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3. Yu. L. Daletskii and S. G. Krein, Dokl. Akad. Nauk SSSR,76, No. 1, 13?16 (1951).
4. Yu. L. Daletskii, Dokl. Akad. Nauk SSSR, No. 4, 234?237 (1951).
5. Yu. L. Daletskii and S. G. Krein, in: Transactions of the Functional Analysis Seminar [in Russian], Voronezh (1956), pp. 1?30.