S. A. Nazarov, “Asymptotics of negative eigenvalues of the Dirichlet problem with the density changing sign”, J. Math. Sci. (N. Y.), 163:2 (2009), 151–175
S. A. Nazarov, “Asymptotics of the solution of the Dirichlet problem for an equation with rapidly oscillating coefficients in a rectangle”, Math. USSR-Sb., 73:1 (1992), 79–110
V. A. Kondrat'ev, O. A. Oleinik, “Boundary-value problems for partial differential equations in non-smooth domains”, Russian Math. Surveys, 38:2 (1983), 1–86