ZHAO Hang1, LU Lingxia2, LI Shenggang1*, TAO Qian1, LI Yaping1
(1 School of Mathematics and Information Science, Shaanxi Normal University,Xi′an 710119, Shaanxi, China;2 School of Mathematics and Sciences, Shijiazhuang University of Economics,Shijiazhuang 050031, Hebei, China)