Affiliation: | 1. Department of Mathematics, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong,China 2. Faculty of Mathematics and Computer Science, University of Bucharest, 14 Academiei Street, 010014 Bucharest, Romania 3. Technical University of Civil Engineering, Department of Mathematics and Computer Science, 124 Lacul Tei Blvd., 020396 Bucharest, Romania |