1. School of Mathematics and Physics, Anhui Jianzhu University, Hefei, Anhui, P.R. China
2. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan, P.R. China
3. Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio, USA
4. School of Life Science, Anhui Medical University, Hefei, Anhui, P.R. China