I have a somewhat unorthodox question. What are some good niche topics that aren't typically covered in courses in math to self-study? Ideally, I'm looking for something that could ultimately be useful in physics (even if it is rather advanced), but if not, I'm still open to suggestions.
My background: real analysis, advanced linear algebra, calculus on manifolds, complex analysis, differential equations, some functional analysis. I'm probably forgetting a few things.