Stability of Impulsive Neural Networks with Time-Varying and Distributed Delays
Abstract
This work is devoted to investigating the stability of impulsive cellular neural networks with time-varying and distributed delays. We use the new method of fixed point theory to obtain some new and concise sufficient conditions to ensure the existence and uniqueness of solution and the global exponential stability of trivial equilibrium. The presented algebraic criteria are easily checked and do not require the differentiability of delays.
1. Introduction
Since cellular neural networks (CNNs) were proposed by Chua and Yang in 1988 [1, 2], many researchers have put great effort into this subject due to their numerous successful applications in various fields such as optimization, linear and nonlinear programming, associative memory, pattern recognition, and computer vision.
Owing to the finite switching speed of amplifiers, there is no doubt that time delays exist in the communication and response of neurons. Moreover, as neural networks usually have a spatial extent due to the presences of a multitude of parallel pathways with a variety of axon sizes and lengths, there is a distribution of conduction velocities along these pathways and a distribution of propagation designed with discrete delays. Therefore, a more appropriate and ideal way is to incorporate continuously distributed delays with a result that a more effective model of cellular neural networks with time-varying and distributed delays proposed.
In fact, beside delay effects, stochastic and impulsive as well as diffusing effects are also likely to exist in neural networks. So far, there have been many results [3–11] on the study of dynamic behaviors of complex CNNs such as impulsive delayed reaction-diffusion CNNs and stochastic delayed reaction-diffusion CNNs. Summing up the existing researches on the stability of complex CNNs, we see that the primary method is Lyapunov theory. However, there are also lots of difficulties in the applications of corresponding theories to specific problems. It is therefore necessary to seek some new methods to deal with the stability in order to overcome those difficulties.
Recently, it is inspiring that Burton and other authors have applied the fixed point theory to investigate the stability of deterministic systems and obtained some more applicable conclusions, for example, see the monograph [12] and the work in [13–24]. In addition, more recently, there have been a few papers where the fixed point theory is employed to investigate the stability of stochastic (delayed) differential equations, for instance, see [25–31]. Precisely, in [26–28], Luo used the fixed point theory to study the exponential stability of mild solutions for stochastic partial differential equations with bounded delays and with infinite delays. In [29, 30], Sakthivel used the fixed point theory to discuss the asymptotic stability in pth moment of mild solutions to nonlinear impulsive stochastic partial differential equations with bounded delays and with infinite delays. In [31], Luo used the fixed point theory to study the exponential stability of stochastic Volterra-Levin equations. We wonder if we can obtain some new and more applicable stability criteria of complex CNNs by applying the fixed point theory.
With this motivation, in this paper, we aim to discuss the global exponential stability of impulsive CNNs with time-varying and distributed delays. It is worth noting that our research technique is based on the contraction mapping principle rather than the usual method of Lyapunov theory. We deal with, by employing the fixed point theorem, the existence and uniqueness of solution and the global exponential stability of trivial equilibrium at the same time, for which Lyapunov method feels helpless. The obtained stability criteria are easily checked and do not require the differentiability of delays.
2. Preliminaries
Let Rn denote the n-dimensional Euclidean space and ∥·∥ represent the Euclidean norm 𝒩≜{1,2, …, n} and R+ = [0, ∞). C[X, Y] corresponds to the space of continuous mappings from the topological space X to the topological space Y.
Throughout this paper, we always assume that fi(0) = gi(0) = σi(0) = Iik(0) = 0 for i ∈ 𝒩 and k = 1,2, …. Thereby, problems (1) and (2) admit a trivial equilibrium x = 0.
Definition 1. The trivial equilibrium x = 0 is said to be globally exponentially stable if for any initial condition φ(s) ∈ C[[−m*, 0], Rn], there exists a pair of positive constants λ and M such that
The consideration of this paper is based on the following fixed point theorem.
Theorem 2 (see [32].)Let Υ be a contraction operator on a complete metric space Θ, then there exists a unique point ζ ∈ Θ for which Υ(ζ) = ζ.
3. Main Results
- (A1)
There exist nonnegative constants lj such that for any η, υ ∈ R,
(6) - (A2)
There exist nonnegative constants kj such that for any η, υ ∈ R,
(7) - (A3)
There exist nonnegative constants pjk such that for any η, υ ∈ R,
(8) - (A4)
There exist nonnegative constants ωj such that for any η, υ ∈ R,
(9)
- (1)
ϕi(t) is continuous on t ≠ tk (k = 1,2, …),
- (2)
and exist; moreover, for k = 1,2, …,
- (3)
ϕi(s) = φi(s) on s ∈ [−m*, 0],
- (4)
eαtϕi(t) → 0 as t → ∞, where α = const and 0 < α < min i∈𝒩{ai},
Theorem 3. Assume that conditions (A1)–(A4) hold provided that
- (i)
there exists a constant μ such that inf k=1,2,…{tk − tk−1} ≥ μ,
- (ii)
there exist constants pi such that pik ≤ piμ for i ∈ 𝒩 and k = 1,2, …,
- (iii)
max i∈𝒩{pi(μ + (1/ai))}≜χ < 1,
and then the trivial equilibrium x = 0 is globally exponentially stable.
