Math A7700, Exam 1
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Math A7700, Exam 1
Oct. 5th, 2016
1. Consider the Markov chain on I = {0, 1, 2, . . . , N} with the following transition matrix:
p(0, 1) = 1 ,
p(i, i + 1) = 1/2 ,
p(N, N - 1) = 1 , p(i, i - 1) = 1/2
for 1 s i s N - 1 .
All other entries of p are zero.
(a) (3 points) Find the stationary distribution π .
(b) (2 points) If I start the chain at X0 = 0, what is the expected return time? That is, what is E0T0 ?
2. (a) (3 points) In class, we proved that if we have a Markov chain on a finite state space I, we can decompose I as a disjoint union
I = T u R1 u . . . u Rk ,
where T is a set of transient states and each Rj is a closed irreducible set of recurrent states.
Find these sets explicitly for the Markov chain on I = {1, 2, 3, . . . , 7} with
1 2 3 4 5 6 7
0.7 0.1 0 0 0.6 0 0 |
0 0.2 0 0 0 0 0 |
0 0.3 0.5 0 0 0 0 |
0 0.4 0.3 0.5 0 0 1 |
0.3 0 0.2 0 0.4 0 0 |
0 0 0 0.5 0 0.2 0 |
0 0 0 0 0 0.8 0 |
(b) (3 points) Find limn→o pn (5, 1).
3. I have two umbrellas, some at my office and some at home. Every day I walk to the office in the morning and back home in the evening (a total of two trips per day); if it is raining, I will take an umbrella if one is there. Otherwise, I get wet. Assume that independently of the past, it rains on each trip with probability 1/3. Let Xn be the number of umbrellas at my current location after the nth trip; then Xn is a Markov chain.
(a) (2 points) Find the transition probability matrix for this Markov chain.
(b) (4 points) What is the limiting fraction of the time I get wet?
4. Suppose we have an irreducible Markov chain Xn on I = {1, 2, . . . , N} (where N > 1)
with transition probability matrix p. Let S0 = 0 and for each k > 0, let Sk+1 = inf{n > Sk : Xn XSk} .
For each m > 0, define Zm = XSm .
(a) (3 points) Show that Zm is also a Markov chain — you don’t have to write a huge amount, but please make the argument / steps clear.
(b) (3 points) By part (a), Zm has some transition probability matrix . Find in terms of p.
2022-10-25