“…Unfortunately, because of the highly oscillatory nature of the wave fields, these solvers lead to discretizations with a large number of unknowns and require the solution of large poorly-conditioned linear systems. Research on accurate and computationally efficient methods is very active: we can mention for instance recent works on high-frequency boundary element methods [7,21,22], high-order finite element methods [14,16,57,68] and domain decomposition methods [9,10,34,77].…”