ST302 Stochastic Processes 2019
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January 2019 Examination
ST302
Stochastic processes
1. Identify the transient and recurrent states, and the irreducible closed sets for the Markov chains with following transiton matrices. [8 marks]
|
1 |
2 |
3 |
4 |
5 |
|
1 |
2 |
3 |
4 |
5 |
1 |
1 |
0 |
0 |
0 |
0 |
1 |
0 |
0 |
0.5 |
0 |
0.5 |
2 |
0.2 |
0 |
0 |
0.8 |
0 |
2 |
0 |
0.2 |
0 |
0.8 |
0 |
3 |
0 |
0.3 |
0.3 |
0.4 |
0 |
3 |
0 |
0.2 |
0.3 |
0.5 |
0 |
4 |
0 |
0.6 |
0 |
0.4 |
0 |
4 |
0 |
0.6 |
0.4 |
0 |
0 |
5 |
0.3 |
0 |
0 |
0 |
0.7 |
5 |
0.3 |
0 |
0 |
0 |
0.7 |
2. consider a Markov chain (xn )nΣ0 with the countable state space {0)1)2). . .} and the following transition probabilities:
p(i)i + 1) = p)i > 0;
p(i)i - 1) = q)i > 1;
p(i)i) = 1 - p - q)i > 1)
p(0)0) = 1 - p)
where p 持 0 and q 持 0. Let Ⅴi := min{n > 0 : xn = i} be the irst time that the chain visits i.
a) Explain why this Markov chain is irreducible. Is it also aperiodic? show your reasoning. [5 marks]
b) Let a)b and i belong to the state space of x such that a < i < b. without using the optional stopping Theorem show that
i - a
b - a .
[8 marks] c) Assume p < q and show that the limiting distribution π is given
by
π(n) = n ) n > 0. [8 marks]
3. Let s be a random walk adapted to (大n )nΣ0 such that
P (sn+1 = sn + 1|大n ) = p)
P (sn+1 = sn - 1|大n ) = q)and
P (sn+1 = sn |大n ) = 1 - p - q)
for some p 持 0 and q 持 0. Deine
φ(j) := j - 1.
a) show that φ(s) is a martingale with respect to (大n )n>0. [4 marks] b) Deine Mn = sn - n(p - q) for n > 0. show that M is martingale with respect to (大n )n>0 . [4 marks]
c) Assume p q and let T = min{n > 0 : sn = b or sn = 0}. show that whenever 0 参 i 参 b we have
E[T |s0 = i] = . [8 marks]
4. Let N be a poisson process with intensity λ and adapted to some iltration (大t )t>0 .
a) show that Nt - λt and (Nt - λt)2 - λt are martingales with respect to (大t )t>0 . [8 marks]
b) consider the time of the n-th arrival Tn := inf{t > 0 : Nt = n} and let m 持 n. show that
P (Tn 参 t|N1 = m) = k(m))(1 - t)m-ktk , At E [0, 1]. [8 marks]
c) suppose there exists another adapted poisson process Z with in- tensity μ, which is independent of N. Let xt = Zt +Nt and deine τ := inf{t > 0 : xt = 1}. show that
P (Nτ = 1) = .
(Hint: consider P (T1 < s1 ) where T1 and s1 are the irst arrivals for N and Z respectively and recall that the time of irst arrival
for a poisson process has exponential distribution.) [6 marks]
5. Let B denote a Brownian motion with B0 = 0.
a) state the deinition of a Brownian motion. [4 marks]
t ) and (Bt - 1o(t) sdBs -
be written as stochastic integrals with respect to B . [6 marks] c) Let δ 持 2 and consider X, which solves the following SDE:
Xt = 1 + lot 2^Xs dBs + δt.
integral with respect to B. (Take that X never hits 0 for granted.) [5 marks]
d) Solve the following stochastic diferential equation:
dyt = aytdt + (b(t) + cyt )dBt ,
where yo = 0. (Hint: Try a solution of the form ztHt where zt = exp(cBt + (a - adapted process F and G which need to be determined.) [8 marks]
e) It is well known that for any deterministic function f(t) the ran-dom variable1a(b) f(s)dBs is normally distributed. Find its mean and variance. [2 marks]
f) use Feynman-kac representation result to ind a function F (t, ①) that solves
?F - a① ?F + 1 σ 2 ?2 F = 0
F (T, ①) = exp(T①),
where a, T and σ are real constants. You may want to use the fact that E[euz ] = random variable. [8 marks]
2023-08-18