1. M. I. Vishik, “On the general boundary-value problems for elliptic differential equations,” Trudy Mosk. Mat. Obshch.,
1, 187–246 (1952).
2. V. I. Gorbachuk and M. L. Gorbachuk, Boundary-Value Problems for Operator Differential Equations, Springer, Berlin (1991).
3. T. Kato, Theory of the Perturbations of Linear Operators [Russian translation), Mir, Moscow (1972).
4. A. N. Kochubei, "The extensions of a nondensely defined symmetric operator," Sib. Mat. Zh., 18, No. 2, 314-320 (1977)
5. English translation: Sib. Math. J., 18, No. 2, 225-229 (1977).