I'm an engineering major who is about to graduate from an undergrad program and will be entering grad school in the fall. I'm interested in doing some sort of summer study of math, though I'm not sure whether I should do a review of the previous math courses that I've taken during my undergrad years or branch out into something completely new to get a head start on some of the things that I might see later on in grad school. I've previously taken (multi-variable) calculus, differential equations, linear algebra, and discrete math. If doing a review, I would lean more towards discrete math since the topics from the class I had seemed very interesting, though what would be some recommendations for further pursuing number theory or discrete math? Also, has anyone previously done a self study or a partner study over the summer? If so, how did it work for you?