Delay-Dependent Dynamics of Switched Cohen-Grossberg Neural Networks with Mixed Delays
Abstract
This paper aims at studying the problem of the dynamics of switched Cohen-Grossberg neural networks with mixed delays by using Lyapunov functional method, average dwell time (ADT) method, and linear matrix inequalities (LMIs) technique. Some conditions on the uniformly ultimate boundedness, the existence of an attractors, the globally exponential stability of the switched Cohen-Grossberg neural networks are developed. Our results extend and complement some earlier publications.
1. Introduction
With the rapid development of intelligent control, hybrid systems have been investigated extensively for their significance, which is in theory and application. As one of the most important classes of hybrid systems, switched systems have drawn increasing attention in the last decade [19–21]. A typical switched systems are composed of a set of subsystems and a logical switching rule whose subsystem will be activated at each instant of time and orchestrates the switching among the subsystems [22]. In general, the switching rule is a piecewise constant function dependent on the state or time. The logical rule that orchestrates switching between these subsystems generates switching signals [23]. Recently, many results on the stability of switched system with time delay and parametric uncertainties have been reported [24, 25]. Switched system in which all subsystems are stable was studied in [26], and Hu and Michel used dwell time approach to analyse the local asymptotic stability of non-linear switched systems in [27]. Hespanha and Morse [28] extended this concept to develop the average dwell time approach subsequently. Furthermore, in [29], the stability results of switched system extended to the case when subsystems are both stable and unstable have been reported, and therefore we derive less conservative results. So, average dwell time (ADT) approach turns out to be an effective tool to study the stability of switched systems [28] and specially when not all subsystems are stable [29].
Meanwhile, neural networks as a special kind of complex networks, the connection topology of networks may change frequently and often lead to link failure or new link creation during the hardware implemtation. Hence, the abrupt changes in the network structure often occur, and switchings between some different topologies are inevitable [30]. Thus, the switched neural network was proposed and has successful applications in the field of high-speed signal processing and artificial intelligence [31, 32], and switched neural networks are also used to perform the gene selection in a DNA microarray analysis in [33]. Thus, it is of great meaning to discuss the switched neural networks. Recently, the stability of switching neural networks has been intensively investigated [34–36]. Robust exponential stability and H∞ control for switched neutral-type neural networks were discussed in [34].
In [35], delay-dependent stability analysis for switched neural networks with time-varying delay was analyzed. In [36], by employing nonlinear measure and LMI techniques, some new sufficient conditions were obtained to ensure global robust asymptotical stability and global robust exponential stability of the unique equilibrium for a class of switched recurrent neural networks with time-varying delay.
The function is a piece-wise constant function of time, called a switching signal, which specifies that subsystem will be activated. N denotes the number of subsystems. The switched sequence can be described as , where t0 denotes the initial time and tk is the kth switching instant. Moreover, σ(t) = i means that the ith subsystem is activated. For any , this means that the matrices (Aσ, Bσ, Cσ) can taken values in the finite set {(A1, B1, C1), …, (AN, BN, CN)}. Meanwhile, we assume that the state of the switched CGNN does not jump at the switching instants; that is, the trajectory xt is everywhere continuous.
However, these available literatures mainly consider the stability property of switching neural networks. In fact, except for stability property, boundedness and attractor are also foundational concepts of dynamical neural networks, which play an important role in investigation of the uniqueness of equilibrium point (periodic solutions), stability and synchronization and so on [13, 14]. To the best of the author’s knowledge, few authors have considered the uniformly ultimate boundedness and attractors for switched CGNN with discrete delays and distributed delays.
As is well known, compared with linear matrix inequalities (LMIs) result, algebraic result is more conservative, and criteria in terms of LMI can be easily checked by using the powerful Matlab LMI toolbox. This motivates us to investigate the problems of the uniformly ultimate boundedness and the existence of an attractor for switched CGNN in this paper. The illustrative examples are given to demonstrate the validity of the theoretical results.
The paper is organized as follows. In Section 2, preliminaries and problem formulation are introduced. Section 3 gives the sufficient conditions of uniformly ultimate boundedness (UUB) and the existence of an attractor for switched CGNN. It is the main result of this paper. In Section 4, an example is given to illustrate the effectiveness of the proposed approach. The conclusions are summarized in Section 5.
