1. M.V. Keldysh, On some cases of degeneracy of elliptic-type equations on the boundary of a domain. Dokl. Akad. Nauk SSSR 77(2), 181–183 (1951)
2. A. Weinstein, Discontinuous integrals and generalized theory of potential. Trans. Am. Math. Soc. 63(2), 342–354 (1948)
3. A. Weinstein, Generalized axially symmetric potential theory. Bull. Am. Math. Soc. 59, 20–38 (1953)
4. R.P. Gilbert, Function Theoretic Method in Partial Differential Equations (Academic, New York, 1969)
5. I.A. Kipriyanov, Singular Elliptic Boundary Value Problems (Nauka, Moscow, 1997, in Russian)