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(description copied from the old thread)

As has been proposed in chat (and seconded by a couple of users), it seems that it is time to create a new thread for reopen and undeletion requests. The old thread (as of now) has over $200$ answers, and it is really hard to scroll through the mass of old and/or possibly outdated answers--voting is a mess, too. (It's especially problematic for 10k users who can see the deleted posts.)

marked as duplicate by quid discussion Jul 15 at 10:45

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

231 Answers 231


This question has recently been undeleted. I have to admit that I'm stunned.

There is nothing good about this Q&A: the questions consist a homework dump from a timed online quiz; they have apparently been copy-pasted here without any effort done to even format them correctly (the sentence "Answer both parts for full credit" is especially bad, I half expected to find "Calculators forbidden" in there); there is a multitude of unrelated questions instead of a clear, detailed, focused question; the title is uninformative (something that takes 20s to fix, but no-one seems to care about that); the "answer" does not even address any of the questions and instead consists of a different problem that A.N. devised and then answered.

Is this what math.SE is for? This thread is for Q&As that require exceptional handling, so I'm asking for this question to be re-deleted. The Q&A will not help any future visitors, it probably didn't even help the OP.

Arthur Fischer's note.

That question was a wreck from the beginning, and probably should have been closed within hours (if not minutes) of its being posted back in 2011. Way too many separate, specific questions to make for a valuable reference going forward. I don't think any amount of effort would be sufficient to polish that turd.

Gerry Myerson's original edit, though well-intentioned, made the question worse, IMHO. It did allow for André Nicolas's answer to match the question, but now instead of simply a "truckload" of separate questions, we have an additional "ask your own similar question and answer it". This takes too broad to another level.

Daniel Fischer, robjohn, Jyrki Lahtonen, Michael Greinecker and myself have discussed this question, and we really couldn't find much redeeming value in it. Were the answer of superlative quality, myself or another mod would have locked the question (undeleted) for "historical significance". At this point the only thing historically significant about the question is the delete-undelete war. We all wanted it to end, and now it has.

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    I purged the comment thread. Sorry that I didn't act earlier. The goal should possibly be to prevent these skirmishes from escalating. I appreciate Gerry's effort in editing the question, but it does look like the question was beyond repair. – Jyrki Lahtonen May 14 '15 at 12:49


I would like this question to be undeleted. It was voluntarily removed by the author although a correct answer was given. Given the OP's history on MSE, it seems the user asks questions, gets answers, and then deletes the questions. I do not think this should be permitted, as some user in the future may have the same question as the OP, but a valid answer will not be present due to a senseless deletion.

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    It's customary that the bold bit is the current state of the question - as it said "Undeleted" before, users encountering this post might have thought it had already been undeleted and hence not cast undelete votes. I have changed it to "Deleted" so that people will recognize that action is desired. – Milo Brandt Feb 24 '15 at 5:24
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    @Meelo Ah, thanks for the pointer. Haven't used meta really at all until now. Thanks. :) – Daniel W. Farlow Feb 24 '15 at 5:25
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    @Meelo actually, if you look at the other requests (on this thread and the previous one), you'll see that the change of status is recorded, not the original status. It makes it easier to browse through the list by making a visual difference between requests for which nothing has changed (and thus still require action), and those for which something has changed (usually the issue has been resolved). – Najib Idrissi Feb 24 '15 at 6:51


Please consider re-closing this question : Is it to the students' advantage to learn the language of infinitesimals?

On the one hand, the question is primarily about pedagogy, which would be a better fit for a different SE site. But, more importantly, the question is essentially a request to engage in a discussion about whether there is an "advantage" to teaching infinitesimals. This is a perfect example of a question that is primarily opinion based, and arguably "subjective and argumentative" as the former close reason said.

This kind of subjective discussion is certainly healthy for the field of mathematics, but it is not what we want on this site. The help center puts it clearly: If your motivation for asking the question is “I would like to participate in a discussion about ______”, then you should not be asking here.

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    IMO this wasn't a question at all but an opportunity for the OP to use the site as a soapbox. – Najib Idrissi Feb 21 '16 at 8:29


Request for reopening: What would change in mathematics if we knew $\pi+e$ is rational?

It was closed as unclear last March. I edited it to try to make it clear, and there are comments that (to me) show that the question can be answered reasonably. I voted to reopen, then it went through review and left with 3 selecting "leave closed."


This question was closed as "off topic". It was originally just a statement of a problem, but not the OP has added some more detail and motivation. So I believe that it should be reopened.


This question was deleted right after OP saw my answer. I just want to have the answer for myself as I used some time to figure out how to solve it.

