I would like to express my gratitude to my supervisor, who has always been available and attentive, and has patiently guided and encouraged me through the steps of my work with many enlightening advices and a maddening attention to punctuation. I thank Gianluca Lamanna, whose <<Would it be easy to try this?...>> suggestions I learned to expect and to treasure (and they will probably never stop until the very day of my graduation!). Thanks to Roberto, Bruno and Jacopo for having always been willing to help me during these long months. My deepest gratitude goes to all of them.

Thanks to Felice, for his valuable help and company during the long afternoons we spent writing and erasing code in front of the lab PCs.

I cannot keep myself from thanking all of my friends. I know that every one of them will bring to themselves and to the people around them the same enjoyable happiness that they have often brought to me.

Thanks to Martina, Alberto, Daniele, Leslie, Massimo, Elena, Paola, Marco, Sara, Andrea, Giovanni, and Paola. I hope they did not mind that I haunted our department as a thesis-writing ghost all the time. Thanks to Francesco and Luca, my fellow coffee-seeking ghosts!
A heartfelt thought goes to Arianna: our paths splitted as we left Siena for new adventures, but I know she will always be there when I need someone to talk to.

I take this opportunity to express my profound gratitude to my family, and especially to my parents, that have been a constant source of support during my university years. It is to them that this thesis is dedicated.

Last but not least, my deepest gratitude goes to Salvo. Without his constant and loving support, I can say I could have never come through these challenging and sometimes stressful times. 
