4106-08-Notes-MCII

4106-08-Notes-MCII - Copyright c 2008 by Karl Sigman 1...

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Unformatted text preview: Copyright c 2008 by Karl Sigman 1 Communication classes and irreducibility for Markov chains For a Markov chain with state space S , consider a pair of states ( i,j ). We say that j is reachable from i , denoted by i → j , if there exists an integer n ≥ 0 such that P n ij > 0. This means that starting in state i , there is a positive probability (but not necessarily equal to 1) that the chain will be in state j at time n (that is, n steps later); P ( X n = j | X = i ) > 0. If j is reachable from i , and i is reachable from j , then the states i and j are said to communicate , denoted by i ←→ j . The relation defined by communication satisfies the following conditions: 1. All states communicate with themselves: P ii = 1 > 0. 1 2. Symmetry: If i ←→ j , then j ←→ i . 3. Transitivity: If i ←→ k and k ←→ j , then i ←→ j . The above conditions imply that communication is an example of an equivalence relation, meaning that it shares the properties with the more familiar equality relation “ = ”: i = i . If i = j , then j = i . If i = k and k = j , then i = j . Only condition 3 above needs some justification, so we now prove it for completeness: Suppose there exists integers n , m such that P n ik > 0 and P m kj > 0. Letting l = n + m , we conclude that P l ij ≥ P n ik P m kj > 0 where we have formally used the Chapman-Kolmogorov equations. The point is that the chain can go from i to j by first going from i to k ( n steps) and then (independent of the past) going from k to j (an additional m steps). If we consider the rat in the open maze, we easily see that the set of states C 1 = { 1 , 2 , 3 , 4 } all communicate with one another, but state 0 only communicates with itself (since it is an absorbing state). Whereas state 0 is reachable from the other states, i → 0, no other state can be reached from state 0. We conclude that the state space S = { , 1 , 2 , 3 , 4 } can be broken up into two disjoint subsets, C 1 = { 1 , 2 , 3 , 4 } and C 2 = { } whose union equals S , and such that each of these subsets has the property that all states within it communicate. Disjoint means that their intersection contains no elements: C 1 ∩ C 2 = ∅ . A little thought reveals that this kind of disjoint breaking can be done with any Markov chain: Proposition 1.1 For each Markov chain, there exists a unique decomposition of the state space S into a sequence of disjoint subsets C 1 ,C 2 ,... , S = ∪ ∞ i =1 C i , in which each subset has the property that all states within it communicate. Each such subset is called a communication class of the Markov chain. If we now consider the rat in the closed maze, S = { 1 , 2 , 3 , 4 } , then we see that there is only one communication class C = { 1 , 2 , 3 , 4 } = S : all states communicate. This is an example of what is called an irreducible Markov chain....
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This note was uploaded on 10/20/2010 for the course IEOR 4106 taught by Professor Whitt during the Fall '08 term at Columbia.

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4106-08-Notes-MCII - Copyright c 2008 by Karl Sigman 1...

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