Ask HN: Any good online sites to continue mathematical education?

I just graduated college, but would love to keep learning/practicing math. Any sites you are familiar with that are good resources? Also, the more interactive the site the better. In an ideal world I would love a site like codeacademy or hackerrank but for math problems. Thanks!

The closest thing you are asking for is https://projecteuler.net/. This isn't purely math and currently there isn't a good way for sites to analyze your proofs which is most of math. You could do previous Putnam problems or something similar and just manually check your solutions if you just want hard undergrad lvl problems, otherwise it's best to just get a textbook.