- Open Access
Some improved criteria on exponential stability of neutral differential equation
© Chen; licensee Springer 2012
Received: 14 February 2012
Accepted: 2 August 2012
Published: 25 September 2012
In this paper, some improved delay-dependent exponential stability criteria of the neutral delayed differential equation
are expressed in terms of linear matrix inequalities (LMIs). Compared with some existing ones, our results are derived without the use of the mode transformation method and the bounding technique. Three numerical examples are provided to show the effectiveness of the proposed method and less conservatism of the obtained results.
Recently, for equation (2), many authors have investigated its properties due to its applications in the research of the dynamic characteristics of neural networks of Hopfield type, see [1–17] and references therein. Although some qualitative stability analysis for equation (2) have been presented in [1, 3, 5–14, 17], their results are mainly concerned with the asymptotic stability, without providing any conditions for exponential stability and any information about the decay rates of equation (2). Besides, it should be pointed out that the advantage of equation (2) with exponential stability in comparison with that with asymptotic stability lies in that the former can provide fast convergence and desirable accuracy. In , Li has obtained the global exponential stability for equation (2), but the sufficient condition is delay-independent. It is well known that current efforts on the problem of stability for delay systems can be divided into two categories, namely delay-independent stability criteria and delay-dependent stability criteria. And generally speaking, the former are more conservative than the latter when the delay is small. Thus, the sufficient condition about the global exponential stability for equation (2) given in  is more conservative. More recently, in , Rojsiraphisal and Niamsup have proposed some delay-dependent LMI-based sufficient conditions ensuring the exponential stability for equation (2), but these conditions are also more conservative since the mode transformation and the bounding technique are both used. On the other hand, to the best of our knowledge, compared with constant-delay systems, the time-varying delay systems are more closed to reflect the reality. So, it is necessary to discuss the global exponential stability for equation (1). In , Chen and Meng have established the LMI-based exponential stability criterion for equation (1) by only constructing a modified Lyapunov functional, but the result in  is sightly more conservative. Thus, there still exists room for further improvement.
In this paper, we reconsider the exponential stability of equation (1). New delay-dependent sufficient criteria ensuring the global exponential stability for equation (1) are given in terms of linear matrix inequalities (LMIs). The criteria here are also discussed from the point of view of its comparison with the earlier results. To show the applicability and effectiveness of the proposed method developed in this paper, three illustrative examples are provided.
Notation Throughout this paper, the notation (respectively, ) means that the matrix is positive semi-definite (respectively, positive definite), where X and Y are symmetric matrices of the same dimensions. The symbol ∗ denotes the elements below the main diagonal of a symmetric matrix.
2 Main results
In this section, we give our main results.
where () are positive scalars to be chosen later.
where and .
where and . The proof of this theorem is completed. □
When , we can easily derive the following corollary.
and , , , are given in Theorem 1.
must be imposed. Besides, note that Theorem 4 in  involves seven decision variables, while Corollary 2 involves five decision variables. Thus, Corollary 2 needs fewer decision variables than Theorem 4 in . What is more, this restrictive condition (11) is removed in this paper. Thus, our LMIs-based sufficient conditions are less conservative than those provided in , which is shown by Example 1 and Example 2 in Section 3. And the technique employed in this paper is different from the previous ones introduced in [1, 3, 7–11, 16, 17].
Remark 3 If , the criteria about the global asymptotical stability for equation (1) are presented as follows:
() are given in Theorem 1.
3 Three illustrative examples
In this section, three illustrative examples are given to show the effectiveness of our result.
The upper bound of b for Example 2 when
Obviously, the result provided in  is infeasible. And even if the delays and are the constant delays, the zero solution of equation (16) is not globally exponentially stable according to the criterion given in  since the inequality (12) is not satisfied. Thus, our result can also complement that in [8, 16].
The author would like to thank the anonymous referees for their very helpful comments and suggestions which greatly improved this manuscript, and the editors for their careful reading of this paper. This work is supported by the National Natural Science Foundation of China under Grant No. 11126278 and the Natural Science Foundation of Jiangxi Province in China under Grant No. 20114BAB211001.
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