A master equation is a phenomenological set of first-order differential equations describing the time evolution of (usually) the probability of a system to occupy each one of a discrete set of states with regard to a continuous time variable t. The most familiar form of a master equation is a matrix form: d P → → d t = A P → → , where P → → is a column vector, and A is the matrix of connections. The way connections among states are made determines the dimension of the problem; it is either