There are many, many facts and properties known about the sequence of Fibonacci numbers $F_n$ , and there is a whole industry devoted to discovering and publicizing these facts. The present book abstracts some of these properties and studies them in general integer sequences. The book is inspired by the work of the late Caltech mathematician Morgan Ward (1901–1963).

For example, one simple but surprising fact is that if $m \vert n$ then $F_m \vert F_n$ (the notation $m \vert n$ means that m divides n, in other words that $n/m$ is an integer). Generalizing, we say that an integer sequence $a_n$ is a divisibility sequence if whenever $m \vert n$ we have $a_m \vert a_n$, and divisibility sequences are studied in Chapter 3 of the present book.

$a_{n+k} = c_{k-1}a_{n+k-1} + .... + c_1 a_{n+1} + c_0 a_n + \alpha$

Usually the $a_n$, $c_k$, and $\alpha$ are considered to be integers, but in some cases (especially Chapter 2) we’ll take them all to be elements of a finite field. For example, the Fibonacci numbers satisfy the linear recurrence \(F_{n+2}=F_{n+1}+F_{n}\).

Chapters 4 and 5 deal with Lucas and Lehmer sequences, respectively. Lucas sequences generalize the Fibonacci sequence by considering a recurrence $L_n=aL_{n−1}−bL{n−2}$ where $a$ and $b$ are integers. Lehmer sequences generalize Lucas sequences by considering the same recurrence except that we take $a=\sqrt c$ where $c$ is an integer; this is a case where the coefficients of the recurrence are not all integers. Chapter 6 deals with primitive prime divisors of a sequence. Primitive in this context means first, so we investigate the first index $n$ for which a given prime divides $a_n$.

Chapter 7 contains exercises, most with solutions. They are not closely related to the first six chapters, although they share some techniques. Most of them deal with divisibility properties of elements of a sequence; for example, Problem 7.14 asks for all positive integers n such that $n^2|2^n+1$ . Many others deal with solutions of Diophantine equations. Most of the problems originate in the International Mathematical Olympiad or the training for the IMO.