1. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal
2. ISCA, University of Aveiro, 3810-500 Aveiro, Portugal
3. CMUC, Department of Mathematics, University of Coimbra, 3001-501 Coimbra, Portugal
4. BIFI, Department of Theoretical Physics and Unidad Asociada IQFR-BIFI, University of Zaragoza, Edificio I+D, Campus Río Ebro, C/ Mariano Esquillor s/n, 50018 Zaragoza, Spain