What kind of book is this? It is a book that was written within the framework of an important informal educational tradition of the former Soviet Union, which encouraged the formation of groups of students, teachers, and mathematicians, known as "mathematical circles." The fundamental idea that underlies it is that the study of mathematics can generate the same excitement that a team sport does – without necessarily being competitive.

The book is aimed at students and teachers who love mathematics and wish to explore its various branches beyond the narrow confines of the school curriculum. It is also a book of recreational mathematics and, at the same time, a work that contains a vast wealth of theoretical material and problems in key areas of "extracurricular mathematics." The book is based on the unique experiences that many generations of Russian educators and university instructors have accumulated.

There is much to discover, learn, and enjoy in this work for both students and teachers… Amateur lovers of mathematics will also be captivated by it… In all chapters, the presentation and writing style are exquisitely appealing and "light," even when more difficult topics are examined. It certainly has a place in every school and personal library.