Harvard University Forward Contract Put Call Parity Math Finance Questions

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1. (10 pt each) XYZ flour company is planning to buy wheat for the next year’s
production. 10,000 bushels of wheat will be purchased each quarter, on the 15th of
January, April, July, and October in 2022. It wants to lock the wheat price using
futures contract, and at the same time use a swap to prepare the expenditure. The
spot price of wheat today (April 15, 2021) is \$3.1 per bushel, and the company
enters futures contracts today. The prevailing interest rate is 1.5%, compounded
continuously. Recall that each futures contract is for 5000 bushels and assume no
storage cost of wheat.
(a) If the company uses a prepaid swap, what is the prepayment?
(b) If the company uses a plain vanilla swap for the quarterly purchase next year,
what is the fixed amount?
2. (30 pt) Prove the Put-Call Parity C − P + d(0, T)K = S of European options.
Mathematics of Finance II-1
1
Forwards, Futures, and Swaps
Definition 1.1. A derivative is financial security whose payoff is explicitly tied to the value of
other variable(s), usually the price of another financial security or a group of securities.
Definition 1.2. An underlying security is the security that determines the price of a derivative.
1.1
Forward Contract
Throughout the chapter, we asume that there is no arbitrage profit opportunity.
Definition 1.3. A forward contract is an agreement to trade a specific amount of asset at a
predetermined price at a future date.
For example, an investor may enter a forward contract by promising to buy an asset at a future
time T . This invester is taking a long position or going long. Nothing changes hands at time 0,
and at t = T , this long position holder pays a price F (0, T ) predetermined at t = 0 and take the
delivery of the asset from his counterparty, the short position holder. The payoff the long position
holder is
S(T ) − F (0, T )
where S(T ) is the market price of the underlying asset at t = T and F (0, T ) the forward price of
the contract that was initiated at t = 0 and has a delivery time T . The payoff of the short position
holder is
− (S(T ) − F (0, T )) = F (0, T ) − S(t).
Theorem 1.1. (Forward price formula: basic version) If an asset can be stored at no cost and sold
short, then
S(0)
F (0, T ) =
or equivalently S(0) = F (0, T )d(0, T ),
d(0, T )
where S = S(0) is the current (t = 0) spot price, F (0, T ) is the current forward price for delivery
at time T , and d(0, T ) the discount factor for the time interval [0, T ].
Proof. Assume the contrary, then derive a contradiction using the strategy “buy low, sell high”.
Case 1. Suppose F >
S
d(0,T ) ,
then buy in the spot market and sell in the forward market.
t=0
t=T
S
d(0,T )
Spot Market:
borrow S to buy a unit
debt has increased to
Forward Market:
contract to short a unit
deliver the unit to receive F
1
Mathematics of Finance II-1
S
S
Therefore, after paying off the debt plus interest d(0,T
) , there is an arbitrage profit of F − d(0,T ) ,
contradicting to no arbitrage profit opportunity assmption.
