More problems like Engel's Problem Solving

Question

I've been working through Arthur Engel's Problem Solving and I've been enjoying it very much. I especially liked Chapters 1, 3, and 4, which cover the Invariant, Extremal, and Pigeonhole Principles respectively. Where can I find more problems using those techniques with solutions given?

(I know about Putnam and IMO but those problems use a much wider variety of techniques...and right now I want to focus more on these three)

Answer 1

I would do the followings:

1. Buy and read the red book by Kenneth Williams by Dover Publisher.

2. Buy math books at the art of problem solving for the future olympiad students.

3. Buy collection of math books written by Titu Andreescu.

4. After that take courses in graph theory, advanced combinatorics, and real analysis at university. Take number theory courses.

I hope this helps.