1. E. C. Titchmarsh, Eigenfunction Expansions Related to Second-Order Differential Equations, Part 2, Oxford, 1958.
2. M. A. Naimark, Linear Differential Operators, Ungar, New York, 1967.
3. E. A. Coddington, and N. Levinson, Theory of Ordinary Differential Equations, New York, Toronto, London, 1955.
4. Necessary and sufficient conditions for the Riesz basis property of the eigen- and associated functions of high-order differential operators on an interval