The purpose of this thread is to help focus the attention of the community on posts that may require exceptional handling. This includes requests for reopen and undeletion votes. A request should be posted as an answer below.

Please do not use this thread to engage in debates on contentious matters (e.g. reasons for closure). That should be done in a separate linked thread. The goal is to keep this meta thread free of tension, so that everyone feels comfortable posting here. Please be polite, and respect the many different viewpoints in our diverse community.

To inform readers of the current (and past) states of the targeted post, please append tags such [REOPENED,RECLOSED] or [UNDELETED] at the start of the answer.

Beware that "short" requests such as "request reopening of <link>" may be automatically converted to comments by the SE software, so you may need to write more (e.g. why you think that the question should be reopened or undeleted).

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@Bill: I saw no reason to wait with that idea. There was no actual discussion in this thread anyway. –  Asaf Karagila Oct 24 '12 at 18:34
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@Gerry: The solution would be to add a few words, I suppose. For example why it should be reopened. –  Asaf Karagila Nov 9 '12 at 12:16
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@Asaf, I opted for cursing the darkness rather than lighting a candle. Anyway, the question has been reopened. –  Gerry Myerson Nov 9 '12 at 21:54
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@Gerry: Darkness is just the light's way of proving the empty set exists. –  Asaf Karagila Nov 9 '12 at 21:55
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@Willie: I think that we should delete old reopening requests and perhaps have one post/additional thread for indexing them. I should also think that any request older than $n$ days for some reasonable $n$ should be deleted. If something has not been reopened and the initial votes expired... well, it makes sense to conclude that there aren't that many people interested in reopening. –  Asaf Karagila Nov 30 '12 at 9:49
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@Willie It's probably useful to have some nontrivial history remain so that folks $\rm < 10K$ can gain some idea about what types of questions do get reopened, and what types don't. By quickly scanning the requests it might help to convey some idea of the community consensus on marginal topics. To keep the unopened requests at the top of the active sort, they could easily be bumped if there is still interest. –  Bill Dubuque Dec 1 '12 at 1:44
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@Belgi: This is why I prefer to browse Meta with answers sorted by activity. –  Rahul Jan 5 '13 at 21:38
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@Belgi Sorting by activity solves the problem. I just bumped the only active discussion to the top with an edit. There are two requests dated by November 2012, which I guess are no longer ongoing conversations (but anyone so inclined can bump them; it's a CW). –  user53153 Jan 5 '13 at 22:07
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Thanks for the workaround, I still think theres no reasons for this to log all reopen request that were/will be made –  Belgi Jan 5 '13 at 22:10
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Per the (short) discussion in this deleted meta post, I have edited this thread to support requests for votes to undelete as well as votes to reopen. –  Alexander Gruber Jul 13 at 14:59

166 Answers 166

I would like to request Motivation for the Tensor Product be reopened as I do not think it is a duplicate of and Motivation for Tensor Product.

The latter question asks "We already have Direct Product, Semi-direct products, so after all why do we need Tensor Product?", which is a question about why multilinearity is in general useful.

The former question (which was closed as a duplicate) asks "What's the reason/motivation to define the tensor product using the free vector space and that quotient to impose linearity? Can someone point me out the motivation for that definition?". These are questions about the specific construction of the tensor prodct, and NOT the general usefulness of multilinearity properties.

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I think one thing that would help is if the two questions are edited to they have more precise and more distinct (from each other) titles. –  Willie Wong Mar 18 '13 at 12:16

[REOPENED]

This question was closed as a duplicate, but the question specifically asks not for a proof (which the OP knows) but, instead, for some intuition behind successful strategies for constructing such proofs - something that is not addressed at all in the proposed duplicate. Probably there is a good chance of further helpful answers appearing if it is reopened.

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[REOPENED]

(Also, as pointed out by Asaf in a comment below, this was a duplicate request. My apologies for the inadvertent dupliaction.)


The question "If every convergent subsequence converges to a then so does the original sequence" should be reopened (in my opinion).

It is a request for clarification/explanation of an argument from a text.

It was closed as off-topic since it is essentially a repost of an earlier, deleted question. The thing is: this earlier question was deleted by the Community User, hence cannot be reopened by 10K+ voters. My view is that the earlier question was reasonable as well; if it could be reopened, then I agree that the current version should be closed as a duplicate, and the earlier one revised for clarity and reopened. Since that doesn't seem to be possible, I have voted to reopen the current question, and I would ask others to do likewise.


In general, it seems a bit unreasonable to me that the OP has been put in a Catch-22, in which their current question is being closed essentially as a duplicate of an earlier question which is unable to be undeleted/reopened.

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@AsafKaragila: Dear Asaf, Thanks, I hadn't seen this. (I somehow missed it when looking down the list.) I'll try to sort out the duplication of posts soon (although I have to rush out right now). Regards, –  Matt E May 5 at 11:31

Will you please to reopen Specific Advice/Tips to Streamline Studying Math by virtue of its upvotes and those on my Answer.

Will you please to appreciate the fact that most of the downvotes in its Meta thread (How to Adapt or Emend : Specific Advice/Tips to Streamline Studying Math) occurred before my edit to my question, and observe Lord_Farin's gracious comment underneath his answer thereunder.

