1. V. Buslaev and V. Fomin, Vestn. Leningr. Univ., No. 1, 56 (1962).
2. E. Ya. Khruslov, Mat. Sb.,99, 261 (1976).
3. A. Cohen, Commun. Partial Diff. Equations, No. 9(8), 751 (1984).
4. L. D. Faddeev, Dokl. Akad. Nauk SSSR,121, 63 (1958), Tr. Mosk. Inst. Akad. Nauk SSSR,73, 314 (1964).
5. V. A. Marchenko, Sturm-Liouville Operators and Their Applications [in Russian], Naukova Dumka, Kiev (1977).