Affiliation: | 1. Université de Brest, LMBA, UMR CNRS 6205, 6 avenue Le Gorgeu, C.S. 93837, 29238 Brest Cedex 3, France;2. Department of Mathematics, Hanoi National University of Education, 136 Xuan Thuy Street, Cau Giay, Hanoi, Viet Nam;3. Department of Mathematics, Thai Nguyen University of Education, Luong Ngoc Quyen Street, Thai Nguyen City, Viet Nam |