Response to a mechanical perturbation and the Green's functions for nonequilibrium systems

A general investigation of the reaction to the external mechanical perturbation for non-equilibrium systems, the state of which can be described by a set of macroscopical variables has been made. The response of such systems is written in the form of functional depending on macroscopical variables or approximate statistical distribution. Closed formulas are obtined for non-linear admittances in the form of integrals of correlation functions over the non-equilibrium state, what represents a general formulation of the fluctuation-dissipation theorem for non-equilibrium systems.