We study the Master equation with time--dependent coefficients, a linear
kinetic equation for the Markov chains or for the monomolecular chemical
kinetics. For the solution of this equation a path summation formula is proved.
This formula represents the solution as a sum of solutions for simple kinetic
schemes (kinetic paths), which are available in explicit analytical form. The
relaxation rate is studied and a family of estimates for the relaxation time
and the ergodicity coefficient is developed. To calculate the estimates we
introduce the multi--sheeted extensions of the initial kinetics. This approach
allows us to exploit the internal ("micro")structure of the extended kinetics
without perturbation of the base kinetics.Comment: The final journal versio