1. Scientific Area of Mathematics, School of Technology and Management of Viseu, Polytechnic Institute of Viseu, Campus Politécnico de Repeses, 3504-510 Viseu, Portugal
2. Department of Mathematics, University of Coimbra, Largo D. Dinis, 3001-454 Coimbra, Portugal
Institute of Systems and Robotics, University of Coimbra - Pólo II, Pinhal de Marrocos, 3030-290 Coimbra, Portugal