Just finished the book, this is a good intro to number theory which is one of the fields I’m interested in mathematics.
The content covers integers, greatest common divisors, lowest common multiples, primes, numbers in base n, and more primarily, modular arithmetic which is the first time I officially studied the topic. Following that is also linear congruence as well as general tricks in number theory. Overall, an interesting book with challenging problems which most I could understand and do well.
The Introduction to Number Theory is the weakest book in the Art of Problem Solving series. Unlike others there, the difficulty level plateaus early, peaking around Chapter 4 without a satisfying progression. Most topics are covered more thoroughly and with better practice problems in H.S. Hall's Elementary Algebra for Schools. The hints provided are largely unhelpful, often merely reiterating the chapter title, which is already obvious to the reader; at least one hint is outright incorrect. And the quotes, as usual in AoPS books (with the exception of Precalculus), are rather depressive, anti-rational, and skeptical, along the lines of wondering if one will ever be able to tell where their back ends and their backside begins. Could as well quote St. Augustine: "The good Christian should beware of mathematicians and all those who make empty prophecies. The danger already exists that the mathematicians have made a covenant with the devil to darken the spirit and to confine man in the bonds of Hell".
Best intro to number theory I've found. Gives you a very solid foundation to delve deeper into the field. Think my 3 favourite ideas were the fundamental theorem of arithmetic, modular arithmetic and solving linear congruences. Each one completely changes how you look at numbers. Great book.