A method for calculating time correlation functions

A device that makes it possible to express a two-time Green's function in terms of higher correlation functions is used to develop a method for constructing a system of equations for the energies of correlation functions. Solutions of the system are considered in the limiting cases of long ($\vert t\vert\to\infty$) and short ($t\to\infty$) times. A calculation of the time of spin-spin relaxation in a rigid lattice by the proposed method yields a finite expression, while the ordinary procedure of decoupling leads to a trivial result.