The level of math amenable to apps isn't really what Su is talking about. I would suggest learning proofs, and maybe pick up some Art of Problem Solving books.

The level of math amenable to apps isn't really what Su is talking about. I would suggest learning proofs, and maybe pick up some Art of Problem Solving books. Or perhaps working through the foundational curriculum of any decent math program (e.g., algebra, analysis, topology, number theory, etc.).