ABSTRACT
Themajorityofthemacroscopicpropertiesofasystemcanberelatedtothefollowinggeneralproblem:Anexternalperturbationvarying inbothspaceandtimeisimposedonthesystem(e.g.,anelectricfield, magneticfield,etc.),andweaskwhattheresponseistothisperturbation (electricpolarizationandcurrent,magnetization,etc.).Theproblemis alwaysthesame;onlytheformoftheperturbationischanged.Fora weakperturbativepotentialH1,theresponseisingenerallinear,proportionaltoH1.Inordertocharacterizeit,itisthensufficienttofindthe coefficientofproportionality.
