Since this issue came up in comments to another questions on meta, I'll ask about the intended issue of the tag.

Question. What is the intended usage of the tag? Should it be used for questions related to some specific open problem? Should it be used for lists of open problems? Or are both usages admissible?

If there is some consensus how the tag is supposed to be used, possibly the tag-info might be improved based on that.

Some information related to the tag in question that I was able to find (and which could also help explain why I see current guidelines on the usage of this tag as unclear):

  • If we check how the tag is currently used in practise, people use it for both possibilities. (For example, there are 26 questions tagged open-problems+big-list.)
  • If you look at the tag-info (here are the links to the current revisions of the tag-excerpt and the tag-wiki), it does not exclude either of the two possibilities, although it does not mention lists of open problems.
  • However, it was pointed out in an older post on meta that the formulation in the tag-info was taken from the old FAQ and some users considered the description in the tag-info outdated: What should be the policy on “open problems” on MO? (See also comments under the corresponding FAQ entry. The post about open problem policy was made in July 2013, shortly after the tag-info was created.)
  • At some point in the past, the tag was called - in Wayback Machine I was able to find a snapshot from July 2017 with the old name. It was changed to the plural at some point by a moderator.
  • In the past, there used to be a separate tag. Wayback Machine shows 42 questions in this tag in March 2017. In October 2017 Alexander Chervov asked on meta where this tag disappeared. It seems that it was merged into open-problem(s) by a moderator.
  • You can find a longer conversation in chat related to these two tags - (open-problems) and (open-problem-list). This is where I learned about the merges and renames I mention above. The moderator who handled these actions said there that: "In any case, using for specific problems is a meta usage and shouldn't be encouraged. I remember adding the plural form in an attempt to discourage this." And in another chat message: "using for specific open problems makes it a "meta-tag", the correct usage is what you intend (whether the intent is a list or something else is to be clarified in the question)." Although shouldn't be encouraged is not too strong phrase, from this it seems that the moderators are in favor of using this tag for lists of open problems rather than using the tag for questions concerning single open problem. (Or at least this particular moderator.)

For the users who are unfamiliar with this terminology, I'll add that the phrase meta tag is used here to mean a tag based not on mathematical content of the question, but describing some feature of the question. (Examples might be tags such as , , , , .) I do not know about some post which offers more details about meta tags (in this sense) on MathOverflow Meta, but you can find Willie Wong's post on Mathematics Meta useful if you want to read a bit more about them: The “meta-tags”.. There is also a blog post The Death of Meta Tags on Stack Overflow blog.

  • 1
    $\begingroup$ It was my understanding that some types of 'open problem' questions (e.g. "does this solve Goldbach") were explicitly unwelcome here, but an "open-problems" tag seems to give the opposite impression. Perhaps this should be mentioned in the tag description to avoid potential downvote avalanches for new users? $\endgroup$
    – Alec Rhea
    Commented Sep 19, 2019 at 2:34


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