1. Theory of Ordinary Differential Equations;Coddington,1955
2. Asymptotic methods in the theory of one-dimensional differential equations;Fedorjuk;Trans. Moscow Math. Soc.,1966
3. On the deficiency index of differential operators;Glasmann;Dokl. Akad. Nauk SSSR,1949
4. The asymptotic nature of the solutions of linear systems of differential equations;Levinson;Duke Math. J.,1948
5. Linear Differential Operators;Naimark,1968