2. Problem Formulation
Throughout this paper, we use the following notations. The superscript “T” stands for matrix transposition; Rn denotes the n-dimensional Euclidean space; the notation P > 0 means that P is real symmetric and positive definite; I and O represent the identity matrix and a zero matrix; diag {⋯} stands for a block-diagonal matrix; λmin (P) denotes the minimum eigenvalue of symmetric matrix P; in symmetric block matrices or long matrix expressions, “*” is used to represent a term that is induced by symmetry. Matrices, if their dimensions are not explicitly stated, are assumed to be compatible for algebraic operations.
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(H1) For any j ∈ {1,2, …, n}, there exist constants lj and Lj, such that
()
Remark 1. The constants lj and Lj can be positive, negative, or zero. Therefore, the activation functions f(·) are more general than the forms |fj(u)| ≤ Kj|u|, Kj > 0, j = 1,2, …, n.
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(H2) For continuously bounded function αj(·), there exist positive constants , , such that
() -
(H3) There exist positive constants bj, such that
()
Definition 2 (see [15].)System (6) is uniformly ultimately bounded; if there is , for any constant ϱ > 0, there is t′ = t′(ϱ) > 0, such that for all t ≥ t0 + t′, t0 > 0, ∥φ∥ < ϱ, where the supremum norm ∥x(t, t0, φ)∥ = max 1≤i≤n sup −δ≤s≤0 |xi(t + s, t0, φ)|.
Definition 3 (see [37].)The nonempty closed set A ⊂ Rn is called the attractor for the solution x(t; φ) of system (6) if the following formula holds:
Definition 4 (see [28].)For a switching signal σ(t) and each T > t ≥ 0, let Nσ(t, T) denote the number of discontinuities of σ(t) in the interval (t, T). If there exist N0 > 0 and Ta > 0 such that Nσ(t, T) ≤ N0 + (T − t)/Ta holds, then Ta is called the average dwell time. N0 is the chatter bound.
Remark 5. In Definition 4, it is obvious that there exists a positive number Ta such that a switched signal has the ADT property, which means that the average dwell time between any two consecutive switchings is no less than a specified constant Ta, Hespanha and Morse have proved that if Ta is sufficiently large, then the switched system is exponentially stable. In addition, in [18], one can choose N0 = 0, but in our paper, we assume that N0 > 0, this is more preferable.
Lemma 6 (see [16].)For any positive definite constant matrix W ∈ Rn×n, scalar r > 0, and vector function u(t):[t − r, t] → Rn, t ≥ 0, then
Lemma 7 (see [38].)For any given symmetric positive definite matrix X ∈ Rn×n and scalars α > 0, 0 ≤ d1 < d2, if there exists a vector function such that the following integration is well defined, then
3. Main Results
Theorem 8. For a given constant a > 0, if there is positive definite matrix P = diag (p1, p2 … , pn), Di = diag (Di1, Di2 … , Din), i = 1,2, …, Q, Si, such that the following condition holds:
Proof. Let us consider the following Lyapunov-Krasovskii functional:
Similarly, taking the time derivative of V2(t) along the trajectory of system (6), we obtain
Computing the derivative of V3(t) along the trajectory of system (6) turns out to be
Denoting that , we obtain
Using Lemma 7, the following inequality is easily obtained:
From assumption (H1), it follows that, for j = 1,2, …, n,
Using (20)–(27) and adding (29), we can derive
By integrating both sides of (31) in time interval t ∈ [t0, t], then we can obtain
If one chooses , then for any constant ϱ > 0 and ∥φ∥ < ϱ, there is t′ = t′(ϱ) > 0, such that e−atV(x(t0)) 2 < 1 for all t ≥ t′. According to Definition 2, we have for all t ≥ t′. That is to say, system (6) is uniformly ultimately bounded. This completes the proof.
Theorem 9. If all of the conditions of Theorem 8 hold, then there exists an attractor for the solutions of system (6), where .
Proof. If one chooses , Theorem 8 shows that for any ϕ, there is t′ > 0, such that for all t ≥ t′. Let . Clearly, is closed, bounded, and invariant. Furthermore, . Therefore, is an attractor for the solutions of system (6).
Corollary 10. In addition to the fact that all of the conditions of Theorem 8 hold, if J = 0, and Fj(0) = 0, then system (6) has a trivial solution x(t) ≡ 0, and the trivial solution of system (6) is globally exponentially stable.
