1. Boyadzhiev, G.: Comparison principle for non-cooperative elliptic systems, Nonlinear Anal Theory Methods Appl 69(11), 3838–3848 (2008). ISSN:0362-546X. IF 1.612 (2013)
2. Boyadzhiev, G.: Existence of classical solutions of linear non-cooperative elliptic systems. C. R. Acad. Bulg. Sci. 68(2), 159–164 (2015). ISSN: 1310–1331 (print), ISSN 2367–5535 (online)
3. Evans, L.C.: Partial differential equations, series Graduate Studies in Mathematics. AMS, New York (1998)
4. Grebenkov, D.S., Nguyen, B.-T.: Geometrical structure of Laplacian eigenfunctions. SIAM Rev. 55(4), 601–667 (2013)
5. Ladyzhenskaya, O., Rivkind, V., Ural’tseva, N.: Classical solvability of diffraction problem for elliptic and parabolic equations with discontinuous coefficients. Trudy Mat. Innst. Steklov 92, 116–146 (1996) (in Russian)