FORMULATION OF NONLINEAR CONDUCTIVITY IN A MANY-ELECTRON SYSTEM IN THE PRESENCE OF AN OSCILLATORY ELECTRIC-FIELD BY A COMBINED PROJECTION TECHNIQUE

A non-linear form for the current in a many-electron system which is s ubject to an oscillatory electric field is derived by starting with th e Liouville equation and with the help of the combined projection tech nique which combines the techniques of Kenkre and of Argyres and Sigel . The primary projection operator yields the general expression for th e current in the Fourier-Laplace transformed space. The secondary comb ined projection operator generates the lowest order of non-linearity i n the expression. The final result is given in a time-retarded form, b ut not in a continued-fraction form.