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**Multilevel Schwarz methods for elliptic problems with discontinuous coefficients in three dimensions.**
*(English)*
Zbl 0857.65131

Authors’ summary: Multilevel Schwarz methods are developed for a conforming finite element approximation of second-order elliptic problems. We focus on problems in three dimensions with possibly large jumps in the coefficients across the interface separating the subregions. We establish a condition number estimate for the iterative operator, which is independent of the coefficients, and grows at most as the square of the number of levels. We also characterize a class of distributions of the coefficients, called quasi-monotone, for which the weighted \(L^2\)-projection is stable and for which we can use the standard piecewise linear functions as a coarse space. In this case, we obtain optimal methods, i.e. bounds which are independent of the number of levels and subregions. We also design and analyze multilevel methods with new coarse spaces given by simple explicit formulas. We consider nonuniform meshes and conclude by an analysis of multilevel iterative substructuring methods.

Reviewer: S.F.McCormick (Boulder)

### MSC:

65N55 | Multigrid methods; domain decomposition for boundary value problems involving PDEs |

65F10 | Iterative numerical methods for linear systems |

65N30 | Finite element, Rayleigh-Ritz and Galerkin methods for boundary value problems involving PDEs |

65F35 | Numerical computation of matrix norms, conditioning, scaling |

35J25 | Boundary value problems for second-order elliptic equations |