We study second-order equations and systems on non-Lipschitz domains including mixed boundary conditions. The key result is interpolation for suitable function spaces. From this, elliptic and parabolic regularity results are deduced by means of Šneı̆berg's isomorphism theorem.
Special Issue: Singularities, Integral Equations and Electromagnetism, dedicated to M. Costabel on the occasion of his 65th birthday - Part 1