If you survived this book, hooray! You made it. You did a great job making it through a whole arc of number theory accessible at the undergraduate level.

Although we really did see a lot of the problems out there, there are many we did not see all the way through. We were able to prove some things about them. Here are just a few problems we started touching on.

• Solving higher-degree polynomial congruences, like \(x^3\equiv a\text{ (mod }n)\). (Chapter 7)

• Knowing how to find the first integer point on hard things like the Pell (hyperbola) equation \(x^2-ny^2=1\). (Chapter 15)

• Writing a number not just in terms of a sum of squares, but a sum of cubes, or a sum like \(x^2+7y^2\). (Chapter 14)

• The Prime Number Theorem, and finding ever better approximations to \(\pi(x)\). (Chapter 21)

It's this last one we will focus on in this extended postscript, for it takes us to the very frontiers of the deepest questions about numbers.