Affiliation: | 1.School of Mathematical Sciences and LPMC, Nankai University, Tianjin 300071, P. R. China;2.Department of Mathematics, Frostburg State University, Frostburg, MD 21532, USA;3.Department of Mathematics, West Virginia University, Morgantown, WV 26506, USA;4.School of Mathematics and Statistics, Jiangsu Normal University, Xuzhou 221116, P. R. China |