There is a following scenario that happens sometimes:

  1. Question is posted on math.SE.
  2. Question don't get any (useful) answers.
  3. Question is crossposted to Mathoverflow and is answered there.

Now, nothing bad have happened, definitely. But the original question still looks unanswered and it's not terribly convenient sometimes: it shows on "unanswered" tabs, sometimes it's bumped by community user (anything else?).

What should we do with such questions (if anything)?

One obvious thing is that posting a link in comments when question is x-posted would certainly be nice. (Maybe we should even explicitly require it in the FAQ — BTW, whatever conclusion we might arrive at, perhaps we should add something about x-posting to MO to the FAQ.)

(Hope, it's clear that I don't think such questions should be closed.)

  • $\begingroup$ Recent example: math.stackexchange.com/questions/45653/hirzebruch-surfaces aka mathoverflow.net/questions/67992/hirzebruch-surfaces $\endgroup$
    – Grigory M
    Commented Jun 17, 2011 at 12:29
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    $\begingroup$ I wish we had "closed as no longer relevant," but we don't. That would be the most appropriate thing to do if we could do it. $\endgroup$ Commented Jun 17, 2011 at 12:34
  • $\begingroup$ @Qiaochu Why do you such question should be closed, actually? (We don't close question after they are answered [here on math.SE] -- what's the big difference?) $\endgroup$
    – Grigory M
    Commented Jun 17, 2011 at 12:37
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    $\begingroup$ @Grigory: okay, fair point. I was thinking that reposting on MO is in some sense choosing to move the discussion elsewhere, but I guess that isn't necessary. Editing in a link seems like a good first step. $\endgroup$ Commented Jun 17, 2011 at 12:38
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    $\begingroup$ @Grigory, the difference is the difference between having all the answers at one site and having the answers split between two sites. Posting links won't stop it appearing as unanswered and won't deter the Community user - only closing will accomplish those goals (I think). Since we can't close "as no longer relevant," close it for some fake reason and leave a comment indicating the real reason for closure. $\endgroup$ Commented Jun 17, 2011 at 12:55
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    $\begingroup$ @Gerry, @all Maybe x-posting the answer from MO (under CW) would be better than closing? $\endgroup$
    – Grigory M
    Commented Jun 17, 2011 at 12:58

2 Answers 2


I think that it would be fine simply to encourage people here to adapt MO answers, whenever they are available, to the math.SE context. It surely often happens that a question is answered on MO in such a way that has a different focus or difficult level of explanation than would be ideal for the math.SE audience, and so this is a nontrivial and important task, which can be rewarded using the usual voting/points mechanism. For example, perhaps the MO answer assumes a certain level of familiarity with a technical method, but on math.SE one would explain it in a different or more elementary way, and so it is desirable that someone perform this "translation" service.

Thus, I believe that the policy should be that links should be provided to the MO answer, but it is fine for someone (anyone) to adapt the MO answer to math.SE. I don't think we should insist that this be done in a community wiki manner, since it is fine for people to get the points for doing this service---they are after all answering a math.SE question using information that is available elsewhere.

Thus, I don't think these questions need any special treatment in comparison with other questions.

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    $\begingroup$ The main thing that this proposal requires is that we support and encourage (rather than criticize) people who simply transfer answers from MO over to math.SE, so that people won't feel inhibited to perform this valuable service. $\endgroup$
    – JDH
    Commented Jun 18, 2011 at 10:39

I would think the ideal outcome is to have the question here either appear to be answered or appear to be closed as duplicate (or migrated). In the first case, my suggestion would be to post a link to the accepted MO answer as a CW answer here. In the second case, I'd like to have it look just as if it were closed as a duplicate of the MO question or as if it had been migrated to MO (note that I said look that way—I don't think the mechanics should necessarily be to close it as a duplicate and if it's already cross-posted, it certainly wouldn't make sense to migrate it, nor is migration between here and MO programmatically possible).

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    $\begingroup$ I'm in favor of the first option you propose of posting the link to the MO answer as a CW answer. That would prevent the question from bumping on the main page and it would still allow users to add answers in the future. $\endgroup$ Commented Jun 17, 2011 at 15:13
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    $\begingroup$ @Adrián: I think you're right—the first option sounds better to me, too, now. $\endgroup$
    – Isaac
    Commented Jun 17, 2011 at 17:19
  • $\begingroup$ not bad advice, but I think JDH's is better (assuming someone is willing to make the effort to do so). I agree that a hanging stub of a question with no answer is a net negative to the internet though, so some action is better than none. $\endgroup$ Commented Jun 17, 2011 at 19:17
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    $\begingroup$ Dear Isaac, I agree with @JDH that if someone can write a nice answer here (influenced/inspired or not by the MO answer) that is all to the good. But if no-one wants to do that, providing an answer with a bare link is a good second choice, just to allow the question to get of the unanswered list. Regards, $\endgroup$
    – Matt E
    Commented Jun 18, 2011 at 0:18

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