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2. M. G. Krein, “On the inverse problems for a nonuniform string,” Dokl. Akad. Nauk SSSR, 82,No. 5, 881–884 (1952).
3. I. S. Kats and M. G. Krein, “On the spectral functions of a string,” Appendix II in F. V. Atkinson, Discrete and Continuous Boundary-Value Problems [Russian translation], Mir, Moscow (1968), pp. 629–647.
4. I. Ts. Gokhberg and M. G. Krein, Theory of Volterra Operators in Hilbert Spaces and Its Applications [in Russian], Nauka, Moscow (1967).
5. I. Ts. Gokhberg and M. G. Krein, “On the triangular representation of linear operators and multiplicative representations of their characteristic functions,” Dokl. Akad. Nauk SSSR, 176,No. 2, 272–275 (1967).