1. M. A. Naimark, Linear Differential Operators (Nauka, Moscow, 1969) [in Russian].
2. M. S. P. Eastham, The Asymptotic Solution of Linear Differential Systems (Clarendon Press, Oxford, 1989).
3. Ph. Hartman, Ordinary Differential Equations (John Wiley, New York–London–Sydney, 1964).
4. S. A. Orlov, “On the deficiency index of linear differential operators,” Doklady Akad. Nauk SSSR (N. S.) 92 (3), 483–486 (1953).
5. R. B. Paris and A. D. Wood, “On the $$\mathscr L_2(I)$$ nature of solutions of $$n$$-th order symmetric differential operator and McLeod’s conjecture,” Proc. Roy. Soc. Edinburg Sec. A 90, 209–236 (1981).