1. School of Economics and Management, Southeast University, Nanjing, Jiangsu, P.R. China;
2. School of Mathematics and Physics, Anhui Jianzhu University, Hefei, P.R. China;
3. Department of Mathematics, COMSATS University Islamabad, Sahiwal Campus, Sahiwal, Pakistan;
4. Department of Mathematics, Division of Science and Technology, University of Education Lahore, Lahore, Pakistan