Professor Han Ping of Shandong University is too low-key. He has taught mathematics for more than 20 years and taught in obscurity. I didn't know there was such a powerful teacher when I took the postgraduate entrance examination. The topic of the lecture is very representative and the thinking is clear. I taught myself chaos before, but it became clear after listening to the lecture. As an experienced person, I recommend you to study math and find an old man in Pinghan, Korea, hoping to help you.