1. School of Mathematics and Statistics, Central South University, Changsha, Hunan 410083, P. R. China
2. School of Science, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, P. R. China
3. School of Mathematics and Information Science, Henan Polytechnic University, Jiaozuo, Henan 454003, P. R. China
4. National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu 215123, P. R. China