A. Zhang et al. / Neural Networks 50 (2014) 98–109 99
The above equations can be written in a reduced form
dx
ki
(t)
dt
= −a
ki
(t)x
ki
(t) +
p(3−k)
j=1
b
(3−k)ij
(t)f
(3−k)j
×
x
(3−k)j
t − r
(3−k)ij
(t)
+
p(3−k)
j=1
p(3−k)
l=1
c
(3−k)ijl
(t)f
(3−k)j
×
x
(3−k)j
t − τ
(3−k)ij
(t)
× f
(3−k)l
x
(3−k)l
t − τ
(3−k)il
(t)
+ I
ki
(t), (2)
where i = 1, 2, . . . , p(k); k = 1, 2 (k = 1 for X -layer and k = 2
for Y -layer);
p(k) =
m
1
, k = 1,
m
2
, k = 2;
a
ki
(t) > 0; x
ki
(t) is the activation of the i-th neuron in the k
layer at the time t; f
ki
is the signal transmission function of the
i-th neuron in the k layer; b
kji
(t) and c
kjil
(t) are the first- and
second-order synoptic weights of the neural network, r
kji
(t) (≥0)
and τ
kji
(t) (≥0) stand for transmission delays; I
ki
(t) corresponds
to the external input of the i-th neuron in the k layer; and
a
ki
(t), b
kji
(t), c
kjil
(t), r
kji
(t), τ
kji
(t), and I
kij
(t) are T -periodic func-
tions in R.
Numerous attempts have been made to show the properties
of the system (2). Cao et al. examined the existence and global
exponential stability of the equilibrium with constant coefficients
(Cao, Liang, & Lam, 2004). Ho et al. studied the dynamic behaviors
with impulse terms (Ho, Liang, & Lam, 2006), and Li et al.
considered the problem of impulsive effects on the stability (Li, Lin,
Liao, & Huang, 2011). Note that a high-order Hopfield-type neural
network is a special case of (2). In other words, (2) becomes a high-
order Hopfield-type neural network when k = 1 or k = 2. Yi et al.
studied the convergence behavior of a high-order Hopfield-type
neural network in Yi, Shao, Yu, and Xiao (2008). They presented
some sufficient conditions that ensure all solutions of the network
converging to zero under a bounded condition on high-order signal
transmission functions.
Since discretization is needed in the implementation of a
continuous-time neural network, it is of both theoretical and
practical importance to study the dynamics of a discrete-time
neural network. Recently, discussions on the properties of low-
order discrete-time BAM neural networks have thrown new light
on the subject. For example, Liang et al. studied the following
discrete-time BAM neural network (Liang, Cao, & Ho, 2005)
x
i
(n + 1) = a
i
x
i
(n) +
m
j=1
c
ij
f
j
(y
j
(n − k(n))) + I
i
,
y
j
(n + 1) = b
j
y
j
(n) +
m
i=1
d
ij
g
i
(x
i
(n − l(n))) + J
j
,
(3)
where a
i
∈ (0, 1), b
j
∈ (0, 1), and k(n) and l(n) are positive in-
tegers. They used a linear matrix inequality and obtained some
sufficient conditions for the existence, uniqueness, and global ex-
ponential stability of the equilibrium point. Zhou et al. investigated
the existence and global exponential stability of periodic solution
of the following discrete-time BAM neural networks (Zhou & Liu,
2006)
x
ki
(n + 1) = α
ki
(n)x
ki
(n) +
p(3−k)
j=1
w
(3−k)ji
f
(3−k)j
×
+∞
l=1
g
(3−k)j
(l)x
(3−k)j
(n − l)
+ I
ki
(n). (4)
To our knowledge, there have no studies been reported on the
properties of high-order discrete-time BAM neural networks un-
til now. This motivated us to carry out a study in this paper. In this
study, we discuss the existence and exponential stability of peri-
odic solution for the following high-order discrete-time BAM neu-
ral networks
1x
ki
(n) = −A
ki
(n)x
ki
(n) +
p(3−k)
j=1
b
(3−k)ij
(n)f
(3−k)j
×
x
(3−k)j
(n − r
(3−k)ij
(n))
+
p(3−k)
j=1
p(3−k)
l=1
c
(3−k)ijl
(n)f
(3−k)j
×
x
(3−k)j
(n − τ
(3−k)ij
(n))
× f
(3−k)l
x
(3−k)l
(n − τ
(3−k)il
(n))
+ I
ki
(n), (5)
where 1x
ki
(n) = x
ki
(n + 1) − x
ki
(n), r
(3−k)ij
(n) and τ
(3−k)ij
(n) are
periodic non-negative integers, and 0 < A
ki
(n) < 1. (5) is obtained
by applying the discretizing method in Mohamad and Gopalsamy
(2003) to the continuous-time system (2). Initial conditions of (5)
are
x
ki
(n) = φ
ki
(n), n ∈ [−τ , 0]
Z
, i = 1, 2, . . . , p(k), k = 1, 2, (6)
where φ
ki
(·) is a continuous function,
τ = max
k,i,j
max
n∈I
N
{r
kji
(n)}, max
n∈I
N
{τ
kji
(n)}
,
I
N
=
{
0, 1, 2, . . . , N − 1
}
,
N is a positive integer,
[a, b]
Z
=
a, a + 1, . . . , b − 1, b
for a, b ∈ Z, and a ≤ b.
First, we introduce some notations for convenience.
A
ki
= min
n∈I
N
{A
ki
(n)}, b
kji
= max
n∈I
N
{|b
kji
(n)|},
c
kjil
= max
n∈I
N
{|c
kjil
(n)|},
I
ki
= max
n∈I
N
{|I
ki
(n)|}, M = max
k,i
sup
x∈R
{|f
ki
(x)|},
Z
+
0
=
{
0, 1, 2, . . .
}
,
u(n) =
1
N
N−1
n=0
u(n),
where u(n) is an N-periodic sequence. We also make the following
assumptions.
Assumption 1. A
ki
(n), b
(3−k)ij
(n), c
(3−k)ijl
(n), r
(3−k)ij
(n),
τ
(3−k)ij
(n), and I
ki
(n) are N-periodic sequences on Z
+
0
.
Assumption 2. There exist non-negative constants L
ki
and e
ki
such
that
f
2
ki
(x) ≤ L
ki
|x| + e
ki
, ∀x ∈ R , i = 1, 2, . . . , p(k), k = 1, 2.
Assumption 3. There exist a real number p (>2) and non-negative
constants L
ki
and e
ki
such that
|f
ki
(x)|
p
≤ L
ki
|x| + e
ki
, ∀x ∈ R, i = 1, 2, . . . , p(k), k = 1, 2.
Assumption 4. f
ki
(x) (i = 1, 2, . . . , p(k), k = 1, 2) are continu-
ous and bounded in R.
Assumption 5. There exist a non-negative constant L
ki
such that
|f
ki
(x) − f
ki
(y)| ≤ L
ki
|x − y|, ∀x, y ∈ R,
i = 1, 2, . . . , p(k), k = 1, 2.