Mathematics with Boris Trushin. Number Theory: from zero to Euler's theorem

From the author: «The book is aimed at a wide range of readers — from sixth graders to people who graduated from school a long time ago. In it, I have gathered years of experience teaching number theory to school... students in lessons, clubs, and electives. The book allows you to get acquainted with number theory and master the basics of this interesting branch of mathematics without any prior knowledge. In this book, I tried, starting from the basic principles of working with integers, to take a journey to substantive theorems, mastering many methods for solving problems along the way. And in the end, you will even learn about one of the important applications of number theory to "real life"».