“…When calculating the non-linear susceptibility by ( 2 ) ) ( 3 ) , and (6), Green functions with the following kinds of matrix elements are obtained: I. J. LALOV, K. T. STOYCHEV, and S. P. PENCHEV I n (8) only products such as (7) take part, the factor with d(l) may be only on the left or on the right of the product (but not in the middle) and the factor with dJ2) may be only in the middle.…”