1. Laboratoire de Mathèmatiques; Université de Sidi Bel-;Abbès; Sidi Bel-Abbès B.P. 89, 22000 Algeria
2. Escola Superior de Tecnologia de Setúbal, Estefanilha, 2910-761 Setúbal, Portugal and Center for Research and Development in Mathematics and Applications (CIDMA); University of Aveiro; 3810-193 Aveiro Portugal
3. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics; University of Aveiro; 3810-193 Aveiro Portugal