Institution: | 1. Computer and Automation Institute of the Hungarian Academy of Sciences, Kende u. 13-17, 1111, Hungary 2. Fakult?t für Mathematik, Universit?t Bielefeld, 4800, Bielefeld 1, West Germany 3. Mathematical Institute of the Hungarian Academy of Sciences, P.O.B. 127, 1364, Budapest, Hungary
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