1. Discrete and Continuous Boundary Problems;Atkinson,1964
2. On self-adjoint extensions of nth order differential operators in the limit circle case;Cao;Acta Math. Sinica,1985
3. Theory of Ordinary Differential Equations;Coddington,1955
4. Linear Operators (II);Dunford,1963
5. Real self-adjoint Sturm–Liouville problems;El-Gebeily;Appl. Anal.,2004