1. CMUC, University of Coimbra, Department of Mathematics, 3000-143 Coimbra, Portugal
2. Department of Mathematics, University of Coimbra, 3000-143 Coimbra, Portugal
3. Institute of Systems and Robotics, UC, 3030-290 Coimbra, Portugal
4. College of Engineering, University of Texas at Arlington, Arlington, TX 76019-0019, USA