the members of the assessment committe: Omid Amini, Matt Baker, Reimundo Heluani and Jaap Top, for the careful reading and valuable comments on a previous version of this thesis. I would also like to thank Jaap Top for the help regarding the Dutch language. Additionally, I thank CNPq and the University of Groningen for the financial support received during the development of this work.I am grateful to my classmates and colleagues from both IMPA and the University of Groningen for making these last years more enjoyable.