An approach to nonlinear response by a closed set of integral equations

Abstract

A nonperturbational theory for nonlinear response of externally driven systems is presented. A set of integral equations for the response of observables, which are slow variables of the system and form a commutator algebra, is derived. This set of integral equations reduces to a tractable form, if some nonlinear memory effects are negligible. The kernels depend only on the linear relaxation functions and the driving field. As an example of the theory the paramagnetic saturation problem is discussed. In a simple way the modified Bloch equations follow as a special result.