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MATH39032
1. A forward contract with delivery price F such that F < SerT (where S is the current value of an asset S that does not pay out a dividend, r is the risk-free rate and T is the time to expiry) is trading on the market with zero cost to enter a short or long position. Show that there exists an arbitrage opportunity. 2. Consider a financial contract depending on two assets, S1 and S2, that has the following payoff at maturity V (S1, S2, t = T ) = max(S1 −X1, S2 −X2). Here X2 > X1 are constants, S1 ∈ [0,∞), S2 ∈ [0,∞), and t ∈ [0, T ]. (a) Provide a sketch of the payoff V against S2 when S1 = X2. (b) Assuming that asset prices follow a geometric Brownian motion, interest rates are constant, and that neither asset pays a dividend, state appropriate boundary conditions for the value of the financial contract V (S1, S1, t) in the following two cases: (i) S1 → 0 and S2 → 0; (ii) S1 → 0 and S2 →∞. 3. Suppose that the interest rate process r is governed by the general stochastic differential equation dr = κ(θ − r)dt+ σrdX, where X is a Brownian motion. V1(r, t) is the value of a zero-coupon bond with maturity T1, and V2(r, t) is the value of a zero-coupon bond with maturity T2. Consider a portfolio comprising long one V1 bond and short ∆ of V2 bonds. Using Itoˆ’s Lemma (which may be used without proof), show that the choice ∆ = ∂V1 ∂r / ∂V2 ∂r eliminates the random component of the portfolio. Hence show that the value of both V1 and V2 must satisfy the equation ∂V ∂t + 1 2 σ2r2 ∂2V ∂r2 + κ(θ − r − λσr)∂V ∂r − rV = 0, where λ(r, t) is the market price of risk. Page 2 of 4 P.T.O. MATH39032 4. (a) Let F (S, t) = KeλtSα, where K, λ and α are constants. Determine λ (in terms of α, r and σ) so that F satisfies the Black-Scholes PDE, ∂F ∂t + 1 2 σ2S2 ∂2F ∂S2 + rS ∂F ∂S − rF = 0, where the volatility σ and interest rate r are constants. (b) Now consider a particular financial instrument with price f(S, t) that satisfies the above PDE and provides a single payoff at time t = T > 0 amounting to Sα; determine f(S, t < T ) using your answer to part (a). (c) Describe the monotonic behaviour of f(S, t) with respect to time for α < 1, α = 1 and α > 1. 5. Consider the Black-Scholes equation for a put option, P , on a stock S which pays no dividends, namely ∂P ∂t + 1 2 σ2S2 ∂2P ∂S2 + rS ∂P ∂S − rP = 0 in the usual notation, where r and σ are both constants. (a) By neglecting the time derivative in the above equation, seek solutions of the form P (S, t) = P (S) = A1S α1 + A2S α2 where A1 and A2 are constants, and α1 > α2. Determine the values of α1 and α2. (b) Consider a perpetual American put option P (S), i.e. an option with no expiry date but where it is possible at any point in time to exercise, which has a (non-standard) exercise value of Ke−S, where K is a constant. This can be valued using the time independent solutions in (i) above. The exercise boundary is denoted by Sf . State and justify the two conditions at S = Sf and the other appropriate boundary condition. (c) Determine Sf and the values of A1 and A2. (d) Describe the behaviour of P (S) and Sf as r → 0. Page 3 of 4 P.T.O. MATH39032 6. You may assume that the Black-Scholes equation (in the usual notation) for a put option on a non- dividend paying asset is ∂P ∂t + 1 2 σ2S2 ∂2P ∂S2 + rS ∂P ∂S − rP = 0 where the interest rate r and the volatility σ are both constant. (a) A binary put option Pb(S, t) has the payoff Pb(S, T ) = { 0 if S > X K if S < X . Show that its value is given by Pb(S, t) = K ∂P ∂X where P (S, t;X) is the value of a vanilla put option with strike X. (b) Given that the value of a European put option is P (S, t;X) = Xe−r(T−t)N(−d2)− SN(−d1), where N(x) is the cumulative distribution function of the standard normal distribution, namely N(x) = 1√ 2pi ∫ x −∞ e− y2 2 dy and d1 = ln(S/X) + (r + σ2/2)(T − t) σ √ T − t , d2 = d1 − σ √ T − t, show that Pb(S, t;X) = Ke −r(T−t)N(−d2). (c) Consider a stock whose price (6 months from the expiration of a binary put option) today is £10.50. The option pays £0.5 if the stock value at expiry is less than £11, and nothing if the stock value at expiry is greater than 11. The risk-free interest rate is 4% per annum (fixed) and the volatility (constant) is 15% per (annum) 1 2 . Using the formula above for Pb(S, t;X), determine its value today. The value of N(x) may be determined by a numerical (online) calculator. END OF EXAMINATION PAPER