856 IEEE TRANSACTIONS ON FUZZY SYSTEMS, VOL. 25, NO. 4, AUGUST 2017
and Karnik [41] defined the union (∪), intersection (∩) and
complements (¯) operations of T2 FSs as follows:
ˆ
A ∪
ˆ
B =
x∈X
μ
ˆ
A∪
ˆ
B
(x)/x, where μ
ˆ
A∪
ˆ
B
(x)
= μ
ˆ
A
(x) μ
ˆ
B
(x)
=
u
w
(μ
ˆ
A
(x, u) ∧ μ
ˆ
B
(x, w))/(u ∨ w),
ˆ
A ∩
ˆ
B =
x∈X
μ
ˆ
A∩
ˆ
B
(x)/x, where μ
ˆ
A∩
ˆ
B
(x)
= μ
ˆ
A
(x) μ
ˆ
B
(x)
=
u
w
(μ
ˆ
A
(x, u) ∧ μ
ˆ
B
(x, w))/(u ∧ w),
¯
ˆ
A =
x∈X
μ
¯
ˆ
A
(x)/x, where μ
¯
ˆ
A
(x)
=
u
μ
ˆ
A
(x, u)/(1 − u).
Here, ∨ and ∧ denote max operation and min operation,
respectively. and defined in fuzzy degrees are called as join
and meet, respectively.
As the generalized T1 fuzzy relation, a T2 fuzzy relation is
defined as follows.
Definition 2: Let X and Y be two universes of discourse.
Then,
ˆ
R = {[(x, y),μ
ˆ
R
(x, y)] | (x, y) ∈ X × Y } (4)
is a binary T2 fuzzy relation in the product space X × Y . Here,
μ
ˆ
R
(x, y) ∈ NCFD denotes the fuzzy degree of (x, y) belonging
to
ˆ
R.LetNCFD
X × Y
denote the set of all T2 fuzzy relations in
the product space X × Y . Then,
ˆ
R ∈ NCFD
X ×Y
with |X| =
m and |Y | = n can also be expressed as an m ∗ n matrix in
which the elements belong to NCFD.
Suppose that
ˆ
R ∈ NCFD
X ×Y
and
ˆ
S ∈ NCFD
Y ×Z
with
|X| = m, |Y | = n and |Z| = k. Then, the composition of
ˆ
R and
ˆ
S is denoted by the m ∗ k matrix
ˆ
R
ˆ
ˆ
S, in which the elements
are obtained by the following meet-join operation:
ˆ
R
ˆ
ˆ
S(x, z)=
y ∈Y
[
ˆ
R(x, y)
ˆ
S(y, z)],x∈ X, z ∈ Z. (5)
By combining classical DESs theory and T2 FSs theory, we
introduce the BFDESs model as follows.
Definition 3: A bi-fuzzy DES is modeled as a bi-fuzzy au-
tomaton, which is a four-tuple:
ˆ
G = {
ˆ
X,
ˆ
Σ,
ˆ
δ, ˆx
0
}.
Here,
ˆ
X is a set of bi-fuzzy states over a crisp state set X with
|X| = n. A bi-fuzzy state ˆx ∈
ˆ
X is denoted by a row vector
{˜x
1
, ˜x
2
,...,˜x
n
}, where ˜x
i
∈ NCFD represents the fuzzy
degree of the system being at the crisp state x
i
.
ˆ
Σ is a set of
bi-fuzzy events. Any ˆσ ∈
ˆ
Σ is denoted by a matrix ˆσ =[˜a
ij
]
n∗n
with ˜a
ij
∈ NCFD. ˜a
ij
denotes the fuzzy transition degree
from state x
i
to x
j
when event ˆσ occurs. δ :
ˆ
X ×
ˆ
Σ →
ˆ
X is
a transition function, which is defined by
ˆ
δ(ˆx, ˆσ)=ˆx
ˆ
ˆσ for
Fig. 2. Computing tree of the BFDES
ˆ
G.
ˆx ∈
ˆ
X and ˆσ ∈
ˆ
Σ.“
ˆ
” denotes the meet-join operation defined
in (5). ˆx
0
=[˜x
0,1
, ˜x
0,2
,...,˜x
0,n
] is a bi-fuzzy initial state,
where ˜x
0,i
∈ NCFD is the fuzzy degree of the crisp state x
i
belonging to initial states.
BFDESs model can directly handle some fuzzy and uncertain
data, whereas FDESs model have to defuzzify these data in
the modeling phase. We have presented an example in [23] to
demonstrate this case. The example also revealed the fact that
sometimes it is much better to process fuzzy data directly than to
defuzzify them too early. Hence, BFDESs have well advantages
over FDESs in some cases.
In general, a BFDES has an infinite states. However, only its
accessible part is our concern. In [23], we provided a method
to obtain the accessible states of a BFDES by constructing a
computing tree. The computing tree is constructed as follows.
The root is labeled by ˆx
0
. Each vertex, labeled by ˆx
0
ˆ
ˆs,may
produce n’s sons vertices labeled by ˆx
0
ˆ
ˆs
ˆ
ˆσ
1
, ˆx
0
ˆ
ˆs
ˆ
ˆσ
2
, ...,
ˆx
0
ˆ
ˆs
ˆ
ˆσ
n
, respectively. If a vertex whose label is equal to that
of anther nonleaf vertex, then it is a leaf and marked by an under-
line. The computing ends with a leaf at the end of each branch.
Then, the labels of the tree vertices contain all the accessible
states. In [23], we have demonstrated that the accessible states
of the BFDESs is finite, if we specify a finite J
x
to NCFD.
Example 1: Let the BFDES
ˆ
G = {
ˆ
X,
ˆ
Σ,
ˆ
δ, ˆx
0
} be a predict-
ing model of a certain clinical testing. For ˆx =(ˆx
1
, ˆx
2
) ∈
ˆ
X,
ˆx
1
(ˆx
2
) denotes the measure of the first (second) physiological
parameter. Here, ˆx
0
=[
1
1
1
0
] is the initial s tate.
ˆ
Σ={ˆσ
1
, ˆσ
2
} is
the events set. Each of the events denotes a certain therapy, and
ˆσ
1
1
=
1
0.6
+
0.6
0.9
1
0.9
+
0.8
1
1
0.3
+
0.7
0.6
1
0.3
+
0.7
0.6
ˆσ
1
2
=
1
0.3
+
0.7
0.6
1
0.8
+
0.7
0.9
1
0.9
+
0.8
1
1
0.6
+
0.6
0.9
are the corresponding matrices of ˆσ
1
and ˆσ
2
in
ˆ
G, respectively.
Each of these matrices summarizes the effects for the physio-
logical parameters when adopting the corresponding therapy.
We obtain the computing tree (as shown in Fig. 2), and get the
accessible states of
ˆ
G (as shown in Table I). Then, the accessible
part of BFDES
ˆ
G can be easily obtained (as shown in Fig. 3),
which can be viewed as an effective predicting model of the
clinical testing.