Affiliation: | aSchool of Mathematics and Computing Science, Changsha University of Science and Technology, Yuntang Campus, Changsha 410114, Hunan, PR China;bSchool of Mathematics and Econometrics, Hunan University, Changsha 410082, Hunan, PR China;cDepartment of Mathematics, Huaihua University, Huaihua 418008, Hunan, PR China;dDepartment of Mathematics, Shili Senior High School in Zixing, Chenzhou 423400, Hunan, PR China |