1. Ya. O. Baranets’kyi, P. I. Kalenyuk, and M. I. Kopach, “Nonlocal multipoint problem for partial differential equations of even order with constant coefficients,” Mat. Met. Fiz.-Mekh. Polya, 61, No. 1, 11–30 (2018).
2. V. P. Burskii and E. A. Buryachenko, “Some aspects of the nontrivial solvability of homogeneous Dirichlet problems for linear equations of arbitrary even order in the disk,” Mat. Zametki, 77, No. 4, 498–508 (2005); 10.4213/mzm2508; English translation: Math. Notes, 77, No. 4, 461–470 (2005); doi: 10.4213/mzm2508.
3. I. Ts. Gokhberg and M. G. Krein, Introduction to the Theory of Linear Nonself-Adjoint Operators in Hilbert Spaces [in Russian], Nauka, Moscow (1965)
4. English translation: I. C. Gohberg and M. G. Krein, Introduction to the Theory of Linear Nonself-Adjoint Operators in Hilbert Space, American Mathematical Society, Providence, RI (1969).
5. B. Yu. Irgashev, “On one boundary-value problem for an equation of higher even order,” Izv. Vyssh. Uchebn. Zaved., Ser. Mat., No. 9, 13–29 (2017); English translation: Rus. Math. (Izv. VUZ), 61, No. 9, 10–26 (2017); doi: 10.3103/S1066369X1709002X.