I've taken all the usual undergrad math courses needed for undergrad physics: calc, diff eq, linear algebra, etc. However, when I read some of the discussions on this board, or when I look at more advanced books, I'm completely lost by the math! What math do I need to understand??? I hear references to group theory, for example. And then there are tensors and such for GR (which I'm just now starting to understand a bit). But none of these maths were offered at my undergrad institution for undergrad students. So, what math do I need to know? Or what are good courses to take so I can understand grad-level physics (say, particle physics, QFT, grad-level quantum) and GR? Thanks, guys.