Institution: | School of Mathematics and Statistics, Xidian Univeristy, Xi''an 710071, China;Department of Mathematics, Lanzhou Jiaotong
University, Lanzhou 730070, China,Department of Mathematics, Morgan State University, 1700 E.Cold Spring Lane, Baltimore, M.D. 21251, USA and Department of Mathematics, Lanzhou Jiaotong University, Lanzhou 730070, China |