1. School of Mathematics and Statistics, Xidian University, Xi’an 710071, P. R. China
2. School of Mathematics and Statistics, Ning Xia University, Ningxia 750021, P. R. China
3. School of Mathematics and Information Science, Bao Ji University of Arts and Sciences, Baoji 721013, P. R. China
4. College of Mathematics Science and Center, for Applied Mathematical Science, Inner Mongolia Normal University, Hohhot 010022, P. R. China