The completion of the thesis is attributed to many people's support andencouragement.

　　First and foremost, I want to extend my heartfelt gratitude to my supervisor,Professor Cao Daogen, whose patient guidance, valuable suggestions and constantencouragement make me successfully complete this thesis. His conscientious academicspirit and modest, open-minded personality inspire me both in academic study and dailylife. He gives me much help and advice during the whole process of my writing, whichhas made my accomplishments possible.

　　Also, I would like to express my sincere gratitude to all the professors who havetaught me in this university. Their instructions have helped broaden my horizon andtheir enlightening teaching has provided me with a solid foundation to accomplish thispaper and will always be of great value for my future career and academic research.

　　My thanks also go to the authors whose books and articles have given meinspiration in the writing of this paper.

　　Last but not least, I would like to express my special thanks to my parents, whosecare and support motivate me to move on and make me want to be a better person.