“…Our original intent was to base the multigrid smoother, M , on the additive overlapping Schwarz method of Dryja and Widlund [11], with local subdomain problems discretized by finite elements having nodes coincident with the GLL nodes, as considered in [3,13,28]. Following Couzy and Deville [6] and Fischer et al [15], one can solve the subdomain problems by using a fast diagonalization method similar to (18), guaranteeing that the cost of M is essentially equivalent to the cost of applying A. However, the use of the FDM frees one from the constraint of using FE-based preconditioners, since the FDM solution cost depends only on the use of the tensor-product form and not on the sparsity of the originating operator.…”