MATH4091/7091: Financial calculus Assignment 1
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MATH4091/7091: Financial calculus
Assignment 1
Semester I
Assignment questions
1. (10 marks) Although in L1, we discussed the perfect replication technique in a one-period binomial model, it is more realistic to consider a multi-period binomial model as we do in this question.
Our model consists of a risky asset S and a bond B. Let St, where t = 0, 1, 2,...,T, denote the time-t price of a tradeable (non-dividend-paying) asset. Let S0 = 100 and let each random increment St+1 − St take value +1 with physical probability 40%, and value -1 with physical probability 60%, independently of all other increments. The bond has a constant price 1 at all times (i.e. r = 0%).
We assume that there exists a call option on S, with strike K = 105 and expiry at time T = 10 (years).
(a.) (5 marks) A student came up with the following argument to prove that the no- arbitrage time-0 value of the call is zero.
Consider the trading strategy ⇥t(rep) , t = 0, 1, 2,...,T, specified as follows.
⇥t(rep) =
This trading strategy replicates the call payo↵, because either ST ≥ K, in which case the call payo↵ matches the time-T portfolio value ST − K, or else ST < K , in which case the call payo↵ matches the time-T portfolio value 0. The time-0 value of the replicating strategy ⇥t(rep) is zero, because S0 < K. So if the time-0 call price is not zero, then arbitrage exists. Specifically, if the time-0 call price is strictly positive, then shorting the call and going long the replicating strategy is an arbitrage; in other words,
⇥t = ⇥t(rep) − ⇥t(call)
is an arbitrage, where we let ⇥t(call) be the portfolio consisting of 1 call at all times. (And, likewise, if the time-0 price of the call is strictly negative, then −⇥t(call) is
an arbitrage.) Therefore the no-arbitrage time-0 call price must be zero. Identify and explain the specific flaw in this “proof”.
(b.) (5 marks) Find the true time-0 value of the call.
Do not induct backwards step-by-step in a tree, and do not use a computer (unless you want to check your answer).
Although your answer should be explicit, you may leave it un-simplified. For example, you may leave binomial coefficients (numbers of the form: n choose k) un-simplified.
2. (10 marks) Suppose that the value of a certain stock at time T is a random variable with distribution P. Note we are not assuming a binary model. An option written on this stock has payo↵ CT at time T. Consider a portfolio consisting of ↵ units of the underlying and β units of bond, held until time T. Let V0 be the portfolio’s value at time-0. Assume that interest rate is zero.
(a.) (6 marks) Show that the extra cash required by the holder of this portfolio to replicate the claim CT is
= CT − V0 − ↵ (ST − S0) .
Find expressions for the values of V0 and ↵ (in terms of E [ST] , E[CT], var[ST] and cov (ST,CT)) that minimise
E ⇥ 2⇤ .
Verify that for these values, we have E[ ] = 0.
(b.) (4 marks) Prove that for the one-period binomial model in L1, any CT depends linearly
on ST − S0. Deduce that in this case, we can find V0 and ↵ such that = 0.
2022-03-25