My area of interest in mathematics is mostly recreational, but I have studied some amount of undergraduate-level number theory, combinatorics, discrete mathematics, algebra, calculus, and geometry.
I was once active at Project Euler, and specifically P299 is from the idea of mine. (That problem has fewer solves than adjacent problems, but it's not that difficult!) Currently, I like to pose mathematically inclined challenges on Code Golf and Puzzling SE.
More recent addition to the list is APL-family languages, especially Dyalog APL and J. I started using them for code golfing, but now I'm also practicing to write more idiomatic, readable, and maintainable APL (e.g. Advent of APL). I'm often active in The APL Orchard, and I'm happy to help other people learn APL and J.