
94 Risk Analysis in Finance and Insurance
Now our aim is to construct a martingale probability P
∗
for this market. We
are looking for a probability of the form of Essher transform:
P
∗
(A)=E
Z
N
I
A
,
where
Z
N
=
N
n=1
z
n
, with z
n
=
exp
a
n
(w
n
− δ
n
)
E
exp
a
n
(w
n
− δ
n
)
,
and (a
n
)
n≤N
is some deterministic sequence. To find (a
n
)
n≤N
,weusethe
martingale property of
S
n
/B
n
n≤N
:
E
∗
S
n
B
n
F
n−1
=
S
n−1
B
n−1
,n=1,...,N,
which is equivalent to
E
∗
exp{μ
n
+ σ
n
n
}
=1,
where μ
n
= μ
n
− δ
n
,n=1,...,N.
Taking into account the expression for Z
n
,weobtain
E
exp{a
n
(μ
n
+ σ
n
n
)+μ
n
+ σ
n
n
}
= E
exp{a
n
(μ
n
+ σ
n
n
)}
and
E
exp{(a
n
+1)(μ
n
+ σ
n
n
)}
= E
exp{a
n
(μ
n
+ σ
n
n
)}
.
Since
n
∼N(0, 1), then
E
exp{a
n
σ
n
n
}