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?

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    $\begingroup$ Was it the post of Mark Sapir here? meta.mathoverflow.net/a/618/2926 $\endgroup$ – Todd Trimble Dec 17 '13 at 13:00
  • $\begingroup$ 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. $\endgroup$ – Amir Asghari Dec 18 '13 at 10:17
  • $\begingroup$ @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. $\endgroup$ – Willie Wong Dec 18 '13 at 10:19
  • $\begingroup$ Oops :) It is @Todd fault, linking to two diffenent places! :) $\endgroup$ – Amir Asghari Dec 18 '13 at 13:31

Hah, my "trivial answer" was converted to a comment. Anyway, you may have been thinking of Mark Sapir's post, as more appropriately linked to here: https://mathoverflow.net/a/137715/2926.

  • $\begingroup$ That's it! Thanks. $\endgroup$ – Willie Wong Dec 17 '13 at 15:11
  • $\begingroup$ Even though this is not directly related -- Mark Sapir has stated "Most probably I won’t be active on mathoverflow.net any longer: too many homework level questions and the community attitude towards these questions has become too liberal." -- see marksapir.wordpress.com/2013/10/25/mathoverflow . $\endgroup$ – Stefan Kohl Jan 3 '14 at 14:05
  • $\begingroup$ @StefanKohl Hm, not sure what he means, since we seem to spend a lot of time closing such questions. Or perhaps I would say he has too liberal an attitude of what constitutes 'homework'. $\endgroup$ – Todd Trimble Jan 3 '14 at 14:28
  • $\begingroup$ @ToddTrimble: I can't tell for sure as well -- just thought it worth to provide the link, without attempting to interpret. $\endgroup$ – Stefan Kohl Jan 3 '14 at 17:18

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