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3. I. M. Gel'fand and B. M. Levitan, "On the determination of a differential equation from its spectral function," Izv. Akad. Nauk SSSR, Ser. Mat.,15, No. 4, 309?360 (1951).
4. V. A. Marchenko, The Sturm?Liouville Operators and Their Applications [in Russian], Naukova Dumka Kiev (1977).
5. Z. S. Agranovich and V. A. Marchenko, The Inverse Problem of Scattering Theory [in Russian], Kharkov (1960).