Many students are frightened by functional language when they study advanced mathematics. What are you afraid of? Newton, Leibniz, Euler, these great gods, have done a lot of work all their lives without understanding this point. You can follow this wild path and do some formal operations first, and then learn. At this time, you may be able to appreciate the elegance in the sentence of "The so-called limit is just ……" by Wilstras.
I once saw my brain opened. After reading it, I looked up at the sky and sighed: God, there is such a method in mathematics. In this book, I learned Ordos-selfridge theorem for the first time. Until today, I don't know which book has talked about this theorem and given a complete elementary proof. If I haven't read this book, I still don't know this theorem, because it is too superficial for modern number theory. The more you know, the more you admire Ordos, because he is a real maverick.