Affiliation: | 1. Institute of Mathematics, Vietnam Academy of Science and Technology (VAST) , Hanoi , Vietnam;2. CEMAT , University of Lisbon , Lisbon , Portugal thanhan@math.ac.vn;4. CEMAT , University of Lisbon , Lisbon , Portugal;5. Department of Mathematics , Vinh University , Vinh City , Vietnam;6. Department of Information Technology , Vinh University , Vinh City , Vietnam |