Textbook
1. Introduction
2. Algebra
3. Geometry
4. Number theory
4.1 Divisibility
4.2 Modular arithmetic
4.3 Diophantine equations
5. Probability
6. Combinatorics
7. What's next?
4.3 Diophantine equations
This is a fancy phrase that means “equations where the only acceptable solutions are integers” (and the coefficients are also all integers).
The key thing to know here is simply that all the rules of modular arithmetic not only work here, but are super-duper helpful.
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