School of Mathematical Sciences;
Shanxi University;
Taiyuan 030006;
P. R. China;
College of Computer Science and Technology;
Nanjing University of Aeronautics and Astronautics;
Nanjing 211106;
P. R. China;
Department of Computer Science;
Royal Holloway;
University of London;
TW20 0EX Egham;
UK;