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Vladimir Balan
University Politehnica of Bucharest, Faculty of Applied Sciences, Department of Mathematics-Informatics I, Splaiul Independentei 313,Bucharest, Romania
Maido Rahula
University of Tartu, Institute of Mathematics, Liivi 2Tartu, Estonia
Nicoleta Voicu
"Transilvania" University of Brasov, Faculty of Mathematics and Computer Science, Department of Mathematics and Computer Science, 50 Iuliu Maniu Str,Brasov, Romania