1. "Institute of Mathematics ""Simion Stoilow"" of the Romanian Academy, P.O.Box 1-764, RO-014700, Bucharest, Romania and the Academy of the Romanian Scientists e-mail: Vasile.Dragan@imar.ro"
2. "Department of Computing, Mathematics and Electronics, ""1 Decembrie 1918"" University of Alba Iulia, 510009 - Alba Iulia, Romania e-mail: lucian.popa@uab.ro"