1. Departament of Mathematics, School of Sciences and Technology, Research Centre in Mathematics and Applications (CIMA), Institute for Research and Advanced Training (IIFA), University of Évora. Rua Romão Ramalho, 59, 7000-671 Évora, Portugal
2. Research Centre in Mathematics and Applications (CIMA), Institute for Research and Advanced Training (IIFA), University of Évora. Rua Romão Ramalho, 59, 7000-671 Évora, Portugal