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XU et al.: ROBUST ESTIMATION FOR NEURAL NETWORKS 3
is impacted by the ith component of x
j
(k) if the condition
r
i
= 0 holds.
Each nonlinear function g
j
(·), j ∈ ψ
N
in (1) is continuous,
bounded, g
j
(0) = 0, and there exist constants
¯
φ
j
and φ
j
,
satisfies the following inequality [40]:
φ
j
≤
g
j
(a) − g
j
(b)
a − b
≤
¯
φ
j
(4)
where a, b ∈ R,anda = b.
Each node randomly occurs the distributed delays indepen-
dently, which is denoted by β
i
(k), i ∈ ψ
N
with the following
stochastic properties:
E{β
i
(k) = 1}=β
i
E{(β
i
(k) − β
i
)
2
}=β
i
(1 − β
i
) β
∗
i
. (5)
The following convergent conditions of the constants μ
d
≥ 0
(d = 1, 2,...) hold:
+∞
d=1
μ
d
< +∞,
+∞
d=1
dμ
d
< +∞. (6)
Remark 1: Parameter uncertainty has been studied by many
researchers in the past several decades, since the accurate
model is always hard to be obtained in general situations,
and systems are frequently disturbed by the environment
noises [34]. It is worth noting that the polytopic uncertainty
describes the parametric uncertainty more precisely than the
norm-bounded one [36], thus studying coupled neural net-
works with polytopic uncertainty is important.
The parameter uncertain model has been used to describe
the networked induced issues. For instances, the norm-
bounded uncertainty has been used to handle the sensor
failure [41]; the polytopic uncertain model has been introduced
to deal with the quantization error [42]; and the multiplicative
noise model has been considered to model the channel fad-
ing [43]. Therefore, the condition of parameter uncertainty can
be extended to the coupled neural networks with communica-
tion constraints.
Remark 2: The kind of distributed delays has become an
interesting topic in the past decade, since it is unavoidable
in many situations [40]. Recently, the randomly occurring
distributed delays have been considered for the T-S fuzzy
systems [44]. In order to further address the randomly occur-
ring distributed delays in coupled neural networks, a set of
distributed delays with different occurring probabilities are
presented.
B. Local Coupling Structure-Dependent State Estimators
The following local coupling structure-dependent estimators
are proposed to estimate the states of coupled neural networks:
ˆx
i
(k + 1) = A
F
i
,ϑ
i
(α(k))
ˆx
i
(k) + B
F
i
,ϑ
i
(α(k))
y
i
(k)
ˆz
i
(k) = C
F
i
,ϑ
i
(α(k))
ˆx
i
(k) + D
F
i
,ϑ
i
(α(k))
y
i
(k), i ∈ ψ
N
(7)
where ˆx
i
(k) represents the state of the estimators. The
matrices A
F
i
,ϑ
i
(α(k))
, B
F
i
,ϑ
i
(α(k))
, C
F
i
,ϑ
i
(α(k))
,andD
F
i
,ϑ
i
(α(k))
are parameters of estimators, which need to be designed.
1 ≤ ϑ
i
(α(k)) ≤ s denotes the local variation of the coupling
Fig. 1. Markovian jump coupling.
condition of the ith neural networks. For example, we consider
a random coupling, which jumps between two states, as shown
in Fig. 1 (i.e., graph 1 and graph 2), where nodes 2 and
4 have the same neighbors for both cases, which means
ϑ
i
(α(k)) = 1fori = 2, 4. However, for nodes 1 and 3, the
neighbors are varying with α(k),thatis,ϑ
i
(α(k)) ∈{1, 2}
for i = 1, 3.
Remark 3: For the coupled systems (i.e., distributed sys-
tems, multiagent systems, and complex networks), the con-
nections among the nodes are always unreliable caused by the
communication constraints, such as packet dropouts, transmis-
sion delays, and so on. This phenomenon motivates people
to study the time-varying connection for coupled systems,
and the Markovian jump coupling is a classic one. In [45],
the coupling-independent controller has been proposed. How-
ever, for Markovian jump systems, how to make full use of
the Markov chain information to design a less conservative
estimator is an important issue. Note that each node cannot
obtain entire coupling information. Thus, a new structure
estimator called local coupling structure-dependent estimator
is presented, where the parameters of the estimator depend on
the local coupling structure.
C. Estimation Error System
Coupled neural networks with randomly occurring distrib-
uted delays (1) can be rewritten as the following augmented
form:
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎩
x(k + 1) = (
¯
A(θ) + W
α(k)
⊗ )x(k) +
¯
B(θ)G(x(k))
+β
¯
F(θ )
+∞
d=1
μ
d
G(x(k − d)) +
¯
E(θ )v(k)
+
N
i=1
(β
i
(k) − β
i
)
¯
I
i
¯
F(θ )
+∞
d=1
μ
d
G(x(k − d))
y(k) =
¯
C(θ)x(k) +
¯
D(θ)v(k)
z(k) =
¯
L(θ )x(k)
(8)
where
x(k) =[x
1
(k)
T
x
2
(k)
T
... x
N
(k)
T
]
T
y(k) =[y
1
(k)
T
y
2
(k)
T
... y
N
(k)
T
]
T
z(k) =[z
1
(k)
T
z
2
(k)
T
... z
N
(k)
T
]
T
G(x(k)) =[g(x
1
(k))
T
g(x
2
(k))
T
... g(x
N
(k))
T
]
T