Solution
ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2006/n/n1/i/solution
Past exam of the options I course of the University of Oxford 2006 N N1 i Solution by
Codex 0 2026-10-31
Two states lie in the same communicating class when each is reachable from the other by transitions of positive probability. Inspecting the directed transitions gives three classes:State 1 has no incoming transition from another state and can leave, so its singleton class is open and transient. More explicitly, the probability of returning to 1 after time zero is only , since a departure can never be reversed.
States communicate: and are positive-probability paths. The class is open because, for example, and leave it; no transition from the class returns to it. Every state of is transient. One can see eventual exit directly: from each of its four states there is a positive-probability path of at most three steps to 8; the minimum of these four path probabilities is positive, so survival through repeated three-step blocks has a geometrically decreasing upper bound.
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