1. Air and Missile Defense College, Air Force Engineering University, Xi’an 710051, P. R. China
2. School of Mathematical Sciences, Beijing Normal University, Beijing 100875, P. R. China
3. Mathematics Group, Xi’an Tieyi High School, Xi’an 710038, P. R. China
4. College of Mathematics and Information Science, Hebei University, Baoding 071002, P. R. China