Math 3589 Introduction to Financial Mathematics Homework Assignment #4
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MATH 3589
Introduction to Financial Mathematics
Homework Assignment #4
1. (Put-call parity) A stock currently costs S0 per share. In each time period, the value of the stock will either increase or decrease by u and d respectively, and the risk-free interest rate is r . Let Sn be the price of the stock at t = n, for 0 < n < N, and consider three derivatives which expire at t = N, a call option VN(call) = (SN _ K)+ , a put option VN(put) = (K _ SN )+ , and a forward contract FN = SN _ K .
(a) The forward price is the strike price such that the price of the forward
contract is F0 = 0. Show that if N = 1 (one-period binomial model), and K = (1 + r)S0 , then F0 = 0.
(b) Show that if N = 1 (one-period binomial model), and K = (1 + r)S0 , then V0(call) = V0(put) .
(c) Explain why in the general N-period model, for any strike price K , VN(call) = FN + VN(put) . That is, if you buy at t = 0 a forward contract and a put option, and hold them until expiration, the payoff you receive is the same as the payoff of a call option. What can you say about the prices of these three options at t = 0? (Hint: Do not use the risk-neutral pricing formula!)
2. Let S0 and S1 be the prices of a stock at t = 0, 1 in the one-period binomial model. Assume the no-arbitrage condition 0 < d < 1 + r < u, and assume
P(H) = p. We define θ = up + d(1 _ p) _ 1. Show that the expected value at t = 0 of is
E0 , ┐ = S0 .
3. Let g, h be two real-valued convex functions on R. Let m(x) = max(h(x), g(x)}. Prove that m(x) is also convex.
4. Let (Ω , P) be a finite probability space. Recall that if A S Ω is an event, then the probability of A is
P(A) = P(ω).
u…A
Let Ac be the compliment of A. Show that
a) P(Ac ) = 1 _ P(A)
b) If A1 , A2 , . . . , AN are a set of events, then prove
P ╱Ak ← < k P(Ak ).
5. Assume S0 = 4, u = 2, d = 1/2, r = 1/4. Fill in the following Binomial tree using the risk neutral probabilities.
S2 [HH] =
2 [S3 |HH] =
S1 [H] =
1[S3|H] =
S0 =
0 [S3] =
2 [S3 |TH] =
S1 [T] =
1 [S3 |T] =
S2 [TT] =
2 [S3 |TT] =
S3 [HHH] =
S3 [HHT] =
S3 [HTH] =
S3 [HTT] =
S3 [THH] =
S3 [THT] =
S3 [TTH] =
S3 [TTT] =
6. One of the assumption we have made is that the risk-free interest rate is constant. In this problem, we will relax that assumption! Consider the following two period binomial model with a random interest rate rn . In this model, we define the risk-neutral pricing formula by
1 1 + rn _ d
(1 + rn ) u _ d .
(Can you explain this formula?) Let V2 = (S2 _ 7)+ .
a) Fill in the following binomial tree.
S2 [HH] = 16
V2 [HH] =
S1 [H] = 8, r1 (H) =
V1 [H] =
S2 [HT] = 4 V2 [HT] =
S2 [TH] = 4
V2 [TH] =
S1 [T] = 2, r1 (T) =
V1 [T] =
S2 [TT] = 1 V2 [TT] =
b) Suppose you sold the option for V0 at time t = 0. However, you now regret your decision and now want to hedge against the risk you are now facing. Compute the amount of stock you should purchase, ∆0 , so that at time t = 1, regardless of what happens to the stock, the value of your portfolio is V1 .
c) Suppose the first coin toss is a head; i.e. ω 1 = H . Calculate how much stock you should buy now, ∆ 1 (H), to ensure that at t = 2, the value of your portfolio is (S2 _ 7)+ .
2022-10-02