“…Let us consider two functions u(x, y) and v(x, y) defined in the domain Q of Under these conditions, it can be shown, using the same demonstration as in § 2 .1 for equation (5), that above yM , the functions u and v can be represented by plane wave expansions of U -, U+ , V -, V + . If yM < y < a, where du/dn and dv/dn are the normal derivatives of u and v on S, s being the curvilinear coordinate on S .…”