  • Yes, this should be undeleted. I will flag the question. – Thomas Mar 15 '16 at 21:27
  • @Thomas Thank you. – Winther Mar 15 '16 at 21:55


Linear Transformation on $\mathbb{R}^6$ was closed for missing context (also by me) because the OP showed no work. This was changed in a comment to my answer. I have copied the relevant comment (of the OP) into the question body to salvage it. Maybe someone TeXifies the image? (Can't be bothered right now)

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    It can be a good idea to wait a bit before posting here. Sometimes the reopen review "just works." If not you could still post here. – quid Jan 29 '15 at 17:41
  • @quid Thanks, will do next time. – AlexR Jan 29 '15 at 17:42
  • It already had a recent review history though @quid so I'm not sure what would happen. I agree it would be good to wait a while to see. But there was a useless edit 11 hours ago that put it in the review queue, which it left with 3 selecting Leave Closed. After that, would a reopen vote put it back in the queue? – Jonas Meyer Jan 29 '15 at 19:28
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    @JonasMeyer reopen votes (in contrast to edits) place a question into review also another time (except there are active reopen votes on it that already triggered a review). See at the end. – quid Jan 29 '15 at 19:41
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    Well it doesn't seem to have shown up in the review queue, so my guess based on what quid says is that the earlier spot on the queue was initiated by a reopen vote instead of that edit that was around the same time. Turns out posting here was a good idea anyway. – Jonas Meyer Jan 29 '15 at 23:57


This question was marked as a duplicate of an identical one by the same user. But the duplicate has been deleted, leaving a question marked as duplicate of a deleted question.

Can we either reopen this one, or undelete the other one? I don't care too much either way, those 2 rep points (because of the downvote) aren't the end of the world.

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    Fishy stuff: 1. User posts twice the same question 14 minutes apart. 2. The same day, mod deletes latter version. 3. Seven months later (=yesterday), user deletes earlier version. – Did Jan 10 '15 at 13:51
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    Not necessarily fishy, @Did. The earlier version was downvoted at some time and never upvoted. If that was shortly before the deletion, you have an un-fishy explanation for the deletion: get back two reps. – Daniel Fischer Jan 10 '15 at 14:51

Deleted by Community Undeleted

I would like this question to be undeleted: Why is there more room in a square room than there is in a rectangular room?

I believe it was deleted as a consequence of having a negative score when the owner's account was deleted, but I think Rahul's answer is a gem worth saving - and the other answers are substantial contributions too. The question is actually pretty good too (though it is hard to see this without first seeing the answers), and I edited it to be a bit less aggressive in tone.

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    "The question is actually pretty good too (though it is hard to see this without first seeing the answers)": so the answers are good, the question isn't. A question doesn't get to -4 for no reason... – Najib Idrissi Feb 4 '15 at 8:34
  • Out of curiosity, what drew your attention to that question a good month after it was deleted? You hadn't posted an answer there, it's not among your "favorites", how did you become aware of it being deleted? – Daniel Fischer Feb 4 '15 at 11:06
  • @DanielFischer I happened to discover the "earned bounties" tab on my profile and was looking through old answers of mine, noticed a comment I left on this other answer (to a question I'd earned a bounty on), clicked the link, and saw that it was deleted, and that that was sad. – Milo Brandt Feb 5 '15 at 0:00
  • Ah, strategic commenting ;) – Daniel Fischer Feb 5 '15 at 0:05

Deleted, undeleted, deleted, undeleted "unilaterally" by a mod

Request for reopening: Probability of first player rolling double six first

It was closed as "missing context or other details," but there were significant thoughts and attempts shown. The original formatting would make it easy to overlook the attempts at first. I have edited to try to make it clearer. I waited out the reopen review queue, and waited for it to leave the front page after my edits, before posting here.


Request for reopening: Norm of the product of an isometry and a bounded operator, closed as unclear, was originally unclear, but no longer is.


Why is $(-1)!!=1$ was deleted. While the question has no context, it does not seem like a homework question, and $(-1)!!$ is probably not that obvious to the neophyte. It is true that the answer can be found on Wikipedia if one knows to look for "Double Factorial", but again, that may not be obvious to the beginner.

Furthermore, this just seems like a question to which we should have the answer here. So unless this question is a duplicate, I think it should be undeleted.

I should note that I have answered this question.

Undeleted - Redeleted - Re-undeleted - Locked

Inequality $\prod\limits_{r=1}^{\infty}(1+(\frac{1}{2})^r)<\frac 52$ was a good question with a couple of decent answers. I think it should be undeleted. Although a bit slim, the OP did state that they had tried induction (and I used induction in one of my approaches, so it is possible). The OP made a strange last minute edit, changing the upper limit of the product to $-\infty$, but I have changed it back to $\infty$.

I should note that I have answered this question.

