The generalizations of the Langevin equation and the fluctuation-dissipation theorem are discussed in nonequilibrium states. The existence of the general form of the fluctuation-dissipation theorem associated with the Langevin type equation in nonequilibrium states is shown on the basis of a kinematical consideration. The equation of motion for the average fluctuations of dynamical variables in the presence of external fields is discussed within the linear response theory and is reduced to a more useful form by means of the Langevin type equation. The scattering cross section of particles from a system of interacting particles in a nonequilibrium state is also considered in connection with the Langevin type equation.
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Hidemitsu Furukawa (1977) studied this question.