GUAN Xue-chong1,2, LI Yong-ming3*
(1 College of Mathematics and Information Science, Shaanxi Normal University, Xi′an 710062, Shaanxi, China;2 School of Mathematical Science, Xuzhou Normal University, Xuzhou 221116, Jiangsu, China;3 College of Computer Science, Shaanxi Normal University, Xi′an 710062, Shaanxi, China)