1. College of Mathematics and Information Science, Hebei Normal University, Shijiazhuang, China 050024.
2. Hebei Key Laboratory of Computational Mathematics and Applications, Shijiazhuang, China 050024.
3. Fachbereich Mathematik, Technische Universität Dortmund, Dortmund, Germany 44221.
4. Institute of Mathematics “Simion Stoilow,” Roumanian Academy, Bucharest, Roumania 014700