We consider a mixed-boundary-value/interface problem for the elliptic operator P = − ∑ ij ∂i(aij∂ju) = f on a polygonal domain Ω ⊂ R2 with straight sides. We endowed the boundary of Ω partially with Dirichlet boundary conditions u = 0 on ∂DΩ, and partially with Neumann boundary conditions ∑ ij νiaij∂ju = 0 on ∂NΩ. The coefficients aij are piecewise smooth… (More)