“…There is a great effort in literature to overcome these difficulties. Among the discretization techniques, there are finite difference [10,24], finite element [1,18,27], discontinuous Galerkin [6,11], virtual element [21], and boundary element methods [19]. At the same time, there is a great effort to develop efficient preconditioners, such as multigrid [4,7,8,15,26] and domain decomposition methods [16,17,25].…”