Case 2. Suppose F < S d(0,T ) , then buy in the forward markett and sell in the market. t=0 t=T Spot Market: short a unit for S and invest it Forward Market: contract to long a share investment has grown to S d(0,T ) take the delivery and pay F After returning the share to close the short selling, there is an atbitrage profit of contradicting to no arbitrage profit opportunity assmption. S d(0,T ) − F, Borrowing money is equivalent to selling bonds. For notational purpose, we introduce a hypothetical zero coupon bond (ZCB) whose face value is one unit cucrrency, and call it a unit ZCB. Denote by B(t, T ) the price at time t of a unit ZCB that matures at T . When we assume that there is only one prevailing interest rate, we can consider B(t, T ) as the discount factor during the time interval [t, T ]. For this reason, we will use the two notations B(t, T ) and d(t, T ) interchangeably. Theorem 1.2. (Forward price formula: general version) If an asset can be stored at no cost and sold short, then the price of a forward contract initiate at t ≤ T for delivery at T is F (t, T ) = S(t) B(t, T ) where S = S(t) is the spot price at time t, F (t, T ) is the forward price for delivery at time T , and d(t, T ) the discount factor for the time interval [t, T ]. Proof. Case 1. Suppose F (t, T ) >
S(t)
B(t,T ) ,
then
– at time t,
buy a share of underlying asset at S(t) which is financed by selling
S(t)
B(t,T )
unit ZCB
sell in the forward market, i.e. take the short position in a forward contract at F (t, T )
– and at time T ,
deliver the asset bought at t in the forward market at F (t, T )
pay
S(t)
B(t,T )
to the bond holder
2
Mathematics of Finance II-1
⇒ profit = F (t, T )−
S(t)
> 0,
B(t, T )
Case 2. Suppose F (t, T ) < S(t) B(t,T ) , contradicts to no-arbitrage profit opportunity assumption. then - at time t, enter a long forward contract at F (t, T ) (i.e., buy in the forward market) short a share at S(t) and buy S(t) B(t,T ) unit ZCB - and at time T, receive S(t) B(t,T ) in the bond market pay F (t, T ) in the forward market to take a share use the share to unwind the short position ⇒ profit = S(t) −F (t, T ) > 0,
B(t, T )
contradicts to no-arbitrage profit opportunity assumption.
Example 1.1. S(0) = \$17, F (0.1) = \$18, r = 8%.
a. Is there an arbitrage profit opportunity?
b. If short selling requires a 30% deposit at d = 4%, what is the artibrage profit (if there is any)?
When the underlying costs money to hold, we call it “cost of carry”. When the underlying
generates income by holding, we call it “dividend”. Cost of carry and dividend are negative of each
other.
Theorem 1.3. When the underlythg asset earns dividend D(t) at t < T , F (0, T ) = S(0) D(t) − . B(0, T ) B(t, T ) Proof. Consider the following intuitively cash flow: buy a share of underlying asset at S(0) at time 0, received the dividend D(t) at t, and deliver the stock to the counterparty at time T to collect F (0, T ). The cash flow equation is −S(0) + D(t)B(0, t) + F (0, T )B(0, T ) = 0 or equivalently 3 Mathematics of Finance II-1 − S(0) D(t)) − + F (0, T ) = 0. B(0, T ) B(t, T ) Therefore F (0, T ) = S(0) D(t) − . B(0, T ) B(t, T ) Precise proof goes as follows. Case 1. Suppose F (0, T ) >
S(0)
B(0,T )

D(t)
B(t,T ) ,
then
– at t = 0,
borrow S(0) by issuing
S(0)
B(0,T )
bonds
buy a unit of underlying asset at S(0)
D(t)
take a long farward position on B(t,T
) unit bonds maturing at T with delivery date t
and forward price B(t, T ) per bond (this is to invest the dividend income D(t))
take a short forward position on the underlying asset for F (0, T )
– at time t,
receive the dividend div and use it to unwind the long forward on bonds:
D(t)
units · \$B(t, T )/unit = \$D(t)
B(t, T )
– at time T ,
sell the underlying asset for F (0, T )
collect \$1 for each bind, totalling
=⇒
Case 2. Suppose F (0, T ) < S(0) B(0,T ) D(t) B(t,T ) , F (0, T ) + − D(t) B(t,T ) , pay off \$1 each for S(0) B(0,T ) units D(t) S(0) − > 0.
