V. Kovtunenko, K. Kunisch. Revisiting generalized FEM: a Petrov–Galerkin enrichment based FEM interpolation for Helmholtz problem. Calcolo 55(3) (2018) DOI 10.1007/s10092-018-0280-5
J. Baiges, R. Codina. A variational multiscale method with subscales on the element boundaries for the Helmholtz equation. Int. J. Numer. Meth. Engng 93(6) (2012) DOI 10.1002/nme.4406
P. Nadukandi, E. Oñate, J. Garcia. A Petrov-Galerkin formulation for the alpha interpolation of FEM and FDM stencils: Applications to the Helmholtz equation. Int. J. Numer. Meth. Engng 89(11) (2011) DOI 10.1002/nme.3291