Numerical homogenization of H(curl)-problems

If an elliptic differential operator associated with an ${H}({curl})$-problem involves rough (rapidly varying) coefficients, then solutions to the corresponding ${H}({curl})$-problem admit typically very low regularity, which leads to arbitrarily bad convergence rates for conventional numerical schemes. The goal of this paper is to show that the missing regularity can be compensated through a corrector operator. More precisely, we consider the lowest-order Nédélec finite element space and show the existence of a linear corrector operator with four central properties: it is computable, ${H}({curl})$-stable, and quasi-local and allows for a correction of coarse finite element functions so that first-order estimates (in terms of the coarse mesh size) in the ${H}({curl})$ norm are obtained provided the right-hand side belongs to ${H}({div})$. With these four properties, a practical application is to construct generalized finite element spaces which can be straightforwardly used in a Galerkin method. In particular, this characterizes a homogenized solution and a first-order corrector, including corresponding quantitative error estimates without the requirement of scale separation. The constructed generalized finite element method falls into the class of localized orthogonal decomposition methods, which have not been studied for ${H}({curl})$-problems so far.