1. I. M. Glazman, Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators [in Russian], Fizmatlit, Moscow (1963); English transl., Daniel Davey, New York (1966).
2. M. S. P. Eastham, The Spectral Theory of Periodic Differential Equations, Scottish Acad. Press, Edinburgh (1973).
3. F. Gesztesy and B. Simon, Trans. Amer. Math. Soc., 335, 329–340 (1993).
4. D. I. Borisov and R. R. Gadyl’shin, Izv. Math., 72, 659–688 (2008).
5. R. R. Gadyl’shin and I. Kh. Khusnullin, St. Petersburg Math. J., 22, 883–894 (2011).