On a mixed FEM and a FOSLS with $H^{-1}$ loads

TL;DR

This paper introduces a regularization approach for mixed FEM and FOSLS methods to handle $H^{-1}$ loads in the Poisson problem, proving quasi-optimality and demonstrating improved convergence with numerical validation.

Contribution

It develops a regularization technique using $H^{-1}$ projectors for mixed FEM and FOSLS, ensuring quasi-optimality and optimal convergence rates under minimal regularity.

Findings

Quasi-optimality of regularized mixed FEM and FOSLS in weaker norms.

Construction of $H^{-1}$ projectors based on weighted Clément quasi-interpolator.

Numerical examples confirming theoretical convergence rates.

Abstract

We study variants of the mixed finite element method (mixed FEM) and the first-order system least-squares finite element (FOSLS) for the Poisson problem where we replace the load by a suitable regularization which permits to use $H^{-1}$ loads. We prove that any bounded $H^{-1}$ projector onto piecewise constants can be used to define the regularization and yields quasi-optimality of the lowest-order mixed FEM resp. FOSLS in weaker norms. Examples for the construction of such projectors are given. One is based on the adjoint of a weighted Cl\'ement quasi-interpolator. We prove that this Cl\'ement operator has second-order approximation properties. For the modified mixed method we show optimal convergence rates of a postprocessed solution under minimal regularity assumptions -- a result not valid for the lowest-order mixed FEM without regularization. Numerical examples conclude this work.