School of Science;
Xi'an Jiaotong University;
Xi'an 710049;
China/Department of Mathematics;
Linyi Teachers College;
Linyi 276005;
China/College of Mathematics;
Qufu Normal University;
Qufu 273165;
China.;
School of Science;
Xi'an Jiaotong University;
Xi'an 710049;
China.;
Department of Applied Mathematics;
Xidian University;
Xi'an 710071;
China.;