It depends on how deep you want to go and what your goals are, but I'd say that CuriouslyC pretty much nailed it. Multi-variable calculus, linear algebra, and probability / stats are definitely the core. If you're interested in finding more "freely available online" maths references, check out: http://people.math.gatech.edu/~cain/textbooks/onlinebooks.ht... http://www.openculture.com/free-math-textbooks https://open.…
Another upvote for 3blue1brown. I just watched his linear algebra series and it's probably the most outstanding math instruction I've encountered.
another nice yt channel about math and physics.