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.
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