Taking the PhD position at the Institut für Numerische und Angewandte Mathematik in Göttingen was a decision that involved many unknowns: I would have to separate from my family, friends and girlfriend, I didn't know German and I hadn't yet met my supervisor. Still, I had a good feeling about it and I can now say it turned out to be the most beneficial event yet to occur in my life. I met amazing people from all over the world that helped me discover my true vocations and the way I should pursue, giving me the most precious gift of friendship when it was most needed. They have made my stay abroad not only tolerable but exciting and I now look forward to the many more adventures that await us together in the future. This thesis is, I hope, the proof that I also couldn't have hoped for a better supervisor than Gerlind Plonka, who by example taught me the value of perseverance and hard work in research. Above all, Gerlind fostered my confidence in my own abilities by always giving me great freedom and being very encouraging and appreciative of my work, even when I thought it didn't hold so much value.I want to dedicate this thesis to my whole "Vitalba" family, with whom I grew up and whom I sorely miss. In particular, I want to express my gratitude to my parents for having always been there like a trusted beacon, even (especially) when the sea was rough or the horizon covered in fog. I also want to dedicate this work to my friend Giulio Regeni, who was killed like a beast and taught me to live like a Man.