1. School of Management & Economics, Beijing Institute of Technology, No. 5, Zhongguancun South Street, Haidian District, Beijing 100081, P. R. China, School of Mathematics & Computer Science, Yan'an University, No. 580, Shengdi Road, Baota District, Yan'an, Shaanxi 716000, P. R. China
2. School of Management & Economics, Beijing Institute of Technology, No. 5, Zhongguancun South Street, Haidian District, Beijing 100081, P. R. China