Area Formulas for σ-Harmonic Mappings

The goal of the present paper is two-fold. First, we review some recent progress concerning generalizations of various classical results, such as sufficient conditions to guarantee univalence of harmonic mappings in dimension two, to certain pairs of elliptic partial differential equations with measurable coefficients. Second, we apply these results to prove new area formulas which are valid for a large class of mappings arising as solutions of these pairs of elliptic partial differential equations. Finally, we briefly discuss some applications to homogenized constants in the context of G-closure problems. To Professor Olga A. Ladyzhenskaya with our deep admiration