MATH4091/7091: Financial calculus Assignment 2 Semester I 2022
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MATH4091/7091: Financial calculus
Assignment 2
Semester I 2022
1. (15 marks) Answer the questions below.
a. (3 marks) Consider a stochastic process {Xt}t∈[0,T], where Xt = cWt , c > 0. Find all values of c for which {Xt}t∈[0,T] satisfies the conditional expectation property of a martingale.
b. (3 marks) Consider a stochastic process {Xt}t∈[0,T], where Xt = e(Wt)4 . Is this process adapted and square-integrable? Mathematically justify your answer.
c. (1 mark) Consider a stochastic process {Xt}t∈[0,T], where Xt = eα(Wt)2 + (T−t), α > 0.
For fixed t ∈ [0,T], is Xt log-normally distributed? Mathematically justify your answer.
d. (3 marks) Let {Xt}t∈[0,T] be a martingale. Assume that the process {tXt}t∈[0,T] is also a martingale.
Prove or disprove the assertion: Xt = 0 almost surely for all t ∈ [0,T].
e. (2 marks) Let A, B, and C be three non-empty disjoint subsets of Ω . Write out the smallest σ-algebra containing A, B, and C .
f. (3 marks) Let T > 0 be fixed. Assume that a random variable ST satisfies
lnST = lnS0 + (r − σ 2 /2)T + σWT ,
where S0 > 0, r > 0 and σ > 0 are known constants.
Find E e −rT (lnST)2 in terms of T, S0 , r and σ .
2. (12 marks) The process {Xt}t∈[0,T] is called non-random and simple if there exists a partition Π = {tn}n(N)=0 of [0,T], where
0 = t0 ≤ t2 ≤ ... ≤ tN = T,
such that, for every n, 0 ≤ n ≤ N , Xtn is non-random and
Xt =
Xt0
Xt1
...
XtN − 1
t ∈ [0,t1 ),
t ∈ [t1 ,t2 ),
...
t ∈ [tN−1,tN ].
For t ∈ [tk,tk+1], define the random sum
k−1
It = X Xtn Wtn+1 − Wtn + Xtk (Wt − Wtk) .
a. (2 marks) Show that whenever 0 ≤ s < t ≤ T, the increment It − Is is independent of
Fs .
b. (4 marks) Show that whenever 0 ≤ s < t ≤ T, the increment It − Is is a normally
c. (6 marks) For each of the following process, mathematically justify whether or not it is
a martingale:
(b.1) {It}t∈[0,T], (b.2) It(2) t∈[0,T] , (b.3) nIt(2) −R (Xu)0(t) 2 du ot∈[0,T] .
Question 3. (18 marks) Let {Xt}t∈[0,T] be a (continuous) stochastic process. We assume that
both {Xt}t∈[0,T] and the stochastic process {Xt(2) − t}t∈[0,T] are martingales with respect to
Let 0 ≤ s < u ≤ T, where s and u are fixed. For fixed positive integer n, let Πn = {ti} , where ti = s + i∆n, ∆n = (u − s)/n, be a partition of the interval [s,u]. For simplicity, we also use the notation ∆Wti = Wti+1 − Wti , i = 0, . . . ,n − 1.
In this question, consider a real-valued function f(x) for which f(x) and the partial derivatives
, , and exist and are continuous and bounded for all x ∈ R. In the below, the notation (Xt) and (Xt) means that the partial derivatives are evaluated at Xt .
a. (6 marks) Show that
E[f(Xu)|Fs] = f(Xs) + E E (Xti) Xti+1 − Xti Fti Fs + E E (Xti) Xti+1 − Xti 2 Fti Fs
+ RΠn , (1)
where RΠn is given by
RΠn = E (X ) ti(∗) Xti+1 − Xti 3 Fs , Xti(∗) ∈ Xti ,Xti+1 .
Hint: Write
n−1
f (Xu) − f (Xs) = f Xti+1− f (Xti) ,
apply conditional expectation.
b. (6 marks) Show that the terms in (1) satisfy: for i = 0, . . . ,n − 1,
E E (Xti ) Xti+1 − Xti Fti Fs E E (Xti ) Xti+1 − Xti 2 Fti Fs
= 0,
= E (Xti) Fs (ti+1 − ti).
Hint: Use the fact that {Xt}t∈[0,T] and {Xt(2) − t}t∈[0,T] are martingales.
c. (4 marks) Let
Yn = E (Xti) Fs (ti+1 − ti),
Y = Zs u E (Xt) Fs dt.
Show that, as n → ∞ , we have Yn L2(−→) Y .
Hint: First, show that |Yn − Y |2 → 0 almost surely, and then apply the Dominated
d. (2 marks) Using the same technique as in part c, it can be shown that, as n → ∞ ,
RΠn −→ 0 in L2 . You do not need to prove this fact. Together with previous results in
E[f(Xu)|Fs] = f(Xs) + Zs u E (Xt) Fs dt. (2)
Which key formula covered in the course is similar to (2)? Briefly explain the connection between the two formulae (one sentence is enough).
2022-04-27