Supervisor

\La forza trainante della Matematica non e il ragionamento, ma l'immaginazione". Augustus de Morgan Aknowledgments I would like to thank Roberto Natalini, my supervisor, for his many suggestions and constant support during this research. He showed me the beauty of Mathematical research by means of his devotion to this work and his enthusiasm when working. The kind hospitality the whole sta of Istituto per le Applicazioni del Calcolo \M. Picone " make me feel as in a big family and contributed to the successful completion of this project. A particular thank goes to the Department of Mathematics \G. Castelnuovo", University of Rome \La Sapienza " for the stimulating meeting and courses I took part and all my collegues, the Ph.D. students, for having shared with me this expe-rience. Of course, I am grateful to my parents and my sister for their patience and love. Without them this work would never have come into existence (literally). I thank them for being always near me understanding my frequent changes of humor. I would like to dedicate this work to Fabio: without his love and constant pres-ence, in bad and good time, I would not be able to pursue this work. Finally, I wish to thank all my friends: Marco and Valeria (for their long date friendship and their constant- even if dierent- presence during the last three years); Maya (for having discussed with me, even strongly: it helps me a lot!); Anna Lisa (for her passion in this work); Corrado and Massimiliano (for having spent some of their time in chatting with me during- and not only- their breaks); Rolando (for his everyday dierent \goodmorning"!)