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I would like to express my gratitude and sincere appreciation to Prof. Peter Gacs, my advisor, for his immeasurable help and assistance during my graduate studies at Boston University. His love of mathematics and science was absolute and without reservation, and his strict adherence to mathematical rigor has been an invigorating in uence on my own mathematical development. Prof. Gacs ' undaunting level of energy and ability to maintain concentration almost ad in nitum is unsurpassed, and his high standards of excellence will stay with me throughout my research career. I am very grateful to Prof. Andre Toom and Prof. Leonid Levin for serving as readers on my thesis committee. Their expertise and insightful comments have been invaluable. I would like to express my special thanks to Prof. Toom for his kindness and generosity in accommodating my hectic schedule from afar in Texas. I am most grateful to Prof. Abdelsalam Heddaya for his friendship and enthusiasm in teaching and pursuing systems research withme. He has been my de facto systems advisor, being instrumental in opening my eyes toward distributed systems research. His active interest and encouragement has been of great help in furthering my e orts in this area, an