1. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics, University of Aveiro, Aveiro, Portugal
2. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics, University of Aveiro, Aveiro, Portugal + Center of Mathematics and Applications of University of Beira Interior (CMA-UBI), Department of Mathematics, University of Beira Interior, Covilhã, Portugal