1. Department of Mathematics and Physical Sciences, Polytechnic Institute of Beja, Beja, Portugal
2. CMA- Center of Mathematics and its Applications, Faculty of Science and Technology, New University of Lisbon, Portugal
3. Department of Mathematics and Center of Mathematics and Applications, University of Beira Interior, Covilhã, Portugal
4. Polytechnic Institute of Portalegre, Portalegre, Portugal
5. Department of Mathematics, Faculty of Science and Technology, New University of Lisbon, Portugal