“…Neighbouring elements need to be subdivided to retain mesh conformity, when not using hanging nodes. Earlier approaches (Ratnajeevan and Hoole, 1990;Fernandez et al, 1993;Golias et al, 1994;Díaz-Morcillo et al, 2000;Tsuji and Koshiba, 2000) concentrate on h-adaptivity by introducing error estimates and criteria whether to refine a mesh cell or not. h-refinement is needed in case of singularities or phase lag at high wave numbers (Ihlenburg and Babuška, 1997;Babuška and Strouboulis, 2001;Monk, 2003;Ainsworth, 2004) and results in polynomial convergence.…”