1. School of Mathematical Science, Nanjing Normal University, Nanjing 210023, P. R. China
2. Institute of Applied Mathematics, Henan University, Kaifeng 475004, P. R. China
3. School of Mathematics, Northwest University, Xi’an, Shaanxi Province 710127, P. R. China
4. Department of Mathematics, College of Science, Swansea University, Singleton Park, Swansea SA2 8PP, UK