MTH3251— Assignment 2 Semester 2, 2022
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MTH3251— Assignment 2
Semester 2, 2022
In what follows (Bt), t > 0 denotes the Brownian motion process started at zero.
1. Let Xt = µt + σBt, with some constants µ and σ, and the stock price be given by St = eXt .
(a) Derive the stochastic differential equation for St .
(b) Show that percentage price changes St/Su for u < t are independent of
Sr, r < u.
(c) Give the mean and the variance of St . Hint: use the moment generating function of Normal distribution.
(d) Give the values of µ for which the process St is a martingale, and show why it is a martingale. Hint: use martingale property of stochastic integral.
(e) Calculate the mean and the variance of the stochastic integral +T et尸BtdBt .
[25 points]
2. Let the wealth of a company at time t be modelled by Xt = x + µt + σBt, where x, σ are positive numbers. In this question we consider two different cases: positive drift µ > 0 and negative drift µ < 0. Let T = T+ be first time when the process (Xt) hits 0. Denote the probability of ruin Ψ(x) = P (T < o).
(a) Let µ > 0. Show that Ψ(x) < e2R北 with R = 2µ/σ2 .
Hint: Show that Mt = e2RXt is a martingale. Then stop it at T A N, and argue just like for ruin probabilities in a Random Walk.
(b) Show that P (T = o) > 0 when µ > 0. Deduce the value of E(T).
(c) Let V be a positive continuous random variable. Show that
E(V) = +o P (V > t)dt.
Hint: E(V) = +o tfV (t)dt. Write t = du, then change the order of
integration in the double integral.
(d) Let µ < 0. Show that E(T) < o. Deduce that Ψ(x) = 1.
Hint: Show that P (T > t) < P (Xt > 0), then use (b) together with the bound for P (Xt > 0) given in Q2 of Assignment 1.
[25 points]
3. Let the process (Xt), t > 0, solve the SDE
dXt = _Xtdt + dBt, X+ e R.
(a) Show that the process Yt = etXt has independent Gaussian increments,
and give the distribution of the increments over the time interval [s, t].
(b) Show that the process (Xt) has Gaussian increments, but they are not
independent.
(c) State with reason whether the process (Xt) is a Gaussian process and give its mean and covariance functions.
(d) Derive the conditional expectation E(XtlXs) for s < t.
(e) Show that if X+ has distribution N (0, 1/2) and is independent of the pro-
cess (Bt), then for any time t, Xt has N (0, 1/2) distribution.
[25 points]
4. Let Xt = (1 _ t) dBs, and Yt = dBs for 0 < t < 1.
(a) Show that the process (Yt) is well defined for t < 1, and it is a martingale.
(b) Show that the process (Xt), 0 < t < 1, solves the stochastic differential
equation
Xt
(c) State with reason whether the process (Xt) is a Gaussian process.
(d) Calculate the mean and covariance functions of the process (Xt).
(e) Let Zt = Bt _ tB1, 0 < t < 1. State with reason why processes (Xt) and (Zt) is the same Gaussian process (called Brownian Bridge).
Hint: Calculate the mean and covariance functions for (Zt). Then use Q2 of Homework 4.
[25 points]
2022-09-16