776 R. Gao / Applied Mathematics and Computation 274 (2016) 774–785
Definition 3 (Liu [5]). Suppose ξ is an uncertain variable. Then the uncertainty distribution of ξ is defined by
(x) = M
{
ξ
≤ x
}
for any real number x.
An uncertainty distribution
(x) is said to be regular if its inverse function
−1
(α) exists and is unique for each α ∈ (0, 1).
Inverse uncertainty distribution plays an important role in the operations of independent uncertain variables. In the following,
the concept of inverse uncertainty distribution will be presented.
Definition 4 (Liu [9]). Suppose
ξ is an uncertain variable with regular uncertainty distribution (x). Then the inverse function
−1
(α) is called the inverse uncertainty distribution of ξ .
Theorem 2 (Liu [12]). A function
−1
(α) : (0, 1) →is an inverse uncertainty distribution if and only if it is a continuous and
strictly increasing function with respect to
α.
The operational law of independent uncertain variables was given by Liu [9] in order to calculate the uncertainty distribution
of a strictly increasing or decreasing function of uncertain variables. Before introducing the operational law, the concept of
independence of uncertain variables is presented as follows:
Definition 5 (Liu [7]). The uncertain variables
ξ
1
, ξ
2
,...,ξ
n
are said to be independent if
M
n
i=1
(ξ
i
∈ B
i
)
=
n
i=1
M
{
ξ
i
∈ B
i
}
for any Borel sets B
1
, B
2
,...,B
n
.
Theorem 3 (Liu [9]). Let
ξ
1
, ξ
2
,...,ξ
n
be independent uncertain variables with continuous uncertainty distributions
1
,
2
,...,
n
,
respectively. If the function f
(x
1
, x
2
,...,x
n
) is strictly increasing with respect to x
1
, x
2
,..., x
m
and strictly decreasing with respect to
x
m+1
, x
m+2
,...,x
n
, then the uncertain variable
ξ = f (ξ
1
, ξ
2
,...,ξ
n
)
has an uncertainty distribution
(x) = sup
f (x
1
,x
2
,...,x
n
)=x
min
1≤i≤m
i
(x) ∧ min
m+1≤i≤n
(1 −
i
(x))
.
Moreover, if
1
,
2
,...,
n
are regular, then ξ has an inverse uncertainty distribution
−1
(α) = f (
−1
1
(α),...,
−1
m
(α),
−1
m+1
(1 − α),...,
−1
n
(1 − α)).
For ranking uncertain variables, the concept of expected value was proposed by Liu [5] as follows:
Definition 6 (Liu [5]). Let
ξ be an uncertain variable. Then the expected value of ξ is defined by
E[ξ ] =
+∞
0
M{ξ ≥ x}dx −
0
−∞
M{ξ ≤ x}dx
provided that at least one of the two integrals is finite.
Theorem 4 (Liu [5]). Let
ξ be an uncertain variable with uncertainty distribution . If the expected value exists, then
E[ξ ] =
+∞
−∞
xd(x).
If the uncertainty distribution is regular, then we also have
E[ξ ] =
1
0
−1
(α)dα.
An uncertain process is essentially a sequence of uncertain variables indexed by time. The study of uncertain process was
started by Liu [6] in 2008.
Definition 7 (Liu [6]). Let T be an index set and let
(, L, M) be an uncertainty space. An uncertain process is a measurable
function from T ×
(, L, M) to the set of real numbers such that {X
t
∈ B}isaneventforanyBorelsetBforeacht.
Definition 8 (Liu [7]). An uncertain process C
t
is said to be a canonical Liu process if (i) C
0
= 0 and almost all sample paths are
Lipschitz continuous, (ii) C
t
has stationary and independent increments, (iii) every increment C
s+t
−C
s
is a normal uncertain
variable with expected value 0 and variance t
2
, whose uncertainty distribution is
(x) =
1 + exp
−πx
√
3t
−1
, x ∈.