This book arose from a course of lectures given by the first author
during the winter term 1977/1978 at the University of Münster (West
Germany). The course was primarily addressed to future high school
teachers of mathematics; it was not meant as a systematic introduction
to number theory but rather as a historically motivated invitation to
the subject, designed to interest the audience in number-theoretical
questions and developments. This is also the objective of this book,
which is certainly not meant to replace any of the existing excellent
texts in number theory. Our selection of topics and examples tries to
show how, in the historical development, the investigation of obvious or
natural questions has led to more and more comprehensive and profound
theories, how again and again, surprising connections between seemingly
unrelated problems were discovered, and how the introduction of new
methods and concepts led to the solution of hitherto unassailable
questions. All this means that we do not present the student with
polished proofs (which in turn are the fruit of a long historical
development); rather, we try to show how these theorems are the
necessary consequences of natural questions. Two examples might
illustrate our objectives.