Chapter 25 Further Up and Further In
ΒΆSolving higher-degree polynomial congruences, like x^3\equiv a\text{ (mod }n)\text{.} (Chapter 7)
Knowing how to find the first nontrivial integer point on hard things like the Pell (hyperbola) equation x^2-ny^2=1\text{.} (Chapter 15)
Writing a number not just in terms of a sum of squares, but a sum of cubes, or a sum like x^2+7y^2\text{.} (Chapter 14)
The Prime Number Theorem, and finding ever better approximations to \pi(x)\text{.} (Chapter 21)