“…ℓ ( ) being the two-level error estimator for BEM [4, 24-26, 39, 41, 45, 50, 51] and the FEM-BEM coupling [5,37,49]. In either case, ℓ ( ) denotes some weighted-residual error estimator, see [11,14,15,19,20] for BEM and [2,18,37] for the FEM-BEM coupling.…”