[mitthrowaway2]

You don't get a free pass to not cite relevant prior literature just because it's in the form of an unreviewed preprint.

If you're writing a paper about a longstanding math problem and the solution gets published on 4chan, you still need to cite it.

[hansvm]

In some fields, sure, cite the 4chan source, ideally with an archived link.

Pure math tends to be much more conservative in citations than other fields though, and even when writing a paper about a longstanding math problem you wouldn't necessarily bother to include existing solutions. You reference the things you actually used, and even then you assume some common background knowledge for your audience and don't reference every little undergrad topology theorem or whatever. The point is to be honest with the reader about what was helpful for this work in particular, both to properly attribute things you actually used and to make any searches based on your work more targeted and fruitful.

[NooneAtAll3]

tbf, you cite the paper that described and discussed said solution in the more appropriate form

[mousethatroared]

You cite the form you encountered and if you're any good of a researcher you will have encountered the original 4chan anon post, Borges' short story, or Chomsky's linguistic paper.