<p>Math 55 at Harvard as little (nothing) to do with theory of sets (?) But to the OP (who sounds like an UG, like me) Number Theory, LA, DE are not advanced math topics. I’d recommend some Introductory Topology, Complex Analysis, PDE, some rigorous treatment of graph theory, and Differential Geometry. Probably an M.Sc publication in math would help.</p>
<p>Edit: a reading course in maths (or some kind of involved senior thesis) I’m sure would help. And naturally, all A’s in your math courses.</p>