1. Mathematics Department, Faculty of Sciences, University of Lisbon, Lisbon, Portugal
2. Center for Mathematics, Fundamental Applications, Computation and Operational Research, University of Lisbon, Lisbon, Portugal
3. Mathematics Department, Technical Institute of Lisbon, University of Lisbon, Lisbon, Portugal
4. Center for Mathematical Analysis Geometry and Dynamical Systems, University of Lisbon, Lisbon, Portugal