NEWCASTLE UNIVERSITY SCHOOL OF MATHEMATICS, STATISTICS & PHYSICS SEMESTER 1 SPECIMEN 2 MAS3904. Stochastic Financial Modelling. Time allowed: 2 hours

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1 NEWCASTLE UNIVERSITY SCHOOL OF MATHEMATICS, STATISTICS & PHYSICS SEMESTER 1 SPECIMEN 2 Stochastic Financial Modelling Time allowed: 2 hours Candidates should attempt all questions. Marks for each question are indicated. However you are advised that marks indicate the relative weight of individual questions, they do not correspond directly to marks on the University scale. There are FOUR questions in Section A and THREE questions in Section B. Statistical tables are provided on pages Calculators may be used.

2 SECTION A A1. A trader buys one European call option with strike price K 1, sells two European call options with strike price K 2 and buys another call with strike price K 3. All options have the same maturity time T. This combination of options is known as a European collar. Suppose that K 1 = 1, K 2 = 2, K 3 = 3. (a) Write explicitly the terminal payoff g(x) of the trader as a function of the stock price x = S T at maturity T. Sketch the graph of g(x). (b) Find those values of the stock price x = S T for which this European collar will be in-the-money. (c) Suppose that S T is continuous and uniformly distributed over the interval (0, 4). What is the probability that the trader is in-themoney? [8 marks] Page 2 of 10

3 A2. A market offers European call and put options, written on the same stock. Let S 0,T,K,r,C 0,P 0 denote the initial stock price, the trading period, the strike price, the risk-free interest rate with continuous compounding, the fair price of a call option and the fair price of a put option respectively. (a) What does it mean to say that there is no arbitrage in the market? (b) Consider two trading positions A and B: Position A: buy one put option with maturity T and strike price K for P 0. Buy one share of stock. Position B: Buy one call option with maturity T and strike price K for C 0. Invest Ke rt in the bank. (i) What is the total cost for setting position A? What is the total cost for setting position B? (ii) Show that the values of positions A and B are the same at time T. Hence or otherwise show that for no arbitrage the European put-call parity relation must hold: S 0 +P 0 = C 0 +Ke rt. (c) Suppose that the European put-call parity relation is not satisfied and that S 0 +P 0 < C 0 +Ke rt. Describe explicitly a trading strategy leading you to risk-free profit. [12 marks] Page 3 of 10

4 A3. Suppose that stock price S t follows a geometric Brownian motion with parameters µ = 0.1 and σ 2 = 0.09 per annum. The current stock price is S 0. (a) Find the probability that the stock price in 6 months time is greater than now. Hint. Calculate Pr(S 0.5 > S 0 ). (b) Find the value S 0 such that the probability of stock price being less than 10 at the end of year one is 5%. (c) Suppose that S 0 = 15. A 95% confidence interval for the stock price after 1 year is (8.8,28.5). An analyst computes a 95% confidence interval for the stock price after 3 years as (25.1, 35.2). Without performing any calculations, briefly explain why the analyst s calculation must be incorrect. [10 marks] A4. Suppose that stock price S t follows a geometric Brownian motion with mean rate of return given by the riskless (continuously compounded) interest rate r and volatility σ 2, both per annum. The initial stock price is S 0. Available in the market are power call options with strike price K, maturity time T and power parameter γ = 2. (a) Define the power call option and give its payoff as a function of the stock price S T at the maturity time. (b) Write down the distribution of 2log(S T /S 0 ). (c) Derive an expression (in terms of S 0,T,K,r,σ 2 ) for the probability that the holder of the power call will not exercise the option at the maturity time T. Hint. Calculate Pr(S 2 T < K). [10 marks] Page 4 of 10

5 SECTION B B5. Suppose that stock price S t follows a geometric Brownian motion with parameters µ and σ 2 per annum. The mean rate of return µ for the stock is the riskless (continuously compounded) interest rate r. The current stock price S 0 is known to all traders. The fair price C 0 of one European call option with a strike price K and maturity T is given by the Black-Scholes formula: C 0 = S 0 Φ(ω) Ke rt Φ(ω σ T), ω = rt +σ2 T/2 ln(k/s 0 ) σ. T (Φ is the standard normal distribution function.) Suppose that S 0 = 10,r = 5% and σ 2 = 1. (a) A trader holds one European call option with strike K = 10 and maturity T = 3 years. Find the fair price of the trader s option. What would be the fair price of a European put option with the same strike and maturity? (b) Explain the effect of increasing S 0 on the fair price of a European put option. Justify your answer. (c) The trader now decides to buy an Asian call option with maturity T = 2 years and strike price K = 8. You should assume that the stock price process is observed daily. (i) Write down the payoff at maturity of the trader s option. (ii) Describe a detailed step-by-step Monte Carlo algorithm for estimating the fair price of the trader s option. [20 marks] Page 5 of 10

6 B6. Available in the market are Barrier call and put options, all of which have barrier ν, maturity time T (in years) and a strike price K. All options are written on the same stock. (a) Define an up-and-in put option and write explicitly the payoff of this option. (b) Trader A holds one up-and-in put option. Trader B holds one upand-out call option and one up-and-in call option. (i) Suppose that K < ν. Can trader A receive a positive payoff? Justify your answer. (ii) Show that the payoff of trader B is equal to the payoff of one European call option. [15 marks] Page 6 of 10

7 B7. Consider a process {X t,t 0} satisfying the following stochastic differential equation, dx t = (1 2X t ) dt+ 2 X t dw t, X 0 = x 0 where x 0 is a positive constant and {W t,t 0} is a standard Brownian motion process. (a) Identify the process {X t,t 0}. (b) Using Itô s formula, write down the stochastic differential equation satisfied by Y t = X t e t. (c) Hence or otherwise, show that Z t = X t can be written as t Z t = z 0 e t +e t e τ dw τ, z 0 = x 0 0 [12 marks] (d) Calculate the expectation and variance of Z t and write down the probability density function of Z t. Hint. You may assume that for real, square-integrable functions g(t), t 0 ( g(τ)dw τ N 0, (e) Calculate the expectation of X t. t 0 ) [g(τ)] 2 dτ. [13 marks] [Total: 25 marks] Page 7 of 10

8 Table 1: The Standard Normal Distribution Values of P(Z z) where Z N(0,1). (a) Negative z values z Page 8 of 10

9 Table 1 continued: The Standard Normal Distribution Values of P(Z z) where Z N(0,1). (b) Positive z values z Page 9 of 10

10 Table 2: Quantiles of Standard Normal Distribution p = Φ(z) z = Φ 1 (p) THE END Page 10 of 10