C.-K. Zhang et al. / Applied Mathematics and Computation 29 4 (2017) 102–120 103
By constructing a special form of LKF candidate, tractable LMI-based stability criteria are derived using necessary tech-
niques to estimate the LKF and its derivative. Therefore, the construction and the treatment for the LKF are the basic issues
related to how conservative the criteria are.
In the early research, the LKFs for the stability analysis of the DNNs were constructed by introducing delay-based single
and double integral terms into the typical non-integral quadratic form of Lyapunov function for delay-free systems [5,6,8–
14]
. Later, researchers have developed many new LKFs by making the previous ones more general in three aspects.
(1) Firstly, based on several subintervals divided from the whole delay region, some scholars have developed the delay-
partition-based LKFs by replacing the original integral terms with multiple new integral terms with smaller domain
of integration [16–30] .
(2) Secondly, by using various state vectors (current, delayed, and/or integrated state vectors etc. ), some scholars have
developed new LKFs by augmenting the quadratic terms of original LKFs [31–42] .
(3) Thirdly, since the triple integral term was found to be helpful for reducing the conservatism of stability criteria for
linear time-delay systems [47] , similar and/or extended forms have also been widely applied to stability analysis of
various DNNs [49–58] .
Although those LKFs have different forms, they all include a common term with the form of
0
−h
t
t+ θ
y
T
(s ) Ry (s ) d sd θ
(Here h > 0 is the scalar, y ( s ) is the system state-based vector, and matrix R > 0.). Then its derivative contains the term as
follows
−
t
t−h
y
T
(s ) Ry (s ) ds (1)
This term was directly dropped in the early literature [5] , but such treatment is very conservative. Later, this term was
retained to improve the results, in which case it must be estimated to represent the criterion in the form of tractable LMI.
As mentioned in [43] , the estimation of the above single integral term is strongly related to the conservatism of criteria.
Therefore, the stability criteria of DNNs have been improved gradually by using more effective techniques for this estimation
task.
The basic inequality was used to estimate the single integral term [6] . Since He et al. [44] proposed the free-weighting
matrix (FWM) approach, which is more effective than the basic inequality, the FWM approach has been widely used in the
stability analysis of DNNs [8–13,28–32] . The slack matrices introduced by the FWM approach provide great freedom of the
criteria.
An alternative method that estimates the original integral terms directly was also used in the stability analysis of DNNs.
The criteria from this type of method are strongly linked to the inequalities used. At the beginning, the Jensen inequality
has been wildly applied to analyze the stability of the DNNs [14–26,33,34,51,52] . In 2013, Seuret et al. [43] presented a
Wirtinger-based inequality and proved that it is less conservative than the Jensen inequality. Since then, Wirtinger-based
inequality has become the most popular method to estimate the single integral term during the investigation of DNNs
[27,37–41,53–56] .
Very recently, Zeng et al. proposed a free-matrix-based inequality (FMBI) in [45] and extended it to the research of DNNs
[42,50] . To the best of the authors’ knowledge, this inequality is the least conservative among the existing inequalities for
estimating single integral term. However, there is further room to be investigated using the FMBI. Some slack matrices in
the FMBI do not seem to contribute to a reduction of the conservatism. In [42] , the FMBI was only used to estimate the
single integral term without any augmented vector, while the augmented integral term was still estimated via the Jensen
inequality.
It can be expected that stability criteria with less conservatism will be obtained by developing and using a more effective
approach to estimate the single integral term. This motivates the present research.
This paper further investigates delay-dependent stability of DNNs following the development of a more effective method
to estimate the single integral term (1) . The contributions of the paper are summarized as follows:
1. A general free-weighting-matrix (GFWM) approach is developed to estimate single integral term. Based on several zero-
value equalities, a new estimation method, named as GFWM approach, is developed by following the basic estimation
procedure of the FWM approach. And a new inequality is derived based on the GFWM approach ( Lemma 5 ).
2. Necessary theoretical studies are carried out to compare the GFWM approach and several previous estimation methods.
It is proved that the inequality obtained from the GFWM approach can encompass the Wirtinger-based inequality and
the FMBI.
3. Several new stability criteria with less conservatism for the DNNs are derived. For generalized neural networks with a
time-varying delay, based on two LKFs (one with delay-product-type terms and the other without similar terms), two
stability criteria are derived by using the GFWM to estimate the single integral term appearing in the derivative of the
LKFs.
The remainder of the paper is organized as follows. Section 2 gives the problem formulation and necessary preliminary.
In Section 3 , the development of the GFWM approach and its comparison to previous methods are discussed in detail. The
GFWM approach is applied to a generalized DNN and several stability criteria are derived in Section 4 . In Section 5 , three
numerical examples are used to demonstrate the benefits of the proposed criteria. Conclusions are given in Section 6 .