“…Speaking about the spectral element method, we adhere to the terminology of the book [13], referring to methods that use the spectral approach inside the subdomains, not only limiting by the Galerkin method in a weak form, as in the original proposal [25]. A formalism based on the application of the Dirichlet-to-Neumann (DtN) map, which we shall use, was previously discussed by us in [29], here we only briefly outline the main points.…”