1. Department of Mathematics, Faculty of Mathematics and Computer Science; University of Bucharest; 010014 Bucharest Romania
2. Institute of Solid Mechanics, Romanian Academy; 15 Constantin Mille 010141 Bucharest Romania
3. Department of Mathematics and Statistics; University of Cyprus/Πανεπιστήμιo Kύπρoυ; 1678 Nicosia/Λευκωσία Cyprus/Kύπρoς
4. Department of Applied Mathematics; University of Leeds; Leeds LS2 9JT UnitedKingdom