1. School of Mathematics and Quantitative economics, Guangxi University of Finance and Economics, Nanning, Guangxi, P. R. China
2. Center for Applied Mathematics of Guangxi, Guangxi Minzu University,
Nanning, Guangxi, P. R. China
3. Center for Applied Mathematics of Guangxi, Yulin Normal University, Yulin, P. R. China
4. Department of Mathematics, National Technical University, Athens, Greece