71. It follows from Theorem 4.2 that for a time reversible Markov chain PijPjkPki = PikPkjPji, for all i,j,k It turns out that if the state space is finite and Pij > 0 for all i,j , then the preceding is also a sufficient condition for time reversibility. [That is, in this case, we need only check Equation (4.26) for paths from i to i that have only two intermediate states.] Prove this.