1. M. A. Naimark, Linear Differential Operators [in Russian], Nauka, Moscow (1968).
2. A. P. Khromov, "Expansion in eigenfunctions of ordinary linear differential operators with irregular separated boundary conditions," Matem. Sb.,70, 310?329 (1966).
3. A. P. Khromov, "Finite-Dimensional perturbations of Volterra operators," Doctoral Dissertation, Avtoreferat, Matem. Zametki,16, No. 4, 669?680 (1974).
4. A. P. Khromov, "Finite-Dimensional perturbations of Volterra operators," Doctoral Dissertation, Novosibirsk (1973).
5. L. A. Sakhnovich, "The inverse problem for differential operators of order n > 2 with analytic coefficients," Matem. Sb.,46, 61?76 (1958).