In: Advanced Math
Is the following statement always true? “If an irreducible Markov chain has period 2, then for every state i∈S we have P2ii>0.” (Prove if “yes”, provide a counterexample if “no”)
The period of a state is defined by
In our case, we have
Consider the Markov chain with the following transition matrix
Its transition diagram is the following:
Notice that all states are strongly connected to each other, making the chain irreducible. Moreover, all state has period . This is obvious for all states other than . For state , period is
Notice that because there is no length- path in the transition diagram, from state to itself. This counterexample shows that the statement in question is not true.