Theory of nonlinear response

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Abstract
The problem of nonlinear response is considered by employing a general time-evolution equation, and a Green’s function which is the transition or conditional probability density for an unperturbed system. Expansion of the Green’s function in terms of orthonormal functions enables us to express the distribution function describing the nonlinear behavior by means of matrix products whose elements are composed of correlation functions in the absence of the perturbation. In other words, it is shown how the distribution function induced by a strong perturbation may be calculated by knowing the Green’s function without the perturbation. As the special case of the linear response, we have obtained Kubo’s relation. The Laplace-transform technique with respect to time is found quite useful in developing the present theory in which the transient effect is also taken into account. As an application of the theory, a new relation valid in the region of the second-order perturbation connecting the transient rise and decay with the stationary alternating perturbations has been obtained.