A generalization of the fluctuation-dissipation theorem for the case of excess noise

Electrical fluctuations proportional to the square of the external field are investigated in the case of quasi-equilibrium conditions. The binary correlation function, proportional to the square of the external field, is expressed in terms of the zero-field quaternary correlation function for both quantum and classical cases. This result can be considered as a generalization of the fluctuation-dissipation theorem. The 1/f spectrum is obtained as a consequence of the symmetry properties of the quaternary correlation function without consideration of the physical mechanisms responsible for the noise. The method of investigation of the 1/f spectrum through zero-field quaternary correlation function is discussed.