1. M.S.P. Eastham, The Spectral Theory of Periodic Differential Equations (Scottish Academic Press, Edinburgh, 1973). (and references therein)
2. J. Weidmann, Spectral Theory of Ordinary Differential Operators (Springer, Berlin, 1987)
3. A. Zettl, Sturm-Liouville Theory (The American Mathematical Society, Providence, 2005)
4. B. Malcolm Brown, M.S.P. Eastham, K.M. Schmidt, Periodic Differential Operators. Operator Theory: Advances and Applications, vol. 230 (Springer, Heidelberg, 2013) (and references therein)
5. G. Bethe, A. Sommerfeld, Elektronentheorie der Metalle (Springer, Berlin, 1967)