ECMT3150: Assignment 2 (Semester 1, 2023)
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ECMT3150: Assignment 2 (Semester 1, 2023)
Due: 5pm, 15 May 2023 (Monday)
NOTE: Please do not write your Önal answers in your R-script. You should summarise the outputs (e.g., plots) and include your discussion and Önal answers in the written response Öle. Both your written response Öle and R-script (i.e., the .R source Öle, not the screenshot) need to be submitted.
[Total: 30 marks (+ bonus)] Carol is undergoing a series of training at the pricing team in Goldman Sachs. She is studying a simple Önancial market consisting of a risk-free money account and a stock called BOB. Here is the single-period model under the risk-neutral probability measure Q:
● Time length of the period is A.
● In the risk-free market account, a dollar at time 0 will grow into a = erA at time A, where r is the continuously compounded risk-free interest rate.
● At time 0, the share price is S0 . At time A, the share price rises to SA = S0u with probability q, and drops to SA = S0 d with probability 1 一 q .
1. [2 marks] Write down the risk-neutral probability distribution of SA , the share price at time A. Express the probability mass function in terms of u;d and q .
2. [3 marks] Show that q = . [Hint: the discounted share price is a martingale under Q.]
3. [3 marks] Find Var(SA ), the variance of the share price at time A? Express your answer in terms of a, u and d.
4. [3 marks] Let u = eg ^A and d = = e-g ^A . Show that Var(SA ) 二 S0(2)a A2 for small
A. [Hint: e北 二 1 + x if x is close to zero. The Önal result is obtained by dropping terms involving higher power of A]
Carol wants to construct a binomial tree model for the price of BOB traded in an n- period market, where n is a positive integer. Here is the binomial tree model under Q (for i = 1;:::;n):
● Time length of a period is A.
● In the risk-free market account, a dollar at time (i 一 1)A will grow into a = erA at time iA, where r is the continuously compounded risk-free interest rate, which remains constant over time.
● At time (i 一 1)A , the share price starts at S(i-1)A . At time iA, the share price rises to S(i-1)Au with probability q, or drops to S(i-1)A d with probability 1 一 q . The probability q is as given in question 2, and u and d are as given in question 4 (i.e. u = eg^A and d = = e-g^A). Assume that the price changes are independent across all n periods.
5. [3 marks] Let j denote the number of times by which the share price goes up over n periods. What is the probability distribution of j? For a given j, show that the share price at the end of period n is given by
SnA = S0uj dn-j :
6. [2 marks] Consider a European call option written on a share of BOB at time 0 with strike price X and time-to-maturity r = nA. Show that its price is given by
C0(bin) = EQ [e-rnA max(SnA 一 X;0)]: (1)
Suppose we are at time 0, and the current share price of BOB is S0 = 50. Suppose r = 0:02 and a = 0:3. Write an R code that simulates 5000 sample paths of share price using the above binomial tree model with the following speciÖcations: n = 63, A = 1=252.1 While simulating the random numbers, set the random seed to be the last 5 digits of your SID.2 [Hint: you may use rbinom(5000,n,p)to generate 5000 random integers from a binomial distribution with parameters n and p.]
7. [3 marks] Using your code, compute the time-0 price of an at-the-money European call option written on a share of BOB at time 0 with strike price X = S0 = 50 and expiring in 63 days (i.e., r = 63A). Correct your answer to 3 decimal places.
8. [3 marks] Compute analytically the time-0 price of the same call option using the Black-Scholes formula instead. Correct your answer to 3 decimal places. Compare it
with your answer in question 7.
Carol has recently moved to the product design team. She is currently designing an exotic option written on a share of BOB at time 0. This option will give the following payo§ as a function of the share price Sr at time r
( X1 一 Sr for Sr < X1 ;
g(Sr ) = 1 0 for X1 ≤ Sr ≤ X2 ;
( Sr 一 X2 for Sr > X2 ;
where X1 < X2 . Carol named this exotic option as ìáy-with-BOB,îafter noting that the graph of the payo§ function looks like the wings of an aeroplane.
9. [3 marks] Using your code, compute the time-0 price of a áy-with-BOB option with strike prices X1 = 45, X2 = 55 and expiring in 63 days (i.e., T = 63A). Correct your answer to 3 decimal places.
10. [3 marks] Compute analytically the time-0 price of a áy-with-BOB option using the Black-Scholes formula instead. Correct your answer to 3 decimal places. Compare it with your answer in question 9.
11. [2 marks] What type of investors will be interested in áy-with-BOB?
12. [Optional question for those who are up to the challenge; bonus marks will be given for correct solutions] Prove mathematically that C0(bin) as deÖned in question 6 converges to the Black-Scholes call price as A o 0 and n o o while T = nA remaining constant.
2023-05-12