Lumpability and marginalisability for continuous-time Markov chains

Abstract

We consider lumpability for continuous-time Markov chains and provide a simple probabilistic proof of necessary and sufficient conditions for strong lumpability, valid in circumstances not covered by known theory. We also consider the following marginalisability problem. Let {X{t)} = {(X 1(t), X 2(t), · ··, Xm (t))} be a continuous-time Markov chain. Under what conditions are the marginal processes {X 1(t)}, {X 2(t)}, · ··, {Xm (t)} also continuous-time Markov chains? We show that this is related to lumpability and, if no two of the marginal processes can jump simultaneously, then they are continuous-time Markov chains if and only if they are mutually independent. Applications to ion channel modelling and birth–death processes are discussed briefly.