
By using product inference engine, singleton fuzzifier, and center-average defuzzifer, the overall discrete-time 2-D T–S
fuzzy systems can be expressed as follows:
x
þ
ðs; lÞ¼
X
r
i¼1
h
i
ðzðs; lÞÞfA
i
xðs; lÞþB
i
uðs; lÞg;
x
h
ð0; lÞ¼f ðlÞ; x
v
ðs; 0Þ¼gðsÞ;
ð4Þ
where h
i
ðzðs; lÞÞ ¼
l
i
ðzðs;lÞÞ
P
r
i¼1
l
i
ðzðs;lÞÞ
,
l
j
ðzðs; lÞÞ ¼
P
L
m¼1
M
mj
ðzðs; lÞÞ. It is easy to see that h(z(s, l)) = [h
1
(z(s,l)) ...h
r
(z(s,l))]
T
belongs to
K
r
¼ k 2 R
r
;
X
r
i¼1
k
i
¼ 1; k
i
P 0
()
: ð5Þ
We first introduce the following definition before proceeding further.
Definition 1. [33]The discrete-time 2-D T–S fuzzy system (4) is asymptotically stable if lim
r?1
sup{kx(s,l)k : r = s + l} = 0 with
the initial and boundary conditions (2).
For simplicity, the following notations will be adopted in the sequel
h
i
¼ h
i
ðzðs; lÞÞ; X
z
¼
X
r
i¼1
h
i
X
i
; X
1
z
¼
X
r
i¼1
h
i
X
i
!
1
: ð6Þ
2.2. Two useful lemmas and homogeneous matrix polynomials
Before presenting the main results, two useful lemmas and some definitions for homogeneous matrix polynomials are
given.
Lemma 1. ([16]). For two symmetric matrices P > 0,P
+
> 0, the inequality A
T
P
+
A P < 0 holds, if there exists a matrix G such that
P ðÞ
GA G þ G
T
P
þ
> 0.
Lemma 2. ([13] (Polya’s Theorem)). Let F(h) ¼
:
F(h
1
,h
1
,...,h
r
) be a real homogenous polynomial which is positive for h 2
K
r
.
Then for a sufficiently large d 2 Z
þ
, the product (h
1
+h
2
+ +h
r
)
d
F(h) has all its coefficients strictly positive, where
K
r
is given
by (5).
The following definitions which are consistent with those in [22] are needed.
A homogeneous matrix polynomial P
g
(h) of degree g can be generally written in the form
P
g
ðhÞ¼
X
k2KðgÞ
h
k
1
1
h
k
2
2
h
k
r
r
P
k
; k ¼ k
1
k
2
k
r
; ð7Þ
where h
k
1
1
h
k
2
2
h
k
r
r
; h 2
K
r
; k
i
2 Z
þ
; i ¼ 1; 2; ...; r, are the monomials, and P
k
2 R;
8
k 2KðgÞ, are matrix-valued coefficients.
Here, by definition, KðgÞ is the set of r-tuples obtained as all possible combinations of nonnegative integers k
i
, i =1,2,...,r,
such that k
1
+ k
2
+ + k
r
= g. Since the number of vertices is equal to r, the number of elements in KðgÞ is given by
JðgÞ¼
ðr þ g 1Þ!
g!ðr 1Þ!
: ð8Þ
To give an example, for homogeneous polynomials of degree g = 4 with r = 2 variables, the possible values of the partial de-
grees are Kð4Þ¼f04; 13; 22; 31; 40g; J ð4 Þ¼5, corresponding to the generic form P
4
ðhÞ¼h
4
2
P
04
þ h
1
h
3
2
P
13
þ
h
2
1
h
2
2
P
22
þ h
3
1
h
2
P
31
þ h
4
1
P
40
. By definition, for r-tuples k and k
0
, we write k P k
0
if k
i
P k
0
i
; i ¼ 1; ...; r. The usual operations
of summation, k + k
0
, and subtraction, k k
0
(whenever k P k
0
), are defined componentwise. We also consider the following
definitions:
e
i
¼ 0 01
|{z}
ith
0 0; recðe
i
Þ¼i;
p
ðkÞ , ðk
1
!Þðk
2
!Þðk
r
!Þ; k 2KðdÞ;
h
k
¼ h
k
1
1
h
k
2
2
h
k
r
r
; h
þ
k
¼ h
þ
1
k
1
h
þ
2
k
2
h
þ
r
k
r
;
X
g
ðhÞ¼
X
k2KðgÞ
h
k
X
k
; X
g
ðh
þ
Þ¼
X
k2KðgÞ
h
þ
k
X
k
:
ð9Þ
D.-W. Ding et al. / Information Sciences 189 (2012) 143–154
145