\chapter{Acknowledgements} \label{ch:ack} Upon the completion of my thesis, I would like to express my deep gratitude to my research supervisors for their patient guidance, enthusiastic encouragement and useful critiques of this research work. I would also like to thank the reporters for their feedback, comments, and time. % \kat{the jury and the reporters do not contribute; thank them for their feedback, comments and time} A special thanks to my department’s faculty, staff and fellow researchers for their valuable assistance whenever needed and for creating a pleasant and creative environment during my studies. Last but not least, I wish to thank my family and friends for their unconditional support and encouragement all these years. \bigskip \noindent Cergy-Pontoise, \thedate