1. Department of Mathematics and Statistics, University of Cyprus/ΠΑΝΕΠΙΣΤHΜΙΟ ΚΥΠΡΟΥ, P.O. Box 20537, 1678 Nicosia, Cyprus
2. Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT, UK
3. Institute of Solid Mechanics, Romanian Academy, 15 Constantin Mille, P.O. Box 1-863, Bucharest 010141, Romania