1. V. A. Marchenko, Sturm-Liouville Operators and Their Applications [in Russian], Naukova Dumka, Kiev (1977).
2. B. M. Levitan, Invertible Sturm-Liouville Problems [in Russian], Nauka, Moscow (1984).
3. V. E. Zakharov and A. B. Shabat, Funktsional. Analiz i Ego Prilozhen.,8, 43 (1974).
4. V. E. Zakharov, ?The inverse scattering method,? in: Solitons [Russian translation], (eds. R. K. Bullough and P. J. Caudrey), Mir, Moscow (1983), pp. 270?309. (Originally published by Springer, Berlin (1980).)
5. V. N. Shapovalov, Differents. Uravneniya.,16, 1864 (1980).