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    I deleted the comments. Sorry about resorting to a blunt instrument in resolving the matter, but the comment exchange took a turn for worse. Very mildly so - only one flag so far - but I'm nipping this one in the bud. Largely because the discussion was no longer relevant to the topic of this un/delete request, – Jyrki Lahtonen Mar 17 '15 at 10:32


I suggest to reopen Translate this proof from German to English

The question was edited so that now the contents of the image is typed as text.

This question already was in the reopen review queue and the result of the review was "Leave Closed". But if I read the timestamps correctly, it was before this edit.

There were several discussions about s and it seems that such questions are allowed on this site.

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    Um, I thought editing a closed question to include the answer in it (which is what Brian M. Scott did) was a no-no? – Najib Idrissi May 15 '15 at 13:21
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    I agree that editing an answer into the question is not good. But I think that this is a separate issue from whether the first half of the edit (i.e., edit of the question, not adding the answer) improved the post enough to be reopened. I have pinged the user who made that edit and asked him to move this to an answer. But for the time being I suppose it is best to leave the answer there, so that somebody else does not waste their effort on the same translation. – Martin Sleziak May 15 '15 at 13:25
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    @NajibIdrissi I agree in principle. However, in the current case at least the question was also improved by typing it up. And, from a purely practical point of view it seems considerably simpler to do it like this. Thus, I consider it as more understandable in this special case. – quid May 15 '15 at 14:22


This Question, about how many real roots a (real) quartic equation has, was closed as "off-topic" qua not about mathematics.

The OP is not a native English speaker, but based on clarifying comments, I have undertaken to rewrite the Question, disposing of distracting elements that may have induced much of the votes to close earlier.

Please consider voting to reopen. The OP mentions discriminants, which is one of several possible approaches, and in my opinion shows sufficient context for the Question to be taken seriously by the Community.


How about we reopen the following?

Is there a non-trivial countably transitive linear order?

The question asks about an interesting, unusual object. As the comments show, several "typical" candidates fail (for easy reasons), and to find an example is really not entirely straightforward.

The problem is tagged "set theory" and "order theory" but it should just as well have the "model theory" tag (as the given answer illustrates). Reopening it should give us the chance to see additional examples. I myself am curious to learn what the "modest mouse" is.

(The poster has edited the question recently, adding some relevant background.)

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    I found this in the re-open review queue and cast the final vote to re-open it. The original question was a PSQ, but it has been edited since then. – Carl Mummert Aug 9 '15 at 0:54


Please vote for the undeletion of this thread. The OP deleted it right after I've posted an answer which I consider useful for other users, knowing that Atiyah and Macdonald is one of the most studied books as an introduction to CA and AG.


This Question Two disjoint closed sets $A,B \subset \mathbb{R}$ such that there does not exist (positive) $\varepsilon$ with $d(A,B) \gt \varepsilon$ was mistakenly marked as a duplicate due to my overlooking the requirement that $A,B$ are subsets of $\mathbb{R}$. The faux duplicate concerns two closed subsets of the plane.

Little help?

Reopened, closed again (off-topic/missing context), deleted and undeleted and reopened

Please vote to reopen this question: Let $I = \{a +\sqrt2b \in \Bbb Z[\sqrt2] : a$ and $b$ are both multiple of $5\}$. Show that $I$ is a maximal ideal.

It was closed as a duplicate, but it isn't: in the linked post the ideal $5\mathbb Z[i]$ is not maximal, while $5\mathbb Z[\sqrt 2]$ it is, so there is a clear difference between the two rings, and the questions themselves.

  • Since the question has been abandoned and has two good answers, I took the liberty to add some context and voted to reopen. – user99914 Feb 21 at 4:03


What is the minimum cardinality for a set intersecting each member of a configurations of pairwise intersecting disks of the same radius? was closed as unclear. I had moderate success in guiding OP to clarify; I think the question can be understood now [and it certainly can, if you are also willing to dive into the comments]. Last time I looked, it had stalled at 4 votes to reopen. Is there a fifth?

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    Is it possible that it stalled at four votes to reopen because you need to read a dozen comments to fully understand the question...? – Najib Idrissi Feb 18 '16 at 7:10
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    I rewrote it now. Hoping to match the intent. – quid Feb 18 '16 at 14:15


I think that Why do people accept the axiom of choice given the well ordering principle? should be reopened.

It's a valid question, and I recall at least two undergraduate classmates that felt the same way back in our sophomore and senior years. It's a clear enough question to anyone who has heard it before, which is how Noah and myself knew to answer it (I can't speak for Noah's understanding, but I immediately knew what's the question).

Reopened, marked as duplicate

Please vote for reopening this question. As I've mentioned into the comments, it is a duplicate, but it's also closed for a wrong reason, that is, "unclear what you're asking": I think it's pretty clear what the OP is asking!

