1. Aveiro Institute of Accounting and Administration of the University of Aveiro (ISCA-UA), 3810-500 Aveiro, Portugal
2. Center for Research and Development in Mathematics and Applications (CIDMA), Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal
3. CMUC, University of Coimbra, Department of Mathematics, 3000-143 Coimbra, Portugal