Probably probability theory (ha!), if your goal is rigor. However, once again, you are splitting hairs here in optimizing for rigor. This is literally a matter of one class in one semester of your senior year.
Probably probability theory (ha!), if your goal is rigor. However, once again, you are splitting hairs here in optimizing for rigor. This is literally a matter of one class in one semester of your senior year.