1. Institute of Mathematics, Johannes Gutenberg-University Mainz Staudingerweg 9 , 55 128 Mainz, Germany
2. Academy for Multidisciplinary studies, Capital Normal University West 3rd Ring North Road 105 , 100048 Beijing, P. R. China
3. School of Mathematics, Nanjing University of Aeronautics and Astronautics Jiangjun Avenue No . 29, 211106 Nanjing, P. R. China