Dear Friends of Numbers: This little book is for you. It should o?er an exquisite int- lectual enjoyment, which only relatively few fortunate people can experience. May these essays stimulate your curiosity and lead you to books and articles where these matters are discussed at a more technical level. I warn you, however, that the problems treated, in spite of - ing easy to state, are for the most part very di?cult. Many are still unsolved. You will see how mathematicians have attacked these problems. Brains at work! But do not blame me for sleepless nights (I have mine already). Several of the essays grew out of lectures given over the course of years on my customary errances. Other chapters could, but probably never will, become full-sized books. The diversity of topics shows the many guises numbers take to ? tantalize and to demand a mobility of spirit from you, my reader, who is already anxious to leave this preface. Now go to page 1 (or 127?). Paulo Ribenboim ? Tantalus, of Greek mythology, was punished by continual disappointment whenhetriedtoeatordrinkwhatwasplacedwithinhisreach. 1 The Fibonacci Numbers and the Arctic Ocean Introduction There is indeed not much relation between the Fibonacci numbers and the Arctic Ocean, but I thought that this title would excite your curiosity for my lecture. You will be disappointed if you wished to hear about the Arctic Ocean, as my topic will be the sequence of Fibonacci numbers and similar sequences.