Affiliation: | aSchool of Mathematics and Physics, University of South China, Hengyang, Hunan 421001, PR China;bDepartment of Mathematics, Shanghai University, Shanghai 200444, PR China;cMathematics and Science College, Shanghai Normal University, Shanghai 200234, PR China;dDepartment of Mathematics, Wilfrid Laurier University, Waterloo, Ont., Canada N2L 3C5 |