Why does a student climb the building of advanced mathematics? Or in other words, why do mathematicians ask all ordinary people to learn mathematics in the way of mathematicians? It's useless except throwing the students black and blue. Every student has to learn seven or eight skills to judge whether the series converges, but he has no chance to get the answer to a simple question: "Why should I learn the series?" This problem is meaningless to mathematicians, who always need series anyway.

But it means something to others. Every time I walk into the classroom and start to discuss the difference between the first kind of generalized integral and the second kind of generalized integral or the difference between absolute convergence and relative convergence, I can't help but imagine the feelings of the students in the audience. Will they think I look like a fool? Anyway, I think I am. I often meet someone who seriously asks, "What's the use of mathematics? Actually, this question is not difficult to answer at all. I can cite countless examples to explain why the most modern mathematical tools are indispensable in every corner of society, but I understand why others ask this question. Because for them, mathematics is the exercise they learned in college. These exercises are really useless to 99% of college students, even the most illusory use of "exercising logical thinking ability". If I were to write a college mathematics textbook, I would try my best to get everyone who had a math class in college to answer such a question: Why can people accurately predict the solar eclipse decades from now, but not the weather tomorrow? Why people can browse the web safely through https without being monitored; Why the speed of global warming exceeds the limit and becomes irreversible; Why does compressing text files into zip reduce a lot of volume, while compressing mp3 files into zip hardly changes the size? Whether people's livelihood statistical indicators should adopt average or median; What do people mean when they say that two musical instruments have the same pitch but different timbres? This is not fun math, but math. Basic, important, profound and beautiful mathematics.