“…For example, spectral coarse spaces for least squares problems and symmetric indefinite saddlepoint systems were proposed in [5,39], where coarse spaces are proposed for related Hermitian positive definite matrices. An exciting new development is that a number of multigrid methods and spectral coarse spaces have been suggested for problems with indefinite or non-self-adjoint operators [9,10,11,12,13,14,22,36,37]. These coarse spaces are mainly based on heuristics and show efficiency on several challenging model problems arising from discretized PDEs.…”