A THEORY OF NONLINEAR SUSCEPTIBILITY BY A COMBINED PROJECTION TECHNIQUE

Starting with the Liouville equation and with help of the combined pro jection technique, we formulate the nonlinear susceptibility for many electron systems subject to-oscillatory external fields. For the itera tive projections, we introduce the primary, secondary, the tertiary pr ojection operators and so on. The final form for the averaged dynamica l quantity is given in the form of a power series in the external fiel d.