Nothing advanced, just Trig, Geometry (proofs based), Algebra and Precalculus. Basically what a sharp high school senior in college prep classes would be familiar with. Geometry was probably the best. If you've never encountered formal proofs before, it's probably the best training for pure logic I can imagine. They start out incredibly frustrating but become addictive once you figure out how to crack them. Precalc was insanely useful too. Learning how to build up a function and shape it with various operators, and work with polynomials is absolutely essential for things like graphics shaders and timing functions and all kinds of other stuff that comes up in UI development (my work). All I can say is that it will give you a much greater appreciation for studying the purely abstract rather than always focusing on practical matters that come up in engineering, as well as making apparent more elegant solutions you may have brute forced in the past.