1528 IEEE TRANSACTIONS ON NEURAL NETWORKS AND LEARNING SYSTEMS, VOL. 24, NO. 10, OCTOBER 2013
Note that, if a ∨ c > b ∧ d,then[a ∨ c, b ∧ d]=∅;in
words, if a ∨ c > b ∧ d, then we assume that the intersection
[a, b]∩[c, d] is the empty set (∅). We remark from [22] that a
preferable (in computing) representation for the least element
O
I1
=∅in lattice (I
1
, ⊆) is O
I1
=[I, O].
Consider a (strictly increasing) positive valuation function
v : L →[0, ∞); furthermore, consider a (strictly decreasing)
dual isomorphic function θ : L → L. Then, function v
1
:
L × L →[0, ∞) given by v
1
([a, b]) = v(θ(a)) + v(b)
is a positive valuation on lattice (L × L, ≥×≤) [25].
Furthermore, based on (1) and (2), two inclusion measures
σ
∩
: I
1
×I
1
→[0, 1] and σ
.
∪
: I
1
×I
1
→[0, 1] can be intro-
duced by σ
∩
(x, y) = σ
(x, x ∩ y) and σ
.
∪
(x, y) = σ
(x, y),
respectively, on the complete lattice (I
1
, ⊆), as will be shown
elsewhere.
Functions θ(.) and v(.) can be selected in different ways.
In the context of this paper, we select a pair of functions v(x)
and θ(x) so as to satisfy the equality “v
1
([x, x]) = v(θ(x)) +
v(x) = Constant” required by a “standard” FLR scheme [25],
[28], [29]. For instance, such pairs of functions v(x) and θ(x)
include, first, v(x) = px and θ(x) = Q − x,wherep, Q > 0,
x ∈[0, Q] and, second, v
s
(x) = A/(1 +e
−λ(x−μ)
) and
θ(x) = 2μ − x,whereA,λ ∈ R
+
0
, μ, x ∈ R. In particular, it
follows, first, v
1
([x, x]) = pQ and, second, v
1
([x, x]) = A.
C. Type-2 Intervals
A Type-2 interval is defined as an interval of Type-1
intervals. Consider the complete lattice (I
2
, ⊆) of Type-2
intervals on a complete lattice (L, ≤) of real numbers with
least and greatest elements O and I, respectively. Recall
that
[[a
1
, a
2
], [b
1
, b
2
]] ∩ [[c
1
, c
2
], [d
1
, d
2
]] =
[[a
1
, a
2
]
.
∪[c
1
, c
2
], [b
1
, b
2
]∩[d
1
, d
2
]],and
[[a
1
, a
2
], [b
1
, b
2
]]
.
∪[[c
1
, c
2
], [d
1
, d
2
]] =
[[a
1
, a
2
]∩[c
1
, c
2
], [b
1
, b
2
]
.
∪[d
1
, d
2
]].
We remark that a preferable representation for the least
element O
I2
=∅in lattice (I
2
, ⊆) is O
I2
=[[O, I],
[I, O]].
Consider a (strictly increasing) positive valuation function
v : L →[0, ∞) as well as a (strictly decreasing) dual
isomorphic function θ : L → L. Recall that function v
1
: L ×
L →[0, ∞) given by v
1
(a, b) = v(θ(a)) + v(b) is a positive
valuation. Furthermore, function θ
1
: L × L → L × L given
by θ
1
(a, b) = (b, a) is dual isomorphic. Therefore, function
v
2
: L×L×L×L →[0, ∞) given by v
2
([[a
1
, a
2
], [b
1
, b
2
]]) =
v(a
1
) +v(θ(a
2
)) +v(θ(b
1
)) +v(b
2
) is a positive valuation on
lattice (L × L × L × L, ≤×≥×≥×≤). In conclusion,
based on (1) and (2), inclusion measures σ
∩
: I
2
×I
2
→[0, 1]
and σ
.
