1. College of Science, Civil Aviation University of China, Tianjin 300300, P. R. China
2. Faculty of Applied Mathematics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland
3. Department of Mathematics, University of Craiova, 200585 Craiova, Romania
4. Department of Mathematics, Heilongjiang Institute of Technology, Harbin 150050, P. R. China