1. College of Information Engineering, Sanming University, 365004, Sanming, P.R. China
2. School of Mathematical and Computer Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK
3. School of Mathematics and Computer Science & Fujian Key Laboratory of Mathematical Analysis and Applications (FJKLMAA) & Center for Applied Mathematics of Fujian Province (FJNU), Fujian Normal University, 350007, Fuzhou, P.R. China