wrote this text for a one semester course at the sophomore-junior level. Our experience with students taking our junior physics courses is that even if they’ve had the mathematical prerequisites, they usually need more experience using the mathematics to handle it efficiently and to possess usable intuition about the processes involved. If you’ve seen infinite series in a calculus course, you may have no idea that they’re good for anything.
If you’ve taken a differential equations course, which of the scores of techniques that you’ve seen are really used a lot? The world is (at least) three dimensional so you clearly need to understand multiple integrals, but will everything be rectangular?