MAFS5010 Stochastic Calculus (2019 Fall)
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MAFS5010 Stochastic Calculus (2019 Fall)
Take-home Final Examination
Time allowed: 3 hours
Instruction: There are 8 problems in this exam. Answer ALL questions. Details must be shown clearly to receive full credits.
Problem 1 (12 marks)
(a) We let X and Y be two random variables.
(i) If X and Y are independent. Show that E[X|Y] = E[X].
(ii) Show that the converse of the statement is not true by providing a counter-
example.
(b) We let X, Y be two random variables such that E[X2] < ∞ and E[Y2] < ∞ . Suppose that E[X|Y] = Y and E[Y|X] = X, prove that X = Y almost surely (i.e. P(X = Y) = 1).
Problem 2 (14 marks)
(a) We let X1, X2, X3, … be a sequence of independent and identically distributed random variable such that each Xi has standard normal distribution (i.e. Xi ~N(0,1)). For any n = 1,2, …, we define Sn = X1 + X2 + ⋯ + Xn and the filtration ℱn = G(X1, … , Xn). Using the definition of martingale, determine which of the following processes are martingales/super-martingales/sub-martingales with respect to {ℱn}n≥1? Complete the following table (with Yes/No) and explain your answer.
Martingale Sub-martingale Super-martingale |
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An = Sn(2) − n |
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Bn = X1(2) + ⋯ + Xn(2) − n |
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Sn n |
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Dn = eSn− 2 |
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(b) We consider a probability space (Ω, ℱ, P) with filtration {ℱt }t≥0 . We let Bt be the Brownian motion and Nt be the Poisson process with parameter 入. We assume that both Bt and Nt are {ℱt }-adopted.
(i) Determine if the process Xt = eBt− is a martingale.
(ii) Determine if the process Yt = eaNt−入t(ea −1) is a martingale for any a ∈ ℝ .
Problem 3 (10 marks)
For any 0 < a < b, we let a = t0 < t1 < t2 < ⋯ < tN = b. We let wt be the standard Brownian motion. Show that
N−1 1 ti+1 2 b − a
(Hint: Think about some properties of stochastic integral studied in the lecture note .)
Problem 4 (12 marks)
We let wt be the standard Brownian motion
(a) Compute the integral wt5 dwt . Leave your answer in term of T and the integral ? ? dt. Hence, determine E[wt6].
(b) Solve the following stochastic differential equation:
dXt = (√1 + Xt(2) + Xt ) dt + √1 + Xt(2)dwt,
where wt is standard Brownian motion.
Problem 5 (10 marks)
We let {Xn}n≥1 be a stochastic process such that E[|Xn |] < ∞ for each n ≥ 1, show that Xn can be expressed as Xn = Yn + Zn, where Yn is a sub-martingale and Zn is super-martingale.
Problem 6 (15 marks)
We let wt be a Brownian motion, and define
Bt = ∫ sign(ws)dws
1 if x ≥ 0
−1 if x < 0.
(a) Show that Bt is a Brownian motion.
(b) Show that E[Bt wt] = 0. In other words, Bt and wt are uncorrelated. (Hint: Apply
Ito’s lemma on the function f(Bt, wt) = Bt wt .)
(c) (i) Show that dwt2 = 2wt dwt + dt. (ii) Hence, verify that
E[Bt wt2] ≠ E[Bt]E[wt2].
(*Note: This result reveals that Bt and wt are not independent. Therefore this problem provides an example that uncorrelated random variables are not necessarily independent.)
Problem 7 (15 marks)
We let wt(1), wt(2), … , wt(d) be d independent Brownian motion and let a and G be positive constants. For j = 1,2, … , d, let Xj) be a stochastic process which satisfy
dXj) = − Xj)dt + dwt(j) .
(a) Show that
Xj) = e −bt [Xj) + t e bs dws(j)].
Given a fixed value of t, determine the distribution of X
(b) We define
Rt = (Xj))2 .
j=1
(i) Show that Bt = ∑j(d)=1 dws(j) is a Brownian motion. (ii) Hence, show that Rt satisfies
dRt = (a − bRt)dt + a√Rt dBt,
where a = .
(c) Using the result in (b)(ii), show that
Rt = e −btR0 + (1 − e−bt) + a te −b(t−s)√Rs dBs .
Problem 8 (12 marks)
We consider a contingent claim which pays an amount f(ST) at time T, where ST is the price of a risky asset (stock). Suppose that the current price of this contingent clam, denoted by V = V(t, St), satisfies the following equation:
+ + (r − q)St − (r + ℎ)V = 0,
where V(T, ST) = f(ST), r, ℎ, q, a are positive constants.
0 ≤ t ≤ T,
Suppose that f(ST) = min(max(ST − X, 0) , M), where X, M > 0. By using Feynman-Kac theorem on suitable function, find V(t, St) for 0 ≤ t < T. Explain your answer.
(Note: You will get 0 mark if you get the final answer using the solution of Black-Scholes equation directly or pricing formula of call/put option.)
2022-11-17