1. Titchmarsh, E. C. [1958] Eigenfunction expansions associated with second-order differential equations, v.II, Clarendon Press, Oxford.
2. Marchenko, V. A. [1986] Sturm-Liouville operators and applications. Birkhäuser, Basel.
3. Levitan, B. M. [1977] Inverse Sturm-Liouville problems, Nauka, Moscow, 1?240.
4. Hochstadt, H. [1963] Estimates on the stability intervals for Hill's equation, Proceed. AMS,14, 930?932.
5. Trubowitz, Eu. [1977] The inverse problem for periodic potentials, Commun. Pure and Appl. Math.,30, 321?337.