1. Ph.D. Candidate, School of Mathematics, Hunan Univ., South Lushan Rd. 1st, Changsha 410082, China.
2. Professor, School of Mathematics, Hunan Univ., South Lushan Rd. 1st, Changsha 410082, China.
3. Professor, School of Civil Engineering, Hunan Univ., South Lushan Rd. 1st, Changsha 410082, China (corresponding author).