To my supervisor, Professor Nicolas Privault. You have helped me develop the aesthetic appreciation of mathematics, and have taught me the optimal way of writing articles. You have also given me the freedom to explore various research topics. I am truly grateful for all your guidance and support through my mathematical journey.To Nanyang Technological University (NTU). Thank you so much for giving me the opportunity to pursue my PhD under the Nanyang President's Graduate Scholarship. I also thank Professor Chi Seng Pun for the useful discussions and feedbacks about my projects.To my family and my partner, Yong Shi. You are always there for me, listening to my problems and helping me go through the difficult times. Your unconditional love is the greatest gift of my life.