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Foguel S., 1964, P AM MATH SOC, V15, P788, DOI [10.1090/S0002-9939-1964-0165362-X, DOI 10.1090/S0002-9939-1964-0165362-X]
[2]
Kreiss Heinz-Otto, 1962, BIT, V2, P153, DOI DOI 10.1007/BF01957330