“…While simple techniques such as block-diagonal preconditioners are easy to construct, they are not always capable of reducing the number of iterations significantly. An incomplete list of more sophisticated preconditioners, which vary in construction time and efficacy, include additive or multiplicative Schwarz [37,38], BPX [39], sparse approximate inverses [40,41], approximate LU decompositions [42], clustering [43,44], integral equations of opposite order [45], Calderon identities [36], or multigrid [46,47,48,49]. In the case of multigrid preconditioners, specialized smoothers must be developed since standard smoothers like Jacobi or Gauss-Seidel will smooth the low, rather than the high, frequency components.…”