1. CIDMA – Center for R&D in Mathematics and Applications, Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal
2. Departamento de Matemáticas, Universidad Nacional de Colombia, Carrera 30 # 45-03, Bogotá, Colombia
3. Department of Mathematics, College of Education, Viet Nam National University, G7 Build., 144 Xuan Thuy Rd., Cau Giay Dist., Ha Noi, Viet Nam
4. Department of Mathematics, Pedagogical College, University of Da Nang, 459 Ton DucThang Str., Da Nang City, Viet Nam