Solvers for mixed finite element problems using Poincaré operators based on spanning trees

Abstract

We propose a decomposition of Hilbert complexes that directly leads to a Poincaré operator. An explicit example is provided that decomposes a finite element differential complex using spanning trees in the grid. The Poincaré operator has three implications. First, it yields a new basis in which the mixed formulation of the Hodge-Laplace problem unravels from a large saddle point system into seven smaller, symmetric positive definite systems. These systems can be solved sequentially, and three of these have the same dimensions as the cohomology classes. Second, we use the operator to construct an explicit basis for the harmonic forms. Third, we propose an auxiliary space preconditioner for problems in weighted Sobolev spaces, that robustly handles the large kernel of the differential operator. These three implications are validated through numerical experiments.