Affiliation: | (1) Department of Mathematics, Auburn University, Auburn, AL 36849-5310, USA (E-mail: sampsgm@mail.auburn.edu), US;(2) Department of Mathematics, Changsha University of Electric Power, Cangsha 410077, China (E-mail: xujs@mail.csuep.edu.cn), CN;(3) Department of Mathematics, Beijing Normal University, Beijing 100875, China (E-mail: dcyang@bnu.edu.cn), CN |