1 Department of Mathematics, The University of Pennsylvania, Philadel- phia, PA 19104-6395 E-mail address:
[email protected] 2 Math480/540, TOPICS IN MODERN MATH. What are numbers? A.A.Kirillov Dec. 2007 2 The aim of this course is to show, what meaning has the notion of number in modern mathematics; tell about the problems arising in connection with different understanding of numbers and how these problems are being solved. Of course, I can explain only first steps of corresponding theories. For those who want to know more, I indicate the appropriate literature. 0.1 Preface The “muzhiks” near Vyatka lived badly. But they did not know it and believed that they live well, not worse than the others A.Krupin, “The live water.” When a school student first meet mathematics, (s)he is told that it is a science which studies numbers and figures. Later, in a college, (s)he learns analytic geometry which express geometric notions using numbers. So, it seems that numbers is the only object of study in mathematics. True, if you open a modern mathematical journal and try to read any article, it is very probable that you will see no numbers at all. Instead, au- thors speak about sets, functions, operators, groups, manifolds, categories, etc. Nevertheless, all these notions in one way or another are based on num- bers and the final result of any mathematical theory usually is expressed by a number. So, I think it is useful to discuss with math major students the ques- tion posed in the title.