Source: http://www.artofproblemsolving.com/Resources/Papers/SatoNT.pdf

Excellent notes on Olympiad Number Theory!

Preface:

This set of notes on number theory was originally written in 1995 for students

at the IMO level. It covers the basic background material that an IMO

student should be familiar with. This text is meant to be a reference, and

not a replacement but rather a supplement to a number theory textbook;

several are given at the back. Proofs are given when appropriate, or when

they illustrate some insight or important idea. The problems are culled from

various sources, many from actual contests and olympiads, and in general

are very difficult. The author welcomes any corrections or suggestions.