1. Department of Mathematics, Northwest Normal University, Lanzhou, Gansu, P.R. China
2. College of Information Engineering, Tarim University, Aral, Xinjiang, P.R. China
3. School of Traffic and Transportation, Lanzhou Jiaotong University, Gansu, P.R. China
4. School of Information Engineering, Lanzhou University of Finance and Economics, Gansu, P.R. China