Answer to what was an open question

I vaguely remember reading a question on main about a possibly "open question", which got an answer which started by affirming that the question is indeed open, and then proceeded to provide a solution to the open question, making it no longer open by the end of the answer.

I was specifically thinking about this because if I remember correctly it is an example of an open question being solved as a corollary of some (later) bigger results, and that no-one until the answerer noticed that the open question can be solved in that manner.

Unfortunately I remember none of the useful information that actually can help me search for the question/answer. In particular I know not what subject this was in, nor who the answerer was! Does the above description ring a bell for anyone?

• Was it the post of Mark Sapir here? meta.mathoverflow.net/a/618/2926
– Todd Trimble Mod
Dec 17, 2013 at 13:00
• Though you have already got your answer, I thougt it is worth mentioning this meta post meta.mathoverflow.net/questions/617/best-of-mathoverflow, that is a collection of successful MO stories, including the one you were interested in. Dec 18, 2013 at 10:17
• @AmirAsghari: you may have failed to notice that that link was exactly the one Todd posted one comment above. :-) He linked to the relevant answer in the thread, while you linked to the question. Dec 18, 2013 at 10:19
• Oops :) It is @Todd fault, linking to two diffenent places! :) Dec 18, 2013 at 13:31