Fast solvers for the high-order FEM simplicial de Rham complex: Extended edition
This work provides incremental improvements for computational scientists dealing with high-order finite element simulations in electromagnetics or fluid dynamics.
The authors tackled the problem of solving high-order finite element discretizations of the de Rham complex efficiently by introducing new basis functions that reduce computational complexity from O(p^9) to O(p^6) flops in 3D, achieving p-robust iteration counts for Riesz maps.
We present new finite elements for solving the Riesz maps of the de Rham complex on triangular and tetrahedral meshes at high order. The finite elements discretize the same spaces as usual, but with different basis functions, so that the resulting matrices have desirable properties. These properties mean that we can solve the Riesz maps to a given accuracy in a $p$-robust number of iterations with $\mathcal{O}(p^6)$ flops in three dimensions, rather than the naïve $\mathcal{O}(p^9)$ flops. The degrees of freedom build upon an idea of Demkowicz et al., and consist of integral moments on an equilateral reference simplex with respect to a numerically computed polynomial basis that is orthogonal in two different inner products. As a result, the interior-interface and interior-interior couplings are provably weak, and we devise a preconditioning strategy by neglecting them. The combination of this approach with a space decomposition method on vertex and edge star patches allows us to efficiently solve the canonical Riesz maps at high order. We apply this to solving the Hodge Laplacians of the de Rham complex with novel augmented Lagrangian preconditioners.