1. University of Minho, Department of Mathematics and Mathematics Center, Braga, Portugal
2. University of Minho, Department of Mathematics and Physics Center, Braga, Portugal
3. Department of Mathematics, School of Engineering, Polytechnic of Porto., LEMA|ISEP and SYSTEC|ISR Portugal
4. University of Minho, Department of Mathematics and Physics Center, Braga, SYSTEC|ISR, Portugal