I have a bachelor's degree in biomedical engineering, but I am looking to go to graduate school for mathematics; I know that the switch would not be easy (obviously an undergrad in math would've been best). I am looking for advice on what courses I should take to both prepare me for graduate school and to show graduate schools that I am "prepared" (if such a thing exists).
I've started reading Spivak's Calculus, but because I don't have an undergraduate degree in math I believe I need to take courses to prove I am ready (again, if I can). I am looking for any general advice for this pursuit and would appreciate if anyone has any advice on how to further strengthen my preparation and application. -Thank you
P.S. I have looked through a few (not all) other posts to see if my question was answered, so I guess I should say I am looking for a more "structured" answer in terms of courses and what graduate schools are looking for. I'm not at all sure that makes any sense.