mas não percamos tempo.

I would like to begin by thanking my supervisor Prof. Jose ́ Barata, for motivation and for giving me the opportunity to work on the subject of this dissertation. I would also like to thank Pedro Santana for all his constant support and guidance throughout the whole work. Next, I want to thank all my colleagues and friends, in particular David and Magno, for all the help, support and friendship along the years we spent together in this university. Samuel, Pedro, André, Zé, and all those who I might not have mentioned, you also have thanks. My parents also deserve a word of appreciation, for all the continuous efforts in my educa-tion, all the support and belief. No words are enough to express my thanks to both of you, and to my brother as well. Last, but not least, a very, very special thank you to all the members of the Comissão A.G.A., you are the best, and I’m glad to have so many friends like you. 3 Não tenhamos pressa