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 [3] and the article by Viro [5].

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.