1. Center for Research and Development in Mathematics and Applications, Department of Mathematics; Universidade de Aveiro; 3810-193 Aveiro Portugal
2. Faculty of Sciences and Technology, Department of Mathematics; Universidade Nova de Lisboa, Quinta da Torre; 2829-516 Caparica Portugal
3. Forest Research Centre, Instituto Superior de Agronomia; Universidade Técnica de Lisboa, Tapada da Ajuda; 1349-017 Lisboa Portugal