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MTH3251/ETC3510/ETC5351 Practice Exam 1
发布时间:2022-06-15
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MTH3251/ETC3510/ETC5351
Practice Exam 1
Throughout the paper, Bt , t > 0, denotes the Brownian motion started at 0.
1. In a game of roulette, there are 37 squares, 18 of which are red and 18 black, 1 square is neither. Suppose you bet on a colour with $1 each bet. Denote by Yn the outcome of the n-th bet with distribution: Yn = 1 with probability 18/37 and Yn = _1 with probability 19/37. Let Xn be the fortune after n bets. You start with $10, X0 = 10, and play until no money is left or you reach $20.
(a) Show that the process Mn = (19/18)Xn , n = 0, 1, . . ., is a martingale.
(b) Show that the duration of the game τ is a stopping time.
(c) Derive the probability of reaching $20 before ruin. Hint: use (without justification) the Optional Stopping Theorem.
2. Denote by Sn , n = 0, 1, 2, . . ., a Random Walk Sn = S0 + Yi , where S0 > 0, is a constant, and the Yi ’s are independent normally distributed random variables N(µ, σ2 ) with µ > 0.
(a) Find a constant C > 0 such that the process Mn = e一CSn , n = 0, 1, 2, . . ., is a martingale.
(b) Let T denote the first time the random walk Sn takes a negative value. Show that for any n, P (T < n) < e一CS0 .
(c) Formulate the Discrete Time Risk Model in Insurance with normally distributed claims. Give a bound on the probability of ruin in terms of the initial funds by using the Random Walk model.
(b) Give the joint distribution of B1 and B3 .
(c) By quoting an appropriate result, find the conditional expectation E(B3 IB1 ).
(d) State with reason whether the process Xt = _Bt is also a Brownian motion.
(e) State with reason whether the process Xt = Bt(3) is also a Brownian motion.
4. The process Xt is given by Xt = 0(t) sdBs , t > 0.
(a) Show that Xt , t > 0 is a martingale. Give the distribution of Xt .
(b) Show that the process eXt 一 t3 , t > 0, is a martingale. Hint: use Itˆo’s formula and check the conditions for Itˆo’s integral to be a martingale.
5. Assume the following market model. The stock price today is $2 and in the next period it can be $4 or $1. The simple interest rate is over this period is 10%, r = 1.1. State what is meant by arbitrage and show that there is no arbitrage in this model.
6. Assume the following market model. The stock price evolves according to St = e0 2t+0 2B..t , where Bt is Brownian motion, 0 < t < 1. The continuously compounding interest in a saving account over this period is 0.1.
(a) Let Xt denote the discounted stock price process. Find a such that e一atXt is a martingale and show that there is only one such a.
(b) Give the distribution of S1 under the equivalent martingale probability measure. Express the price at time t = 0 of a call option on this stock with exercise price $10 and expiration time T = 1 as an expectation of its payoff . Explain briefly how to obtain the Black-Scholes formula from that expression.
7. Assume that the spot rate rt follows Vasicek’s model, given by
t
rt = 0.1 + 0.1e一t + e一t e dBss .
0
(a) Give the distribution of rt and state its mean and variance.
(b) Derive the stochastic differential equation for rt .
(c) Explain briefly what is meant by mean reversion and give the level to which the spot rate rt reverts.
Find the covariance function of the process rt , t > 0. Hint: Show that the process Xt = rt et _ 0.1et _ 0.1 is a martingale thus obtain the covariance function of Xt .
8. Explain how to do the following and justify your choice.
(a) How to simulate an observation from a Pareto distribution with the cu- mulative distribution function F (x) = 1 _ 2/x, x > 2, and F (x) = 0 for x < 2.
(b) How to simulate an observation from a bivariate Normal distribution with
mean 0 and covariance matrix Σ = ┌ 1(1) 4(1) ┐ . In other words, simulate a
pair of correlated normal random variables with these means and covari- ances.
How to evaluate the integral 01
by simulations.