ZHU Shi-huan ,LI Yong-xin, BAI Lu4
1.School of Science, Xidian University, Xi′an 710071, China;2.School of Mathematical Sciences, Anyang Normal University, Anyang 455000, Henan, China;3. Department of Mathematics, Ping Ding Shan Institute, Pingdingshan 467002, Henan, China; 4. Science Institute, Air Force Engineering University, Xi′an 710051, ChinaO221.4