  • That question had a bit of bad luck. Since it was closed, five reopen votes aged away. – Daniel Fischer May 10 '16 at 12:19


Please consider reopening this question. The original version was dismally unclear. The first revision (not much better) came after it was put on hold, so that window of opportunity for getting it reopened came and went.

I edited it recently to make it clear what is being asked (reverse engineering the notation was not difficult for me, but I do have a tiny bit of experience with that). Arguably the question is still not good (lack of context?), so I quite understand, if you want to leave it closed. At least it should now be clear what is asked.


Requesting more opinions about this closure. If I got it right the votes to close it as unclear were partly (if not solely) motivated by the unusual notation for the Legendre symbol. User Did fixed that, but that was too little too late.

Initially there were also comments from users who missed the fact that the question was not about the usual alternating variant of the harmonic series but rather a variant were the signs follow a periodic pattern of length seven. I made an edit trying to make the pattern of signs clear. Since then the question has received two votes to reopen.

Disclaimer: I answered because IMHO the question was quite clear with the Legendre symbol explained in the accompanying text. Arguably we would like to see more context. You be the judge (new user to whom English is not the first language). My answer came well before the question was closed, so the reopening is for the benefit of those users who want to see Did evaluate the series as a definite integral involving a rational function :-)


Request to reopen this question:

If the map induces identity on all homotopic groups then it is homotopic to identity

It appears that the question was closed because the closers did not read it carefully: At least the first close vote stemmed from the incorrect assumption that the question was about the proof of Whitehead's theorem. Even in its original form the question was about something else.


This question has been deleted by its author. However, I don't see anything wrong with the question; the statement of the problem is well posed and understandable, as well as there is some context.

Furthermore, one user has already posted an answer although it has not received any upvotes yet, I don't think it should be wasted.

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    Indeed a good question, with context and all (and a good answer as well...). – Did Mar 19 at 13:04

Undeleted, reopened, reclosed, re-reopened

I would like to see this question undeleted. It received three upvotes and four reopen votes and several good answers. There is a reasonable amount of work shown by the asker, and it is mathematically interesting.

There is a related meta post (10k+ only).

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    The question lacks key context, most notably where the OP came across the problem and why it is of interest. – Carl Mummert Mar 18 at 22:46


On mathematical grounds, I'd like to voice that a mistake was made to mark this post #2766434 as a duplicate of this one from 2014.

The linked older question in 2014 is the basic case where all the rates (of underlying Poisson process) are constant and identical. In #2766434 not only the rates are not identical, they are to be modeled with distinct distributions (which I think is the main point of the question).

  • "Marked as duplicate" means it is closed. – Gerry Myerson Jun 12 at 10:44
  • I previously thought "marked as duplicate" is not the same as "closed as duplicate". Thanks for pointing that out. – Lee David Chung Lin Jun 12 at 10:51
  • Even though it's not my post, here's my thanks to all of you who participated in reopening it. – Lee David Chung Lin Jun 12 at 12:35

Closed, Reopened, Closed, Deleted, Undeleted, Deleted, Undeleted, Deleted, Undeleted, Deleted

I would like my question to be undeleted and reopened:

My question was initially closed for missing context. After I provided the requested context, it was reopened. Then it was closed for the second time, now for being off-topic. Apparently some members of the community are of the opinion that mathematical puzzles have no place at this site. Yet there are countless examples of well-received puzzles at this site that prove otherwise.


Update, september 2018. After three years I return to this website, only to see that my question has not been reopened or undeleted (in contrast to many others). Strange. If I click on the link, I am lead to my old question. In the column on the right I see a whole list of similar puzzle-type of questions which have been approved and awarded points. Bizarre.

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    I voted to undelete, for two reasons: a) A comment under the question (which received $4$ upvotes and thus played a significant role in the discussion) says: "We're here to help people, and you don't need help with this." I disagree; in fact it's quite normal for questions to be posted for reasons other than needing help, and most such questions are not closed or deleted. – joriki Aug 13 '15 at 11:46
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    b) The partial answer given by Emisor demonstrates that thinking about prime factorizations is required to solve the puzzle. This is not "elementary, using only every day ideas and notions" (as described in the question Najib linked to above). There are many more trivial questions about divisibility and prime factorizations, and they're generally considered mathematical and not closed or deleted. – joriki Aug 13 '15 at 11:46
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    FWIW I cast a vote to undelete this. Memories. – Jyrki Lahtonen Sep 23 at 5:24


This question is not a duplicate of "proof by induction" because it is explicitly asking for a proof that is not by induction, but rather one that explains the method used in the included GIF. (The question was edited to clarify.)

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