1. P. Hartman and C. R. Putnam, ?The gaps in the essential spectra of wave equations,? Am. J. Math.,72, No. 4, 849?862 (1950).
2. M. S. P. Eastham, ?Asymptotic estimates for the lengths of the gaps in the essential spectrum of the self-adjoint differential operators,? Proc. R. Soc. Edinburgh,74A, 239?252 (1976).
3. V. I. Feigin, ?Continuous spectrum of differential operators,? Funkts. Anal. Prilozhen.,11, No. 1, 43?54 (1977).
4. E. D. Belokolos and I. M. Pershko, ?The relationship between smoothness of a potential and the size of gaps in the essential spectrum of a Schr�dinger operator,? Preprint, Inst. Teor. Fiz., Akad. Nauk Ukr. SSR, Kiev (1977).
5. I. M. Glazman, Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators [in Russian], Gos. Izd. Fiz. Mat. Lit., Moscow (1963).