1. S. G. Krein, Linear Differential Equations in Banach Space [in Russian], Nauka, Moscow (1967).
2. S. G. Pyatkov, “Properties of eigenfunctions of a certain spectral problem and their applications,” in: Some Applications of Functional Analysis to Equations of Mathematical Physics [in Russian], Inst. Mat. (Novosibirsk), Novosibirsk, 1986, pp. 65–84.
3. I. Ts. Gokhberg and S. G. Krein, Introduction to the Theory of Linear Nonselfajoint Operators in Hilbert Space [in Russian], Nauka, Moscow (1965).
4. S. G. Pyatkov, “Solvability of boundary value problems for a mixed-type equation of second order,” in: Nonclassical Partial Differential Equations [in Russian], Inst. Mat. (Novosibirsk), Novosibirsk, 1988, pp. 77–80.
5. M. M. Lavrent'ev, Conditionally Well-Posed Problems for Differential Equations [in Russian], Novosibirsk Univ., Novosibirsk (1973).