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First and foremost I would like to thank my supervisor Dr. Alexandros Milionis. Working with him has been a great opportunity for expanding my knowledge and learning new things.His eagerness and enthusiasm for research was a great example for me. All of these years his door was always open, guiding me every time I was lost. In every stage of this thesis his patience and support have been invaluable. I would like also to thank the other two members of my advisory committee Dr. Georgios Vamvoukas and Dr. Athanasios Yannacopoulos for their time and help. I want to thank Eleni Klaoudatou for our long conversations as she always was there to hear my concerns. I owe special thanks to Dimitris Geneiatakis for supporting me in every aspect and encouraging me through the way. No matter how far, he always offered his help generously.Finally, I am deeply grateful to my family George Soldatos, Grigorios Papanagiotou and my father Thomas Papanagiotou. Although my choices affected their lives they unconditionally supported me.