# Math Homework 7 Solutions

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1 Math Homework 7 Solutions 1 Find the invariant distribution of the transition matrix: P = p 1 p 0 The equation πp = π, and the normalization condition π 1 + π 2 + π 3 = 1 give the system: π 1 = pπ 3 π 2 = π π 2 + (1 p)π 3 π 3 = 1 3 π 2 π 1 + π 2 + π 3 = 1 We can disregard one of the first 3 equations, since they are dependent; let us discard the second equation It is now a simple matter to solve the non-homogeneous linear system The result is π = ( p 4 + p, p, p 2 Exercise 184, page 46 of textbook There are 6 states for this problem, corresponding to the 6 permutations of the books on the shelf I will represent a state by triples such as 312, or 213, and will order them in the following (arbitrary) order: ) s 1 = 132, s 2 = 312, s 3 = 213, s 4 = 123, s 5 = 321, s 6 = 231 For this ordering of states, the transition probabilities matrix is P =

2 The transition diagram is given if figure 1 The probability that on day n the student finds the books in the order 123 is now p n = P (X n = 4) As n goes to, this number must converge to π 4, where π is the stationary probability distribution of P Figure 1: Diagram for the book shuffling exercise Note that each state leads to itself in one step, so the chain is necessarily aperiodic It is also clear from the diagram that the chain is irreducible From this, and a theorem shown in class, we obtain P (X n = s) π s, where π is the unique stationary distribution for the chain The condition πp = π consists of the the six equations below, where I write x i = π i : x 1 = (x 1 + x 2 + x 5 ) x 2 = (x 1 + x 2 + x 4 ) x 3 = (x 1 + x 3 + x 4 ) x 4 = (x 3 + x 4 + x 6 ) x 5 = (x 3 + x 5 + x 6 ) x 6 = (x 2 + x 5 + x 6 ) We can discard the last equation since these are linearly dependent equations, and we add the normalization condition x 1 + x 2 + x 3 + x 4 + x 5 + x 6 = 1 Solving this system by hand is a rather tedious but elementary problem After quite a bit of work, I got the following answer For convenience, define η i = α i /(1 α i ) Then x 1 = η 1, x 2 = η 3, x 3 = η 2, x 4 = η 1, x 5 = η 3, x 6 = η 2 2

3 Therefore, the probability that the order of books is 123 on day n converges to π 4 = 1 Note that if = = = 1/3, the probabilities simplify to π s = 1/6 for all s 3 Exercise 191, (a), (b) and (c), page 51 of textbook In each case, determine whether the stochastic matrix is reversible (a) I claim that the stochastic matrix ( ) 1 p p P = q 1 q is reversible First, as already noted ( in class ) in different notation, P has invariant distribution π = q p+q, p p+q Now observe that π 1 p 12 = pq p + q = π 2p 21 The other cases of π i p ij = π j p ji are trivial since they involve i = j Therefore, P is reversible (b) Consider now the stochastic matrix P = 1 p 0 p 0 p 1 p p 1 p 0 I claim that it is reversible if and only if p = 1/2 Note that P is doubly-stochastic, ie, the entries in each column also add up to 1, so π = (1/3, 1/3, 1/3) (all entries equal) is an invariant vector for P It is unique by theorem 176, since the chain is irreducible (and recurrent since it has a finite number of states) The condition π i p ij = π j p ji, for all i, j, imply p ij = p ji for all i, j, since all the π i are equal Therefore, P is reversible if and only if P is symmetric, ie, it is equal to its own transpose But this is only the case if p = 1 p, or p = 1/2 (c) Let the set of states be {0, 1,, N} and p ij = 0 if j i 2 I claim that P is reversible in this case Again, in this case, there is a unique invariant probability distribution, which is given by the following set of equations: π 0 = π 0 p 00 + π 1 p 10 π 1 = π 0 p 01 + π 1 p 11 + π 2 p 21 π 2 = π 1 p 12 + π 2 p 22 + π 3 p 32 π N = π N 1 p N 1,N + π N p NN 3

4 It suffices to show that π i p i,i+1 = π i+1 p i+1,i for i = 0, 1,, N 1 In all other cases the reversibility condition holds trivially, either because p ij are 0 or because i = j We prove reversibility by induction From the first equation, we have π 1 p 10 = π 0 π 0 p 00 = π 0 (1 p 00 ) = π 0 p 01 Suppose that the equation holds, and look at the equation π i 1 p i 1,i = π i p i,i 1 π i = π i 1 p i+1,i + π i p ii + π i+1 p i+1,i We combine these two equations with p i,i 1 + p ii + p i,i+1 = 1 and obtain: π i p i,i 1 + π i+1 p i+1,i = π i 1 p i 1,i + π i+1 p i+1,i = π i (1 p ii ) Canceling the term π i p i,i 1 gives = π i (p i,i 1 + p i,i+1 ) = π i p i,i 1 + π i p i,i+1 π i+1 p i+1,i = π i p i,i+1 This is the induction step The last step is shown similarly Therefore, the transition matrix P in this case is reversible 4 (Book shuffling) Do a simulation of the situation described in exercise 184 More precisely, assume that each morning the student takes one of n = 3 books from his shelf, each with equal probabilities, independently of the previous day s choice In the evening he replaces the book at the left-hand end of the shelf We want to find how often the shelf returns to the initial state In your simulation, assume that the books are initially ordered as 1, 2,, n from left to right Each move consists of a permutation of the form (1, 2,, i 1, i, i + 1,, n) (i, 1, 2,, i 1, i + 1,, n) Repeat the operation times (This took about 9 seconds on my laptop) The following program gave for the relative number of occurrences of the initial state: f = 01660, or 1/f = The correct value is 1/f = 6 4

5 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% tic rand( seed,317) n=3; %number of books on the shelf N=100000;%total number of shufflings s=1:n; %initial configuration of books in shelf f=0; %fraction of times state (1 2 n) is observed s_new=s; for k=1:n i=ceil(n*rand); a=find(s_new==i); s_new=[s_new(a) s_new(1:a-1) s_new(a+1:n)]; f=f+(sum(s_new==s)==n); end f=f/n; toc %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 5

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