1. Mathematics Department, University of Monastir, Faculty of Sciences, 5019 Monastir, Tunisia. E-mail: bahrounianouar@yahoo.fr
2. Institute of Applied Mathematics, University of Economics Ho Chi Minh City, 59C, Nguyen Dinh Chieu Street, District 3, Ho Chi Minh City, Vietnam. E-mail: kyhn@ueh.edu.vn