [34]). *The considered network (1) is said to be stochastically synchronized in finite-time with respect to if there exist positive definite matrix and positive constants , , with such that the following condition holds:

#### 3. Main Results

Based on the Lyapunov-Krasovskii stability theory, this section aims to develop a new set of delay-dependent sufficient conditions that can guarantee the stochastic synchronization of the considered network model (1) over a finite-time interval. Moreover, based on these conditions, a design of the robust nonfragile state feedback control (3) for the network model under consideration is provided in terms of linear matrix inequalities (LMIs).

Theorem 5. *Consider the network model (1) with Assumptions 1–3. For given positive scalars , , , , , , , , , symmetric matrix , and diagonal matrices , , the considered network (1) is stochastically synchronized in finite-time under the nonfragile controller (3), if there exist symmetric matrices , , and positive scalars , such that the following matrix inequalities hold:whereand the rest of elements of are zero.*

*Proof. *To develop the finite-time stochastic synchronization criterion for the network model (1), it is enough to establish the finite-time stochastic stability criterion for the closed-loop error system (5). For this purpose, we select the Lyapunov-Krasovskii functional as follows:wherewith , , and .

Based on Ito’s differential formula [9], the stochastic derivative of can be obtained aswhere and .

Now, by calculating the time derivative of along the solution trajectories of the error system (5), we can getFurther, by applying Jensen’s single integral inequality [6] to the integral terms in (16), we can get the following inequalities:On the other hand, it follows from Assumption 2 and condition (7) thatwhere are positive scalars and , are known constant matrices.

Moreover, according to Assumption 1, we can obtain the following inequality:Then, by combining (13)–(19) and taking mathematical expectation, it can be obtained thatwhere and the elements of , , and are defined in the theorem statement. Moreover, based on Lemma in [6], for any positive scalar , the right-hand side of (20) can equivalently be written asBased on the Schur complement, it is noted that (22) is equivalent to the left-hand side of (8). Thus, it can be observed that if the LMIs (7) and (8) hold. Furthermore, if there exists a constant , it yields that . From which, it can be obtained that , where . Next, define the following new parameters: , , , , , . Then, it follows from condition (11) and thatwhere are defined in the theorem statement and is given in Definition 4.

On the other hand, from (11), we can haveNow, by combining the inequalities (23) and (24), we can getIt is clear to see that the inequality (25) is the same as that in (9) which is the desired condition. Hence, it can be concluded that the closed-loop error system (5) is finite-time stochastically stable which means that the considered network model (1) is stochastically synchronized within a prescribed finite-time interval. Thus, the proof of this theorem is completed.

Theorem 6. *Consider the network model (1) with Assumptions 1–3. For given positive scalars , , , , , , , , , symmetric matrix and diagonal matrices , , the considered network (1) is stochastically synchronized in finite-time under the non-fragile state feedback controller (3), if there exist symmetric matrices , , , any matrices with appropriate dimensions and positive scalars , , , , , such that the following matrix inequalities hold:where and the remaining parameters of are zero. Moreover, if the obtained LMIs are feasible, then the desired state feedback controller gain matrices in (3) are computed by .*

*Proof. *Let and pre- and postmultiply the matrix by diag. Now, we introduce the following new variables: , , and .

Moreover, from Theorem 5, it is noticed that , , , , and . According to the congruence transformation, these relations can be changed into , and . Now, if we set , , and , then the constraints in (28) can easily be deduced. Moreover, the conditions in (26), (27), and (29) can be obtained from (7), (8), and (9), respectively, which are the desired conditions. Hence, the proof is completed.

*Remark 7. *It should be mentioned that the constraints in (27) are cannot be solved directly via MATLAB LMI control toolbox due to the existence of the time-varying terms . To overcome this difficulty, the transition rates are assumed to be bounded and satisfy , since they are partially measurable in practice which is mentioned in [26]. Moreover, in this case, the following assumptions are made as in [26]:

Now, we are able to present the sufficient conditions guaranteeing the stochastic synchronization of the considered network (1) over a finite-time interval in terms of LMIs in which all the elements are either constants or constant matrices according to Remark 7. Thus, we have the following theorem.

Theorem 8. *For given positive scalars , , , , , , , , , symmetric matrix and diagonal matrices , , the considered network (1) with Assumptions 1–3 is stochastically synchronized in finite-time under the non-fragile controller (3), if there exist symmetric matrices , , , any matrices with appropriate dimensions and positive scalars , , , , such that the following matrix inequality, (26), (28), and (29) hold:where and the remaining elements of are the same as those defined in Theorem 6. Further, the nonfragile state feedback controller gain matrices in (3) are calculated by .*

*Proof. *Based on Remark 7, the time-varying element may take values in the interval . Then, by using (31) and following the similar lines in the proof of Theorem 6, it is easy to obtain the inequality (32) which completes the proof.

*Remark 9. *It should be noted that, so far in the literature, several control approaches have been proposed for the synchronization problem of several CDNs [3–6], wherein the interconnection topology among the nodes are assumed to be fixed. However, in practice, this assumption is practically difficult or even impossible. However, yet now, there were no results reported in the existing literature for the synchronization analysis of stochastic CDNs with switching topology. According to this fact, in this paper, finite-time synchronization problem of stochastic CDNs with switching topology is investigated. Furthermore, due to random behavior in the dynamics of stochastic CDNs, it is very difficult to determine the exact fixed control value. Therefore, in this paper, the feedback control gain is considered with uncertain terms, which is more significant to reflect the realistic scenarios.

#### 4. An Illustrative Example

This section provides an illustrative example to verify the developed theoretical results in the previous section. For the sake of simplicity, consider a class of CDNs in the form of (1) with five identical nodes and the state vector of each node being three-dimensional, that is, and .

Let us select the network matrix and the nonlinear function as It is clear to see that satisfies Assumption 1 with . In this example, we consider the semi-Markov jump topology with two modes, whose connectivity graph is shown in Figure 1. The inner coupling matrix is assumed to be and the coupling strength is chosen as . The time-varying delay is taken as from which it can be obtained that , and .