1. Sturm-Liouville Theory
2. The Schrödinger Equation
3. Schrödinger Operators
4. E. C. Titchmarsh, Eigenfunction Expansions Associated with Second-Order Differential Equations. Part I, 2nd ed. (Clarendon Press, Oxford, 1962), p. vi+203.
5. M. A. Naimark, Linear differential operators. Part II: Linear differential operators in Hilbert space, With additional material by the author, and a supplement by V. È. Ljance. Translated from Russian by E. R. Dawson. English translation edited by W. N. Everitt, Frederick Ungar Publishing Co., New York, 1968, p. xv+352.