Published online by Cambridge University Press: 14 July 2016
When {X n } is an irreducible, stationary, aperiodic Markov chain on the countable state space X = {i, j,…}, the study of long-range dependence of any square integrable functional {Y n } := {y X n } of the chain, for any real-valued function {y i : i ∈ X }, involves in an essential manner the functions Q ij n = ∑r=1 n (p ij r − πj ), where p ij r = P{X r = j | X 0 = i} is the r-step transition probability for the chain and {πi : i ∈ X } = P{X n = i} is the stationary distribution for {X n }. The simplest functional arises when Y n is the indicator sequence for visits to some particular state i, I ni = I {X n=i} say, in which case limsupn→∞ n −1var(Y 1 + ∙ ∙ ∙ + Y n ) = limsupn→∞ n −1 var(N i (0, n]) = ∞ if and only if the generic return time random variable T ii for the chain to return to state i starting from i has infinite second moment (here, N i (0, n] denotes the number of visits of X r to state i in the time epochs {1,…,n}). This condition is equivalent to Q ji n → ∞ for one (and then every) state j, or to E(T jj 2) = ∞ for one (and then every) state j, and when it holds, (Q ij n / πj ) / (Q kk n / πk ) → 1 for n → ∞ for any triplet of states i, jk.