Ok, here's my issue; I'm a physics major just finishing up my first year, but I also love math, and i've been considering doing a double major. What math courses, other than those required by a physics degree, should I take? I have read that learning fourier series would come in handy. I'm basically just looking for advice on what additional math topics will make my life easier down the road. What math courses did you (grad students and beyond) wish you had learned as an undergrad? I'm considering theoretical physics and from what I have read, a solid understanding of math is a must.