Proof. If J = 0, and Fj(0) = 0, then it is obvious that system (6) has a trivial solution x(t) ≡ 0. From Theorem 8, one has
Therefore, the trivial solution of system (6) is globally exponentially stable. This completes the proof.
In this section, we will present conditions for uniformly ultimate boundedness and the existence of an attractor of the switching CGNN by applying the average dwell time.
Theorem 11. For a given constant a > 0, if there is positive definite matrix P = diag (pi1, pi2 … , pin), Di = diag (Di1, Di2 … , Din), , such that the following condition holds:
Proof. Define the Lyapunov functional candidate of the form
The system state is continuous. Therefore, it follows that
Theorem 12. If all of the conditions of Theorem 11 hold, then there exists an attractor for the solutions of system (39), where .
Proof. If we choose , Theorem 11 shows that for any ϕ, there is t′ > 0, such that for all t ≥ t′. Let . Clearly, is closed, bounded, and invariant. Furthermore, .
Therefore, is an attractor for the solutions of system (39).
Corollary 13. In addition to the fact that all of the conditions of Theorem 8 hold, if J = 0 and Fi(0) = 0, then system (39) has a trivial solution x(t) ≡ 0, and the trivial solution of system (39) is globally exponentially stable.
Proof. If J = 0 and Fi(0) = 0, then it is obvious that the switched system (39) has a trivial solution x(t) ≡ 0. From Theorem 8, one has
It means that the trivial solution of the switched Cohen-Grossberg neural networks (39) is globally exponentially stable. This completes the proof.
Remark 14. It is noted that common Lyapunov function method requires all the subsystems of the switched system to share a positive definite radially unbounded common Lyapunov function. Generally speaking, this requirement is difficult to achieve. So, in this paper, we select a novel multiple Lyapunov function to study the uniformly ultimate boundedness and the existence of an attractor for switched Cohen-Grossberg neural networks. furthermore, this type of Lyapunov function enables us to establish less conservative results.
Remark 15. When N = 1, we have Pi = Pj, Qi = Qj, , , i, j ∈ Σ, then the switched Cohen-Grossberg neural networks (4) degenerates into a general Cohen-Grossberg neural networks with time-delay [15, 17]. Obviously, our result generalizes the previous result.
Remark 16. It is easy to see that τa = 0 is equivalent to existence of a common function for all subsystems, which implies that switching signals can be arbitrary. Hence, the results reported in this paper are more effective than arbitrary switching signal in the previous literature [16].
Remark 17. The constants li, Li in assumption (H1) are allowed to be positive, negative, or zero, whereas the constant li is restricted to be the zero in [1, 15], and the non-linear output function in [5, 18, 34–37] is required to satisfy Fj(0) = 0. However, in our paper, the assumption condition was deleted. Therefore, assumption (H1) of this paper is weaker than those given in [1, 5, 15, 18, 34–37].
4. Illustrative Examples
In this section, we present an example to show the effectiveness and advantages of the proposed method and consider the switched neural networks with two subsystems.
From assumptions H1, H2, we can gain li = 0.5, Li = 1, , α = 1.5, , bi = 1.2, τ* = 0.15, h* = 0.3, and δ = 0.3 and i = 1,2.
5. Conclusion
In this paper, the dynamics of switched Cohen-Grossberg neural networks with average dwell time is investigated. A novel multiple Lyapunov-Krasovskii functional is designed to get new sufficient conditions guaranteeing the uniformly ultimate boundedness, the existence of an attractor, and the globally exponential stability. The derived conditions are expressed in terms of LMIs, which are more relaxed than algebraic formulation and can be easily checked by the effective LMI toolbox in Matlab in practice. Based on the method provided in this paper, stochastic disturbance, impulse, and reaction diffusion for switched systems will be considered in the future works.
Acknowledgments
This work was jointly supported by the National Natural Science Foundation of China under Grant no. 11101053 and 11101435, the Key Project of Chinese Ministry of Education under Grant no. 211118, the Excellent Youth Foundation of Educational Committee of Hunan Provincial no. 10B002, the Hunan Provincial NSF no. 11JJ1001, and National Science and Technology Major Projects of China no. 2012ZX10001001-006, the Scientific Research Fund of Hunan Provincial Education Department of China no. 12C0028.