The rigor of mathematical analysis lies in that in the19th century, Wilstrass and others thought about infinitesimal logical basis and established calculus on the rigorous basis, that is, analytical arithmeticization. In the axiomatic system of modern mathematics, as long as there is a definition of natural numbers, all other analysis contents can be drawn from it. The rational number is obtained by the ratio of natural numbers, and the real number is defined by Dedeking division or the equivalence class of rational number Cauchy column, so the whole analysis is based on the most basic natural numbers. So some people say: God created natural numbers and created the whole universe. If you are interested in these contents, I recommend you to read the history of mathematics, and you can appreciate the beauty of mathematics without understanding profound mathematical theorems.