For us too, beauty is the first test in the choice of topics for our own research, as well as the subject for popular articles and lectures, and consequently, in the choice of material for this book. We did not restrict ourselves to any particular area (say, number theory or geometry), our emphasis is on the diversity and the unity of mathematics. If, after reading our book, the reader becomes interested in a more systematic exposition of any particular subject, (s)he can easily find good sources in the literature. About the subtitle: the dictionary definition of the word classic, used in the title, is �judged over a period of time to be of the highest quality and outstanding of its kind�. We tried to select mathematics satisfying this rigorous criterion. Lecture Notes Collection FreeScience.info ID2280 Obtained from http://www.math.psu.edu/tabachni/Books/taba.pdf http://www.freescience.info/go.php?pagename=books&id=2280