After two and a half years of study, the life of graduate students is coming to an end. In these two years, I not only gained knowledge, but also cultivated my way of thinking in reading and practice. The date of leaving school is approaching, and the completion of graduation thesis is drawing to a close. A few months later, the thesis got the help and support from the tutor and classmates from the topic selection to the finalization. Without these tutors, my thesis could not have been completed so smoothly. Here, I would like to express my most sincere thanks!

First of all, I want to thank my tutor, Professor Sheng. From the beginning of the topic selection, my tutor gave me careful guidance, and the structure and demonstration methods of the following papers all contained my tutor's painstaking efforts and expectations. The tutor not only helped me improve my academic ability and broaden my academic horizons, but also deeply influenced me by his rigorous academic research attitude, pragmatic work style and serious and loving academic research.

From the beginning of the research, Mr. Sheng taught us to strengthen physical exercise while studying. Good health is the capital of life and study. I always do what my tutor says, and I don't forget to love life while studying. My tutor's guidance in all aspects has benefited me a lot in the postgraduate stage, and the serious and positive attitude I learned from him has also benefited me a lot in my future study and life. Thanks to all the teachers of Anhui University of Finance, I was lucky enough to listen to their teachings during my study and benefited a lot.

Here, I would like to express my heartfelt thanks for your hard cultivation for more than two years. Thank you for teaching us more about being human. Thank you for your tireless teaching for more than two years!

I have been in a cold window for more than two years. I want to thank my classmates and roommates who studied with me. They not only helped me in my life, but also helped me understand a lot of knowledge points in my study. At the same time, they also gave me some valuable paper suggestions. It is they who let me gain valuable friendship, which will be a valuable asset in my future life.

I also want to sincerely thank my parents who raised me and care about my family. For a long time, they gave me the greatest support in obscurity, enabling me to successfully complete my studies.

Finally, I would like to express my heartfelt thanks to all the experts and professors of the evaluation and defense Committee! I hereby urge all experts and professors to criticize and correct the shortcomings of this article.