1. Department of Mathematics and Physical Sciences, Polytechnic Institute of Beja, Beja, Portugal
2. Department of Mathematics and CMA-Center of Mathematics and Applications, NOVA, NOVA University of Lisbon, Lisbon, Portugal
3. Department of Mathematics and CIMA-Center for Research on Mathematics and its Applications, University of Évora, Evora, Portugal
4. Forest Research Center, Institute of Agronomy, University of Lisbon, Lisbon, Portugal