Y. Ohfuti et K. Cho, GENERAL SEPARABILITY OF LINEAR AND NONLINEAR-OPTICAL SUSCEPTIBILITIES, Physical review. B, Condensed matter, 52(7), 1995, pp. 4828-4832
It is shown that all components of the electric susceptibilities act a
s separable integral kernels in the nonlocal theory of radiation-matte
r interaction. This reduces the problem of solving the self-consistent
motions of the radiation and matter to a set of polynomial equations
for a given order of nonlinearity.