1. A. V. Bitsadze and A. A. Samarskii, “On some simple generalizations of linear elliptic boundary problems,” Sov. Math. Dokl., 10, 398–400 (1969).
2. G. Fikera, “To unite theory of boundary-value problems wor elliptically parabolic equations of second order,” Matematica, 7, No. 6, 99–121 (1963).
3. T. Kato, Perturbation Theory for Linear Operators [Russian translation], Mir, Moscow (1972).
4. M. V. Keldysh, “On some cases of degeneration of elliptic type on the boundary of a domain,” Dokl. Akad. Nauk SSSR, 77, No. 2, 181–183 (1951).
5. S. G. Krein, Linear Equations in Banach Spaces, Birkhäuser, Boston–Basel–Stuttgart (1982).