∪
: I
2
× I
2
→[0, 1] can be introduced by σ
∩
(x, y) =
σ
(x, x ∩ y) and σ
.
∪
(x, y) = σ
(x, y), respectively, on the
complete lattice (I
2
, ⊆) of Type-2 intervals.
D. Type-1 Intervals’ Numbers
Consider the following definition.
Definition 2.1: A Type-1 IN is a function F :[0, 1]→I
1
which satisfies
F(0) = I
I1
h
1
≤ h
2
⇒ F(h
1
) ⊇ F(h
2
)
∀P ⊆ [0, 1] :∩
h∈P
F
(
h
)
= F
P
.
We will denote the set of INs by F
1
andequipitwithan
order relationship such that F G ⇔ (∀h ∈[0, 1]:
F(h) ⊆ G(h)). Furthermore, we will denote an IN by a capital
letter in italics, e.g., F
1
F = F(h) =[a
h
, b
h
], h ∈[0, 1].In
practice, an IN is interpreted as an information granule. It turns
out that (F
1
, ) is a complete lattice whose least element ∅ is
preferably represented as O
F1
= O(h) =[I, O], h ∈[0, 1].
Definition 2.1 implies that an IN can be represented by a set
of intervals, i.e., its interval representation. In addition, an IN
can, equivalently, be represented by a membership function;
i.e., the membership-function representation [25].
E. Type-2 Intervals’ Numbers
Another information granule of interest is an interval [U, W]
of Type-1 INs U and W , where interval [U, W] by definition
equals [U, W]
.
={X ∈ F
1
: U X W}. In the latter sense,
we say that X is encoded in [U, W].Interval[U, W] is called
Type-2 IN. It follows the complete lattice (F
2
, ) of Type-
2 INs. Recall that the least (empty) interval ∅ is preferably
represented in computing as O
F2
= O(h) =[[O, I], [I, O]],
where h ∈[0, 1]. A Type-2 IN will be denoted by a double-
line capital letter, e.g., F ∈ F
2
.
The lattice (F
2
, ) join operation is demonstrated in Fig. 1.
In particular, Fig. 1(a) shows the trivial Type-2 INs C
1
=
[C
1
, C
1
], C
2
=[C
2
, C
2
],andC
3
=[C
3
, C
3
].Fig.1(b)
displays the join C
1
C
2
=[C
1
C
2
, C
1
C
2
] in its
membership-function representation. Note that, since Type-1
INs C
1
and C
2
overlap, the Type-1 IN C
1
C
2
is not empty.
More specifically, it is (C
1
C
2
)(h) =∅,forh ∈[0, 0.6471];
nevertheless, for h ∈ (0.6471, 1],itis(C
1
C
2
)(h) =∅.
Fig. 1(c) displays the join C
1
C
2
in the (equivalent) interval
representation. Fig. 1(d) displays the join C
2
C
3
=[C
2
C
3
, C
2
C
3
] in its membership-function representation. Note
that, since Type-1 INs C
2
and C
3
do not overlap, the Type-1
IN C
2
C
3
is empty, i.e., (C
2
C
3
)(h) =∅,forallh ∈[0, 1].
We point out that there are similarities as well as differences
between Type-1 and 2 INs and Type-1 and 2 fuzzy sets [30].
Our interest here is on the inclusion measure σ
.
: F
2
×
F
2
→[0, 1], given as [22]
σ
.
(E
1
, E
2
) =
1
0
σ
.
∪
(E
1
(h), E
2
(h))dh. (3)
F. Extensions to More Dimensions
An N-tuple IN of Type-1/2 will be indicated by an “over
right arrow.” More specifically, a Type-1 IN will be denoted
by
−→
E = (E
1
,...,E
N
) ∈ (F
N
1
, ), whereas a Type-2 IN will
be denoted by
−→
E = (E
1
,...,E
N
) ∈ (F
N
2
, ).
The previous section has shown how to define inclusion
measure functions on lattice (F
2
, ). The latter functions