Theory of frequency dispersion of the conductivity of a system of charged particles

The longitudinal and transverse conductivities of the disordered system of charge particles is considered. The linear response theory with respect to weak external field and the perturbation theory for the interaction between the particles and photons are investigated. The expressions for conductivities in these approximations are derived. The problem of long-wavelength limit of conductivities and the connection with the corresponding limits of correlation functions are discussed.