E column vector with all 1s we can write this as i rg

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Unformatted text preview: gives ✓ ◆x ✓ ◆x q q Px (V0 = 1) = 1 and Px (V0 < 1) = . (1.23) p p To see that the form of the last answer makes sense, note that to get from x to 0 we must go x ! x 1 ! x2 . . . ! 1 ! 0, so Px (V0 < 1) = P1 (V0 < 1)x . 49 1.9. EXIT TIMES 1.9 Exit Times To motivate developments we begin with an example. Example 1.45. Two year college. In Example 1.39 we introduced a Markov chain with state space 1 = freshman, 2 = sophomore, G = graduate, D = dropout, and transition probability 1 2 G D 1 0.25 0 0 0 2 0.6 0.2 0 0 G 0 0.7 1 0 D 0.15 0.1 0 1 On the average how many years does a student take to graduate or drop out? Let g (x) be the expected time for a student starting in state x. g (G) = g (D) = 0. By considering what happens on one step g (1) = 1 + 0.25g (1) + 0.6g (2) g (2) = 1 + 0.2g (2) where the 1+ is due to the fact that after the jump has been made one year has elapsed. To solve for g , we note that the second equation implies g (2) = 1/0.8 = 1.25 and then the first that...
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