I assume that most likely this was just an oversight - but since it was a result of a moderator's action, it is probably better to ask than just remove the tag singlehandedly.1

Question: At the moment there are two separate tags and . (The tag is on a single question.) Is it just a mistake, or does MO have two tags with a very similar names on purpose? If this is intentional, what is the distinction between the two tags?

As far as I can tell, the tag was recreated when a moderator recently undeleted this question: Alternating colors on a line: infinitely often or converge? This tag is still listed among the new tags at the moment.

The history of this tag is a bit complicated. According to Wayback Machine, it existed back in 2017. To the best of my recollection,2 at some point in 2017 a moderator merged the tag into this tag and renamed the tag to . (However, the tag remained on the deleted questions - merging only influences questions which are not deleted.)

1After all, if this question is not needed, it can be simply deleted. But I did not want simply revert an action taken by a moderator. (One additional advantage of having this question around is that it documents that a tag can be (re)created by undeleting a question which used to have that tag before it was revmoved/renamed.)

2I am unaware of some place where merging of tags is recorded in the system. (For example, merging changes also the tags displayed in the revision history. For example, you can look at this question: Is there any progress toward solving Gilbreath's conjecture? The revision history shows , despite the fact that when the tag was added, it was called .)

The description of the history of the tag is based on the past discussions: Alexander Chervov's post on meta and a related discussion in chat. You can find a bit on the history of this tag also in this question: What should the tag (open-problems) be used for?

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    $\begingroup$ I would assume that when that question was recently undeleted, the fact that it had an outdated tag was simply not noticed, and that the tags should be merged. Perhaps @@StefanKohl can confirm? $\endgroup$
    – Tim Campion Mod
    Commented May 4, 2021 at 13:21
  • 2
    $\begingroup$ @TimCampion Yes, indeed reviving the tag was completely unintentional. I removed it from the question, as I didn't see that the question is about a known open problem / known open problems. $\endgroup$
    – Stefan Kohl Mod
    Commented May 4, 2021 at 15:08


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