my master's advisor, who since then has always been by my side on my academic journey, advising and encouraging me.Thank you for all your guidance, suggestions, teachings, and support.To Prof. and colleague Giselle Queiroz, for being a new source of energy, for all her suggestions and for her practical and optimistic outlook.To Prof. Dr. Luiz Bloem for his suggestions and comments from the very beginning of this thesis.To Prof. Dr. Roberto Sbragia for his invaluable contributions to the qualifying exam.