MATH3975: FINANCIAL DERIVATIVES
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Semester 2, 2021
MATH3975: FINANCIAL DERIVATIVES (Advanced)
1. [20 marks] Single-period market model
Consider a single-period market model M = (B, S) on the space Ω = (ω1 , ω2 , ω3 }. We assume that the savings account B equals B0 = 1, B1 = 1 + r = 2 and the stock price S is given by S0 = 11 and S1 = (S1 (ω1 ), S1 (ω2 ), S1 (ω3 )) = (24, 20, 16). The real-world probability P is such that P(ωi ) = pi > 0 for i = 1, 2, 3.
(a) Find the class M of all martingale measures for the model M and check if the market model M is arbitrage-free and complete.
(b) Show that the contingent claim X = (8, 6, 4) is attainable and compute its arbitrage price π0 (X) using two methods:
– the replicating strategy for X ,
– the risk-neutral valuation formula.
(c) Consider the contingent claim Y = (4, 2, -3).
– Find the range of arbitrage prices for Y in M. Is the claim Y attainable in M?
– Find the minimal initial endowment x for which there exists a portfolio (x, ϕ) with V0 (x, ϕ) = x and such that the inequality V1 (x, ϕ)(ωi ) > Y (ωi ) is satisfied for i = 1, 2, 3.
(d) Consider the extended market M~ = (B, S1 , S2 ) where S1 = S and S2 is an additional risky asset given by: S1(2) = Y = (4, 2, -3) and S0(2) = 1.35.
– Find a unique martingale measure for the extended market M~ = (B, S1 , S2 ).
– Compute the price of the claim Z = (-2, 5, 3) in the extended market M~= (B, S1 , S2 ). Is the claim Z attainable in M~?
2. [20 marks] CRR model: European contingent claim.
Consider the CRR model of stock price S with T > 1 periods and parameters d, u satisfying d < 1 + r < u where r is the one-period interest rate. We denote by a unique martingale measure for the discounted stock price = B 一1 S.
(a) Consider the binary call and put options with expiry date T and payoffs
T (K) := ≠[K,&) (ST ) =., 1 if ST > K,
and
P~T (K) := ≠[0,K)(ST ) = . 0
. 1
if ST > K,
if ST < K,
respectively. By examining the sum of the payoffs T (K) and P~T (K), find the put-call parity relationship for binary options for any strike K > 0.
(b) Consider the binary call option with the payoff T (K) at expiry date T and strike K = S0 (1 + r)T . Compute the arbitrage price 0 (K) for this option at time t = 0 using the risk-neutral valuation formula.
(c) Find a unique probability measure on (Ω , rT ) such that the process S一1 B is a martingale under with respect to the filtration FS and show that the following equality holds for any European contingent claim X and any date t = 0, 1, . . . , T
πt (X) = St E ╱XS一T1 | rt、.
(d) Let Y and Z be two European contingent claims with maturity T. Assume that the equality πU (Y) = πU (Z) holds for some date U such that 0 < U < T. Does this assumption imply that πt (Y) = πt (Z) for every U < t < T?
3. [20 marks] CRR model: American contingent claim.
Consider the CRR model with T = 2 and S0 = 45, S1(u) = 49.5, S1(d) = 40.5. Assume that the interest rate is negative, specifically, r = -0.05. Consider an American claim Xa with the reward process gt = (St - Kt )+ where K0 = 40,
K1 (ω) = 35.5 for ω e (ω1 , ω2 }, K1 (ω) = 38.5 for ω e (ω3 , ω4 }. and K2 = 36.45.
(a) Find the unique martingale measure on (Ω , r2 ) and compute the price pro- cess Ca for the American claim Xa using the recursive relationship, which holds for t = 0, 1,
πt (Xa ) = max !gt , Bt E ╱ B1 πt+1(Xa ) | rt、(.
Find the rational exercise time τ0(*) for the holder of the American claim Xa .
(b) Find the replicating strategy ϕ for the American claim Xa up to the rational exercise time τ0(*) and check that the wealth V (ϕ) of the replicating strategy matches the price computed in part (b).
(c) Find the early exercise premium for the American claim Xa .
(d) Determine whether the discounted arbitrage price B 一1 π(Xa ) is a super- martingale or a submartingale under with respect to the filtration F. Find a probability measure Q on (Ω , r2 ) under which the process B 一1 π(Xa ) is a martingale with respect to the filtration F.
4. [20 marks] Black-Scholes model: European contingent claim.
Assume that the stock price S is governed under the martingale measure by the Black-Scholes stochastic differential equation
dSt = St ╱r dt + σ dWt、
where σ > 0 is a constant volatility and r is a constant short-term interest rate. The savings account B is given by Bt = ert for all t e R+ .
Let K and L be arbitrary real numbers such that 0 < L < K. Consider a Euro- pean claim with the payoff X at time T given by the following expression
X = max ╱|ST - K|, K - L、.
(a) Sketch the profile of the payoff X as the function of the stock price ST and find the decomposition of this payoff in terms of cash and payoffs of stan- dard call and put options with expiry date T and various strikes.
(b) Using the decomposition from part (a), compute the arbitrage price and the replicating portfolio at time t = 0 for the claim X. Take for granted the Black-Scholes pricing formulae for call and put options and respective hedge ratios.
(c) Show that the arbitrage price of X at time t = 0 is a monotone function of the parameter L and compute the limits lim L→0 π0 (X) and lim L→K π0 (X).
(d) Find the arbitrage price at time 0 of the claim Y = ST(2) with maturity T using the property that the process Mα , which is given by
Mt(α) = exp ╱αWt - α2 t、, V t e [0, T],
is a martingale under for any choice of α e R.
2021-12-17