“…Such structures are widely used in many engineering applications and their mathematical analysis has received a lot of attention in recent years. Besides, problems involving thin layers have been extensively investigated by several authors and a large amount of research has been carried out in this area (see [2][3][4][5][6]13,16,26,27] for acoustic or electromagnetic problems, [1,24] in the thermic framework, [14,19,21,22,28,29,31] for mechanical applications). These studies are in particular devoted to the question of the derivation of approximate boundary conditions that replace "in an approximate way" the effect of the thin layer.…”