“…This idea has been applied to both finite element (e.g., [4,13,41,29,14,43]) and boundary element schemes [26,1,24,44,45,20,43,27,10,46,28]. Promising numerical results are reported, but most of the papers are lacking in mathematical analysis, especially with regard to how any error estimates depend on the wavenumber k. As the present paper follows the same general approach of enriching the approximation space, we survey this body of work in a little more detail.…”