Admitance functions and thirdtime moments in nonlinear, nonequilibrium thermodynamics

A method of finding admittance functions from nonlinear relaxational equations with external forces is set forth which does no require the introduction of a small parameter. The method is used to find the quadratic triple - subscript admittance functions in an arbitrary multi-component case. The resulting functions are substituted in a nonquantum version of the formula used to express the quadratic fluctuation-dissipation theorem. As a result, the triple - time moment of the equilibrium thermal fluctuations are computed in an arbitrary multi - component case. A comparison is made with the results of Markov nonlinear nonequilibrium thermodynamics. In particular, it is finally proved that a relaxational equation with external forces may be restored, in a quadratic- linear approximation, in terms of arelaxational equation without forces.