viii FOREWOR D
further stud y of topology, appears at the end of the book. Among the books
and articles that had the greatest influence on this book, I would like to name
the book by Rolphsen  and the article by Viro .
As is usually done in mathematical books and papers, the symbol • mark s
the end of the proof of a proposition or a theorem.
It should be mentioned that the present text is based on a series of lectures
given by the autho r i n the academi c year 1990-199 1 t o student s o f High
School no. 5 7 in Moscow.
I am grateful to N. M. Fleischer for useful discussion s of the manuscript.