1. F. V. Atkinson, Discrete and Continuous Boundary Value Problems, Academic Press, New York, 1964.
2. E. A. Coddington and N. Levinson, Theory of Ordinary Differential Equations, McGraw-Hill, N. Y., 1955.
3. W. N. Everitt, Integrable-square solutions of ordinary differential equations, Quar. J. Math., Oxford (2) 10 (1959), pp. 145–155.
4. —, Integrable-square solutions of ordinary differential equations II, Quar. J. Math., Oxford (2) 13 (1962) pp. 217–220.
5. —, A note on the self-adjoint domains of second-order differential equations, Quar. J. Math., Oxford (2), 14, (1963), pp. 41–45.