B(t, T ) B(0, T )
then
– at time 0,
take a long forward on a unit of underlying asset with price F (0, T ), delivery at T
short a unit of asset at S(0), invest it in
4
S(0)
B(0,T )
unit ZCB maturing at T
Mathematics of Finance II-1
(to pay dividend at t), take a short forward positnon on
1
with a forward price B(t,T
) , delivery time E
D(t)
B(t,T )
unit ZCB maturing at T
– at time t,
sell
D(t)
B(t,T )
unlts of ZCB at \$B(t, T ) per unit, to collect D(t)
Pay it to the asset holder who lent it for the short selling
– at time T ,
pay out \$1 each for
S(0)
B(0,T )
D(t)
B(t,T )
units of ZCB (from time 0)
units of ZCB (from time t)
buy a unit of underlying asset stock for F (0, T ) and return it to clsoe the short position
=⇒
D(t)
S(0)

− F (0, T ) > 0.
B(0, T ) B(t, T )
Corollary 1.4. When there is a stream of dividends that are earned at the end of each periods,
T
X D(k)
S(0)

F (0, T ) =
d(0, T )
d(k, T )
k=1
where D(k) is the dividend per unit of underlying asset at time k, 1 ≤ k ≤ T , and d(k, T ) is the
discount factor between k and T .
Corollary 1.5. When there is cost of carry C(k) to be paid at the end of each periods,
T −1
X C(k)
S(0)
F (0, T ) =
+
d(0, T )
d(k, T )
k=0
where C(k) is the cost of carry per unit of underlying asset at time k, 0 ≤ k ≤ T − 1, d(k, T ) is a
discount factor between k and T .
Example 1.2. Consider a stock whose price on 2000 January 1 is \$120 and which will pay a
dividend of \$1 on 2000 July 1 and \$2 on 2000 October 1. The interest rate is 12%, compounded
continuously. Is there an arbitrage opportunity if on 2000 January 1 the forward price for delivery
of the stock on 2000 November 1 is \$131? If so, compute the arbitrage profit
5
Mathematics of Finance II-1
Example 1.3. The cunent price of sugar is 12 cents per pound. The carrying cost of sugar is 1 cent
per pound per month, to be paid at the beginning of each month, and the interest rate is constant
at 9% per annum, compounded monthly. What iis the forward price of sugar to be delivered in 5
months?
Example 1.4. (A bond forward) Consider a Treasury bond with a face value of \$10,000, semianual
coupons at 8% per annum, and several years to maturity. Currently this bond is selling for \$9,260,
and the previous coupon has just been paid. What is the forward price for delivery of this bond in
1 year? Assume that the yield for one year out is 9%(2).
1.2
Value of a Forward Contract
Given an asset, let S(t) be the spot price at t and F (t, T ) the forward price at t for delivery at T .
Theorem 1.6. For all t such that 0 ≤ t ≤ T , the time t-value of the “long” forward contract with
forward price F (0, T ) is
fl (t) = [F (t, T ) − F (0, T )]d(t, T ).
Proof. Without any faward contract, a buyer (long position holder) will pay S(T ) at trme T .
– at time 0, take the long forward position with forward price F (0, T )
– at time t, the same person takes the short position with forward price F (t, T )
– at time T , s/he buys the asset at F (0, T ), then sells it at F (t, T )
Then the total gain is
= {S(T ) − F (0, T )}
(from the long position)
+ {F (t, T ) − S(T )}
(from the short position)
= F (t, T ) − F (0, T ),
which is the risk-free arbitrage profit.
Corollary 1.7. For all t such that 0 ≤ t ≤ T , the time t-value of the “short” forward contract with
forward price F (0, T ) is
fs (t) = [F (0, T ) − F (t, T )]d(t, T ).
Example 1.5. Suppose that the price of a stock is \$45 at the beginning of the year, the risk-free
rate is 6% compounded continuously, and a \$2 dividend is to be paid after half a year. For a long
forward position with delivery in one year, find its value after 9 months if the stock price at that
time turns out to be a) \$49, b) \$51.
References
[1] M. Capiński, T. Zastawniak, Mathematics for Finance, 2nd ed., Springer, (2011), Ch.4
[2] D. G. Luenberger, Investment Science, 2nd ed., Oxford University Press, (2014), Ch.12
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