I reproduce my (deleted) answer here:

Use SFFT to factor $$xy-8x-4y = 0$$.

The answer received $$2$$ upvotes and was also accepted by the OP. A while later, however, a moderator converted the answer to a comment. In my opinion, the answer should have remained an answer because of the following reasons:

• Hints are allowed as answers on math.SE. Considering that my answer provided sufficient information to answer the question, it did not simply provide "minor or transient information" that would make it suitable as a comment.
• According to this meta post, an answer is one that provides some useful information even if you strip the markup. If one is aware of SFFT, stripping the markup still provides all necessary information to answer the question.
• Admittedly, the answer is short, but I do not think it lacks coherence or fails in accurately answering the question. IMO, adding additional information would merely detract from the clarity of the answer.

In light of the above, I'd like to understand what criterion my answer fails to meet that classifies it as a comment. Further, on a more general note, I'd appreciate it if one can make more distinct the line between answers and comments.

• I think one big flaw that comes to mind is that of "link rot." Hypothetically, APoS could go down 10 years from now, next month, or even today; what value does your answer have then? The link - and the answer - become next-to-useless. Hence I would usually elaborate on the material being linked to, e.g. the general method and perhaps some examples (different from the OP's problem, in the interest of being more in the nature of "hints" than an outright solution). I wouldn't say your post is not an answer, though, just a "link-only answer" post, which is generally discouraged. Aug 4 at 21:28
• In the days of Enforcement of Quality Standards, I'd advise the following rule of thumb: if your answer can fit in the comments, then do so. It doesn't matter the quality of the question or your answer. If it can fit in comments, then that's what you should do. Aug 5 at 0:43
• @PrincessEev Perhaps I could reproduce the AoPS text. Nonetheless, SFFT isn't unique to AoPS, it can be found on other sites such as here. It is an especially common Olympiad trick and hence didn't need elaboration IMO.
– user1082389
Aug 5 at 2:16
• If someone didn't know how to solve the problem because they didn't know what SFFT was, that text is not an answer to the question. You could have explained what SFFT is, or the details of the technique applied to this example. Deletion was certainly appropriate, conversion to a comment is better than I would have expected.
– Nij
Aug 5 at 3:12
• @BeKind In case it's not super clear, Theo is being somewhat sarcastic/hyperbolic, and habitually answering or providing hints to PSQs, in the comments, does fall afoul of the EoQS, because it enables them and perpetuates their cycle. Incidentally, < deleted the rest of my comments and posted them here instead. > Aug 5 at 6:40
• @ryang that’s more of a response than what I got way back then (shouldn’t ask more than one question at a time…). I think you are right in spirit. If so, it would be good for the letter of the law to coincide Aug 5 at 8:02
• Doesn't MSE also have the risk of going down like so @PrincessEev Aug 5 at 9:49
• "Doesn't MSE also have the risk of going down like so" - Well, yes, but that'll hardly make your answers on MSE useless to users on MSE (since no users and no answers will then exist). Aug 5 at 16:59
• @PrincessEev To be fair, citing questions and answers on Math.SE does run the risk of those posts getting deleted at some point later (for reasons of moderation or by the choice of the authors). Though I still trust the community's robustness and my ability to identify high quality posts that will most likely stay. Aug 5 at 18:11
• As a practical matter, a single short sentence is unlikely to be a good answer. Aug 9 at 0:37
• Tangential comment, but it would've been very nice to explain in your answer that SFFT stands for "Simon's favorite factoring trick", and that this trick is popular in the Art of Problem Solving community. Don't just throw around acronyms without explaining what they stand for. "Acronyms Seriously Suck." Aug 10 at 8:11

The actual issue I see with the answer is that it's a link to a third-party blog post instead of either a self-contained hint (which in this case could also be a one-liner, e.g. "Hint: write your equation in the form $$(x+a)(y+b)=c$$. Do you see how to do this, and how to then find the values of $$x$$ and $$y$$?") or a canonical reference (to a section or exercise in a textbook or journal, etc.). The blog post could move or disappear at any time.