Hello, I am currently an undergrad physics major at a US university, and I am planning on going to grad school to get my phd in astronomy or astrophysics. However, I am very interested in theoretical astrophysics and GR and the like, but I feel as though my university doesn't give us as much preparation in mathematics as I would hope for, as we are only required to take single and multivariable calculus, and a "math methods" class using "Mathematical Methods in the Physical Sciences" by Boas. I therefore am uncertain whether I should try to self-study some upper level math classes, or take extra courses or what. I would really like to avoid adding extra time onto my degree, so that makes my dilemma a bit more difficult. Any advice would be great.