Calculation of two-time Green's functions for disordered crystals

A scheme is proposed for calculating expressions such as the complex admittance or Fourier transforms of two-time retarded Green's functions for disordered crystals. The calculations are made in terms of functions that are called $z$ functions. In the case of Hamiltonians that do not diagonalize, the $z$ functions of the higher orders are expressed recursively in terms of the $z$ functions of lower orders. As an application, the scheme for solving problems relating to the integrated intensity of local modes in disordered crystals is considered.