Nott that this was your situation, but do you have any recommendations for learning advanced math as an adult hobbyist? I sort of missed the boat on doing advan

Nott that this was your situation, but do you have any recommendations for learning advanced math as an adult hobbyist? I sort of missed the boat on doing advanced math in college, and while I likely won't be able to go back to school for it, I'd like to invest a lot of my free time in learning to better think about problems.