Affiliation: | College of Mathematics and Econometrics, Hunan University, Changsha, Hunan 410082, China (Permanent address: Department of Mathematics, Faculty of Science, National University of Laos, Vientiane Capital, DongDok Campus, P.O. Box 7322, Laos) and College of Mathematics and Econometrics, Hunan University, Changsha, Hunan 410082, China |