1. Departamento de Ciências Exatas e Engenharia, Academia Militar, 2720-113 Amadora, Portugal
2. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal
3. Department of Mathematics, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Kattankulthur 603 203, Tamil Nadu, India