ST302 Stochastic Processes 2017
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2017 LT Examination
ST302
Stochastic processes
1. Let sn be a simple random walk: s0 = s, sn = s0 + y1 + . . . + yn , where yk are iid with r(y1 = 1) = 1 - r(y1 = -1) = P e (0, 1). suppose 大0 = {Q, Ω} and 大n = σ(y1 , . . . , yn ). Deine
( 1 sign(①) =〈 0
( 1
if ① 持 0,
if ① = 0,
if ① < 0,
a) Let Mn = sn - nμ and Zn = (sn - nμ)2 - nσ 2 , where μ = 2P - 1 and σ2 = E[(yn - μ)2]. show that Z and M are martingales. [6 marks]
b) suppose s 持 0 is an integer, μ < 0, and let T = inf{n : sn = 0}. show that E[T] = - and Ⅴ aT(T) = -s . (Hint: You may use The optional stopping Theorem without justiication.) [6 marks]
c) show that
E [(|sn+1 | - |sn |)1[Sπ持0] I 大n]= 1[Sπ持0](2P - 1). [3 marks]
d) show that
E [(|sn+1 | - |sn |)1[Sπ<0] I 大n]= 1[Sπ<0](1 - 2P). [3 marks]
e) It is known that there exist a martingale M with M0 = 0 and a predictable process A such that |s| = |s| + M + A. compute A in this decomposition using your results from parts c) and d) . (Hint: First compute An - An-1 assuming the decomposition and using the fact that A is predictable and M is a martingale. Next
observe that An =Σk(n)=1 Ak - Ak-1.) [8 marks]
2. Identify the closed sets, and transient and recurrent states in the follow- ing Markov chain in discrete time. Is the chain irreducible? [5 marks]
1 2 3 4 5
1 0 0 .5 .5 0
2 0 .5 .5 0 0
3 .3 .2 .5 0 0
4 0 0 0 .4 .6
5 0 0 0 0 1
3. a) Bonnie,s restaurant business luctuates in successive years between three states: 0 (bankruptcy), 1 (verge of bankruptcy) and 2 (sol- vency). The transition probabilities of evolving from state to state are given by the following matrix:
0 1 2
0 1 0 0
1 .5 .25 .25
2 .25 .5 .25
Is the chain irreducible? Is it aperiodic? what is the expected number of years until Bonnie,s restaurant goes bankrupt if she starts from the state of solvency? (Hint: consider the function h(i) = E[T |xo = i], where T = inf{n : xn = 0}.) [8 marks] b) Now suppose that Bonnie has a partner, clyde, who infuses her
business with cash whenever state 0 is reached in order to return her to solvency immediately. Therefore the new transition matrix for the states of Bonnie,s business is given by the following:
0 1 2
0 0 0 1
1 .5 .25 .25
2 .25 .5 .25
Is the new chain irreducible? Is it aperiodic? what is the ex- pected number of years between the cash infusions from clyde? (Hint: Think about whether this new chain possesses a limiting distribution.) [8 marks]
4. Let N be a poisson process with intensity λ .
a) show that, for 0 < s < t,
P[N (s) = k|N (t) = n] = (k(n)k(1 - n-k .
[5 marks] b) suppose (Yn )n>1 is an iid sequence of random variables indepen-
dent of N. Let x be a compound poisson process deined by
xt = Y1 + Y2 + . . . + YNt .
show that there exists a function c(a) such that
E[eaxt] = ec(a)t.
(Hint: You may want to use the fact that if f is a continuous function such that f (t + s) = f (s)f (t) for all s and t, then, either f = 0 or f (t) = exp(at) for some constant a.) [5 marks]
c) show that M deined by
Mt = ,
is a martingale with respect to the iltration generated by X, where c(a) is as in part b). [5 marks]
5. Let B be a standard Brownian motion. For this question you may ind it useful to note that 石Z3 = μ3 + 3μσ2 if Z … N (μ, σ2 ).
a) prove that the following processes can be written as stochastic integrals with respect to B:
i)
exp t) cos(aBt ) - 1; [4 marks]
ii)
(Bt + sin(at)) exp (-alot cos(as)dBs - t cos2 (as)ds) . [5 marks]
b) compute for 0 < t < T
i)
石 [loT Bs(3)ds' 大t] ; [5 marks]
ii)
石 l (loT sdBs )2 '大t] ; [5 marks]
c) solve the following stochastic diferential equation:
dyt = (ayt + β)dt + (σ(t) + cyt )dBt ,
with Yo = g E R. (Hint: Try a solution of the form ztHt where zt = exp(CBt + (a - 2 )t)) and dHt = F (t)dt + G(t)dBt for some adapted process F and G which need to be determined.) [8 marks]
d) It is well known that, for any deterministic function f , the random variable'a(b) f (s)dBs is normally distributed. what are its mean and variance? [2 marks]
e) use the Feynman-kac representation result to ind a function F (t, 从) that solves
+ (b + a从) ?(?)从(F) + σ 2?(?)从2(2 F) = 0,
F (T, 从) = k从3 ,
where a,b,k and σ are real constants. [9 marks]
2023-08-19