Time flies, time flies, and the two-and-a-half-year postgraduate stage will be over in a blink of an eye. Looking back on my experience at this stage, I am rich and full, and occasionally I will encounter setbacks. With the help and encouragement of teachers and classmates, the concern of friends and the support of family, I can't express my gratitude.

This paper is completed under the guidance of the tutor Professor Xing. From topic selection, conception, writing to finalization, every progress can not be separated from teacher Xing's careful guidance, so that the paper can be successfully completed. During my two and a half years as a graduate student, both academically and in life, Mr. Xing's rigorous and serious academic attitude and approachable attitude towards life have benefited me a lot and will inspire me to keep making progress in my later life. I would like to express my heartfelt thanks to my respected tutor Xing!

Thanks to the teachers who taught us in graduate school. Their earnest guidance further improved my study and successfully completed all my studies. Thanks to my dear roommates and classmates, their kindness, enthusiasm and diligence infected me, and they made my life so full and fulfilling during the time when I worked hard together. Thank my friends for always supporting and helping me without hesitation and constantly urging me to make progress.

Thank my parents and relatives who have worked hard for me. Thank my parents for giving me life and educating me to grow up. In the past 20 years, my parents have done a lot for me, and their understanding and support are the biggest driving force for my progress. I am grateful to my aunt for her help in study and life, and I will always remember it.

On the occasion of leaving school, I wish all teachers and students all the best in their future work and life!