AMS 513 Financial Derivatives and Stochastic Calculus Final Exam Spring 2021
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Final Exam Spring 2021
AMS 513
Financial Derivatives and Stochastic Calculus
Problem 1. (20p) (Brownian Motion and Time Reversal)
Let (Ω , F, P) be a probability space and let Bt , for t > 0, be a Brownian motion.
(i) (5p+5p=10p) Show that
Cov(Bs , Bt ) = min(t, s),
and that
Corr(Bs , Bt ) = 《 .
(Hint: Use the properties in the definition of the Brownian motion.)
(ii) (10p) Show that the following process defined via time reversion
Wt = B| _ B| ¥ t
is also a standard Brownian motion.
(Hint: Show the three properties from one of the definitions of the Brownian motion, use the first result from (i) above.)
Problem 2. (20p) (Minimum of Brownian Motion)
Let (Ω , F, P) be a probability space and define
MT = max Bt
t桂卜1,Ti
where Bt , for t > 0, is a Brownian motion. In the Practice Final Exam you showed, using the reflection principle, that for all T > 0
,)))Φ( !T(x)) _ Φ( x¥!a ) P(MT < a, BT < x) = (Φ( !T(a)) _ Φ(_ !T(a))
)
)
),0
a > 0, x < a
a > 0, x > a
a < 0
(1)
where Φ is the standard normal cdf; and that the joint probability density function of (MT , BT ) is
,
(
Now, define
T(2|)!2π(2a¥) e ¥ 达(|) | 达幺 0达
0
a > 0, x < a
otherwise
(2)
mT = min Bt
t桂卜1,Ti
where Bt , for t > 0, is a Brownian motion, and modify your argument to show that for all T > 0,
(i) (10p)
,)))Φ(_ !T(x)) _ Φ(_ x¥!b ) P(mT > b, BT > x) = (Φ(_ !(b)T ) _ Φ( !(b)T )
)
)
),0
b < 0, b < x
b < 0, b > x
b > 0
(3)
(ii) (10p) the joint probability density of (mT , BT ) is
fmr,Br (b, x) = ( T!2πT e
,0
b < 0, b < x
otherwise
(4)
Hints:
●
mT = min Bt = _ max (_Bt )
t桂卜1,Ti t桂卜1,Ti
● the Reflection Principle gives us that for τa = inf {t > 0 : Bt = a} we have P(τa < T, BT < x) = P(τa < T, BT > 2a _ x)
Problem 3. (20p) (Ito’s Integrals)
Let (Ω , F, P) be a probability space and let {Wt : t > 0} be a standard Wiener process.
(i) (10p) Compute the integral
.1 t 2Ws dWs .
Hint: Use the Ito’s lemma or the definition of Ito’s integral.
(ii) (10p) Using integration by parts, show that
.1 t Ws ds = .1 t (t _ s)dWs
and prove that
E ┌.1 t Ws ds┐ = 0 and E ┌ ╱.1 t Ws ds、2 ┐ = .
Hint: The integration by parts formula is given by
. u ╱ 、ds = uv _ . v ╱ 、ds.
Problem 4 (20p) (Digital Option - Probabilistic Approach)
Let {Wt : t > 0} be a P-standard Wiener process on the probability space (Ω , F, P) and let the stock price St follow a GBM with the following SDE
dSt = St µdt + St σdWt ,
where µ is the drift parameter, σ > 0 is the volatility parameter, and let r > 0 denote the risk-free interest rate.
A digital (or cash-or-nothing) call option is a contract that pays $1 at expiry time T if the spot price ST > K and nothing if ST < K. In contrast, a digital (or cash-or-nothing) put pays $1 at expiry time T if the spot price ST < K and nothing if ST > K .
(i) (5p) Find an equivalent martingale measure Q under which the discounted stock price e ¥ rt St is a martingale (discuss why this is a martingale).
(ii) (5p) By denoting Cd (St , t; K, T) and Pd (St , t; K, T) as the digital call and put option
prices, respectively at time t, for t < T show that
Cd (St , t; K, T) = e ¥ r|T ¥ t0Q (ST > K | Ft )
and
Pd (St , t; K, T) = e ¥ r|T ¥ t0Q (ST < K | Ft ) .
(iii) (5p) Using Ito lemma find the distribution of ST given Ft under Q, and show, using
the risk-neutral valuation approach from (i) and (ii) above, that
Cd (St , t; K, T) = e ¥ r|T ¥ t0Φ(d ¥ ) and Pd (St , t; K, T) = e ¥ r|T ¥ t0Φ(_d ¥ ),
where
d ¥ = and Φ(x) = .¥x& e ¥ 达(|)u达 du.
(iv) (5p) Verify that the put-call parity for a digital option is
Cd (St , t; K, T) + Pd (St , t; K, T) = e ¥ r|T ¥ t0 .
Problem 5. (20p) (Brownian Bridge Process)
Let (Ω , F, P) be a probability space and let {Wt : t > 0} be a standard Wiener process. Suppose Xt follows the Brownian bridge process with SDE
y _ Xt
where the diffusion is conditioned to be at y at time t = 1.
(i) (10p) By applying Taylor’s formula to Yt = and taking integrals, show that under an initial condition X1 = x, for 0 < t < 1
Xt = yt + (1 _ t) ╱x + .1 t dWs 、.
(ii) (10p) Using the properties of stochastic integrals on the above expression, find the
mean and variance of Xt , given X1 = x.
Hints:
(i.1) Using Taylor’s formula we can expand Yt = as
dYt = dt + dXt + (dt)2 + (dXt )2 + . . .
(i.2) Compute the appropriate partial derivatives and use the properties of Ito’s processes
to get that
dYt = _ ╱ 、dWt
(i.3) Take the integrals and solve for Xt .
(ii.1) Use the properties of stochastic integrals to get the mean and the variance of Xt .
2022-05-11