Problems in Mathematics
yutsumura.com
yutsumura.com
I thought this was going to be about the abc conjecture.
https://www.quantamagazine.org/titans-of-mathematics-clash-o...
https://www.amazon.com/Challenging-Problems-Probability-Solu...
Almost every math major covers an intro to Group Theory, Ring Theory, Field Theory. That's just about all you can do in pure math immediately after Linear Algebra (except number theory and some probability), and the foundation for almost everything you'd try.
If "applied math" was not separated from "pure math" in your school, and you did non-math electives after the minimum core math classes, then you might avoid these topics in favor of a broad curriculum of differential equations, graph theory, probability, and such.
Galois Theory, or Module Theory would be electives or touched on in high-expectations program.