“…Some more recent methods are the 'averaged boundary conditions' (Kontorovich 1972, Astrakhan et al 1975, Astrakhan and Fersman 1979, the 'asymptotic anticipation' (Neureuther and Zaki 1968), the 'least square' (Millar 1973), the 'variation of constants ' (Fel'd 1976), and the 'spectral iteration' . The case of a periodic structure adjacent to a dielectric layer has been treated by Adonina and Shestopalov (1963), Montgomery (1975), Kalhor and Armand (1975) and Kalhor and Ilyas (1982). Among the literature covering the analysis of reflection-type gratings the works of van den Berg (1971Berg ( , 1981 and Petit (1980) should be mentioned.…”