A special thanks to Professor Nuno Costa for his dedication, guidance, motivation and valuable professional constructive suggestions during the arrangement and execution of this dissertation work. Furthermore, I am grateful for the patient guidance and judgment given by Professor Carlos Grilo and I am especially indebted with Professor Jorge Veloz for his useful critiques and advices provided during this work. All of them have actively worked to provide me with the help to pursue my goals. It has been a privilege to have their guidance in a work of this magnitude.

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