Proof. Multiplying both sides of (1) with gives, for t > 0 and t ≠ tk,
Letting ε → 0 in (12), we have, for t ∈ (tk−1, tk) (k = 1,2, …),
Setting t = tk − ε (ε > 0) in (13), we get
Noting xi(tk − 0) = xi(tk), (15) can be rearranged as
Combining (13) and (16), we derive that
We therefore conclude, for t > 0,
Note that xi(0) = φi(0) in (20). We then define the following operator π acting on ℋ, for :
In what follows, we will apply the contraction mapping principle to prove the existence and uniqueness of solution and the global exponential stability of trivial equilibrium at the same time. The subsequent proof can be divided into two steps.
Step 1. We need to prove that π(ℋ) ⊂ ℋ. For yi(t) ∈ ℋi (i ∈ 𝒩), it is necessary to show that π(yi)(t) ⊂ ℋi. As defined above, we see that π(yi)(s) = φi(s) on s ∈ [−m*, 0]. Owing to the continuity of φi(s) on s ∈ [−m*, 0], we immediately know that π(yi)(t) is continuous on t ∈ [−m*, 0].
Choose a fixed time t > 0, and it is then derived from (22) that
Since yi(t) ∈ ℋi, we know that yi(t) is continuous on t ≠ tk (k = 1,2, …); moreover, and exist, in addition, .
Letting t ≠ tk (k = 1,2, …) in (23), it is easy to see that Qi → 0 as r → 0 for i = 1, …, 5. Thus, π(yi)(t + r) − π(yi)(t) → 0 as r → 0 holds on t > 0 and t ≠ tk (k = 1,2, …).
Letting t = tk (k = 1,2, …) in (23), it is not difficult to find that Qi → 0 as r → 0 for i = 1, …, 4. Letting r < 0 be small enough, we compute
According to the above discussion, we see that π(yi)(t) : [−m*, +∞) → R is continuous on ≠tk(k = 1,2, …), while for t = tk (k = 1,2, …), and exist; moreover, .
Next, we will prove that eαtπ(yi)(t) → 0 as t → ∞ for i ∈ 𝒩. To begin with, we give the expression of eαtπ(yi)(t) as follows:
-
,
-
,
-
,
-
, and
-
.
First, it is obvious that limt→∞W1 = 0 as ai − α > 0. Furthermore, for yj(t) ∈ ℋj (j ∈ 𝒩), we see limt→∞eαtyj(t) = 0. Then, for any ε > 0, there exists a Tj > 0 such that s ≥ Tj implies |eαs yj(s)| < ε. Choose T* = max j∈𝒩 {Tj}. It is derived form (A1) that
Similarly, for the given ε > 0 above, there also exists a such that implies |eαsyj(s)| < ε. Select . It follows from (A2) that
Since eαζ|yj(ζ)| → 0 as ζ → ∞, we know that, for any ε > 0, there exists a such that implies eαζ|yj(ζ)| < ε. Selecting , it follows from (30) that
Furthermore, from (A3), we see that |Iik(xi(tk))| ≤ pik|yi(tk)|. So,
As yi(t) ∈ ℋi, we have lim t→∞ eαtyi(t) = 0. Then, for any ε > 0, there exists a nonimpulsive point Ti > 0 such that s ≥ Ti implies |eαsyi(s)| < ε. It then follows from conditions (i) and (ii) that
Now, we can derive from (27) that eαtπ(yi)(t) → 0 as t → ∞ for i ∈ 𝒩. It is therefore concluded that π(yi)(t) ⊂ ℋi which results in π(ℋ) ⊂ ℋ.
Step 2. We need to prove that π is contractive. For and , we estimate
Note that
It is then derived from (36) that
In view of condition (iii), we know that π is a contraction mapping, and hence, there exists a unique fixed point of π in ℋ which means that is the solution to (1)–(3) and as t → ∞. This completes the proof.
Lemma 4. Assume conditions (A1)–(A4) hold. Provided that
- (i)
inf k=1,2,… {tk − tk−1} ≥ 1,
- (ii)
there exist constants pi such that pik ≤ pi for i ∈ 𝒩 and k = 1,2, …,
- (iii)
max i∈𝒩 {pi(1 + (1/ai))}≜χ < 1,
then the trivial equilibrium x = 0 is globally exponentially stable.
Remark 5. In Theorem 3, we use the fixed point theorem to prove the existence and uniqueness of solution and the global exponential stability of trivial equilibrium all at once, while Lyapunov method fails to do this.
4. Example
From Theorem 3, we conclude that the trivial equilibrium x = 0 of this two-dimensional impulsive cellular neural network with time-varying and distributed delays is globally exponentially stable.
5. Conclusions
This article is a new attempt of applying the fixed point theory to the stability analysis of impulsive neural networks with time-varying and distributed delays, which is different from the existing relevant publications where Lyapunov theory is the main technique. From what have been discussed above, we see that the contraction mapping principle is effective for not only the investigation of the existence and uniqueness of solution but also for the stability analysis of trivial equilibrium. In the future, we will continue to explore the application of other kinds of fixed point theorems to the stability research of complex neural networks.
Acknowledgment
This work is supported by the National Natural Science Foundation of China under Grant no. 61174077 and Natural Science Foundation of Jiangsu of China under Grant no. BK2012682.