The theme of this book, on the one hand, is in contact with the alphabet of mathematics itself, and on the other hand, it reaches areas of a rather high order, like the concept of a logarithmic function, the idea of ​ ​ a differential equation, etc. Therefore, a book can be useful for a beginner mathematician, like a springboard that can immediately raise it from basics to a relatively high height. In the presentation, the author seeks not so much to logically substantiate the subject, but with the help of various examples to convince the reader of the need for new concepts introduced by him. This is the main value of the book. The second feature of the book is the very wide use of geometric models, which develop a geometric imagination in the reader. One hopes that this book will develop for many a taste for mathematical thinking.