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3. Levitan, BM, Sargsyan, IS: Sturm-Liouville and Dirac Operators. Nauka, Moskva (1988) (in Russian)
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5. Marchenko, VA: Concerning the theory of a differential operator of the second order. Dokl. Akad. Nauk SSSR 72, 457-460 (1950) (in Russian)