I would also like to thank my parents, Nancy and Jorge, because with their love, selfdenial, sacrifice and example, they help me and support me when I make mistakes, they advise me when I doubt, and when I need them, they are always by my side.I would like to express my gratitude to Prof. Dr. Celso de Almeida, for his support, patience, and encouragement throughout my graduate studies. To him I owe moments of kind dedication and guidance during the development of this work. His technical and editorial advice was essential to the completion of this dissertation and he has taught me innumerable lessons and insights on the workings of academic research and life in general.My thanks also go to the members of my dissertation committee, Prof.