1. L. D. Fadeev, Tr. Mat. Inst. im. V.A. Steklova, Akad. Nauk SSSR 73, 314–336 (1964).
2. V. A. Marchenko, Dokl. Akad. Nauk 104, 695–698 (1955).
3. V. A. Marchenko, Sturm-Liouville Operators and Their Applications (Naukova Dumka, Kiev, 1977) [in Russian].
4. G. Sh. Guseinov, Dokl. Akad. Nauk 227, 1289–1292 (1976).
5. L. A. Takhtadzhyan and L. D. Faddeev, The Hamiltonian Approach in Soliton Theory (Nauka, Moscow, 1986) [in Russian].