It was a pleasure to have smart friends and colleagues to discuss interesting problems with. Let me say "Thank you" to Ding Yang, Tan Yin, Zhang Xiande, and many others. Inspiration usually comes to the mind during the discussion with them. Some of the results in this dissertation were inspired by their suggestions. During the course of my candidature, I was supported by NTU Research Scholarship. I appreciate very much the various academic events in our division in NTU, and the opportunities to be supported to attend oversea conferences. Last, but not least, I thank my parents, who showed me the fascination of mathematics during my childhood. I would not have persisted in learning mathematics without their support and encouragement.