1. A. M. Molchanov, "On conditions for discreteness of the spectrum of selfadjoint differential equations of the second order," Tr. Mosk. Mat. Obshch.,2, 169?199 (1953).
2. F. Martinelli and E. Scoppola, Preprint, Rome (1986).
3. R. S. Ismagilov, "The continuous spectrum of second-order ordinary differential operators," Mat. Zametki,34, No. 4, 535?547 (1983).
4. R. S. Ismagilov, "On the spectrum of the Sturm-Liouville equation with an oscillating potential," Mat. Zametki,37, No. 6, 869?879 (1985).
5. S. Kotani, "Lyapunov exponents and spectra for one-dimensional random Schr�dinger operators," in: Random Matrices and Their Applications (Brunswick, Maine, 1984), Contemp. Math., No. 50, Am. Math. Soc., Providence, R.I. (1986), pp. 276?286.