Calculation of single-particle strengths of states

The Green's function method is used to calculate the fragmentation of single-particle states over nuclear levels. Chains of equations are obtained for these functions in a model based on the interaction of quasiparticles with phonons. It is noted that truncation of this chain of equations by neglect of the higher Green's functions corresponds to an approximation which reduces to neglecting the higher components of the wave function in the variational method. The one- and two-phonon approximations are discussed. The convenience of the Green's function method for this problem is noted.