1. Hebei Key Laboratory of Computational Mathematics and Applications, P. R. China
2. School of Mathematical Sciences, Hebei Normal University, P. R. China
3. Hebei International Joint Research Center for Mathematics and Interdisciplinary Science, P. R. China
4. Fachbereich Mathematik, Technischen Universitat Dortmund, Dortmund, Germany
5. Romanian Academy, Bucharest, Romania