1. Department of Applied Mathematics, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China
2. Department of Computer and Information Sciences, University of Strathclyde, 26 Richmond Street, Glasgow G1 1XH, UK
3. IRIF, CNRS UMR 8243, Université Paris Diderot – Paris 7, 75205 Paris Cedex 13, Paris, France
4. College of Mathematical Science, Tianjin Normal University, Tianjin 300387, P. R. China