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@TobiasKildetoft It may be broad, but it is also of general interest. Broad questions are frowned upon, but often are looked less critically if they are of general interest. For example: math.stackexchange.com/questions/71874/… is incredibly broad, as are many of the top-voted soft-questions. –  user1729 Oct 28 '13 at 19:11

[REOPENED]

My question here was said to be too broad, so I tried to edit it with a more specific example: How can you tell when you need to use the binomial coefficient for probabilities?

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[REOPENED. Thank you.]

Will you please to reopen Infer distance from a point to a line, from the distance from a point to a plane [Stewart P793 12.4.44]?

My question is tendered at the bottom of my question and there are no comments on why it is avowedly "unclear." I've made an ancillary edit that should only clarify the OP.

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[REOPENED] Show that floating point sqrt(x*x) >= x for all long x.

I would like to see this question reopened. I believe it is a valid numerical analysis question.

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I am trying to get

Let $G$ be a finite abelian group. Let $a\in G$ be an element of maximal order. Prove $|b|$ divides $|a|$ for all $b\in G. A\: different \: proof\: $

reopened. It was closed as a duplicate of Prove that for any element $b$, $|b|$ divides $|a|$ (order of $b$ divides order of $a$). and Finite abelian group generated by elements of maximal order. True the question has been answered, but I haven been given another outline of a proof for this and would really appreciate it if i could get some feedback on this version of the proof for this question

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Please re-open Undefinably large algebra? . It has been substantially revised and should be sufficiently clear at this point. Also, note that you can address the question without addressing my semi-formalization of a hopeful answer. Suggestions for helping the question be re-opened can be put here: Working Towards Re-Opening (Undefinably large algebra?)

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Please reopen Intuition. If f differentiable and $\lim_{x\to0} f'(x) = L$, then $f'(0) = L$. (S.A. pp 137 q5.2.8c,d)

This question is for intuition only and no proofs. The other is for proofs. Ergo they aren't duplicates. The closers left no comments?

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May be I'm wrong, but it seems to me the pleas to reopen work better, when they come from someone other than the original poster? I rather thought that was the whole point of having this thread. –  Jyrki Lahtonen Mar 1 at 17:27
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@JyrkiLahtonen It works fine for the OP to post here if the question has either been edited to address whatever got it closed, or if the closure was based on a misunderstanding (I didn't check if either was the case here). –  Tobias Kildetoft Mar 1 at 18:38

Please reopen Prove nth root of k exists with supremum. (S.A. pp 27 1.4.6b).

My question is long but I put my questions in the gray shaded boxes. Questions are numbered too and spaced. Ergo it should not be 'unclear what you're asking.' None of the closers commented?

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[REOPENED]

Becoming Better at Math was closed on the grounds of being primarily opinion-based. It is community wiki with and tags. Though there are many possible answers (hence the CW), I do not find the question to be opinion based, as it is a call for guidance from experienced mathematics students, who in general do not disagree about most fundamental aspects of learning mathematics. (In other words, though opinion-based answers to this questions may exist, certainly not all answers are opinion-based.) This question could serve as a useful collection of practical advice specifically aimed at high school aged students preparing to major in math in college.

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[REOPENED]

The question Is there a highest order of infinity? was closed as a duplicate. Its original version asked whether "there is a limit to the orders of infinity", something that has been asked here several times already. The person asking the question has since clarified that what they are asking for is something very different,

Does there exist an infinite set of cardinality such that it can never be reached by taking power sets of a set with cardinality aleph-null?

(By the way, if reopened, an answer may as well clarify that $\beth_{\omega}$ is far from being "highest" among the infinite cardinalities.)

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[REOPENED] I would like this question:

Field Extensions of cos and sin

to be reopened. The question was closed as a duplicate of Degree of field extension. Bit IMO, the questions are different. The closed one is about whether a specific proof is correct, not just a question about how to prove it (like the other question).

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[Re-opened] I'd like to suggest reopening What is importance of the Bunyakovsky conjecture?. The question in the body, "how important do you consider the answer to this problem", may well be "too soft", but the question in the title seems to me to be a solid mathematical question.

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I cast the fifth and final vote. –  Willie Wong Mar 13 '13 at 12:32

[REOPENED] This question should be re-opened as the OP has edited the post to try to tell us what they have done.

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[REOPENED] Thank you.

The question was undeleted by the OP, and has now been closed. I think it does more good being open and have myself voted to reopen. As I told the OP elsewhere, "The thing is, many (oh so many!) people think that mathematics is just about computations (long and tedious and often times pointless). And many among those that know better think that the only way to use a computer is to help with these computations. But there is much more to math than that, and many more uses of a computer in mathematical research. And there are the incompleteness theorems, showing that not all mathematical practice can be automated anyway. Good answers to your question could be quite insightful!"


This question was actually deleted, but I think it is interesting and can be useful to many, not just the OP: Why are there mathematicians that do not use computers?

I was watching a video on Andrew Wiles and his proof of Fermat's Last Theorem and I quite liked the video, especially the complexity of the proof only to prove a simple concept which can be understood by most people. I also liked the graphics they used to illustrate elliptic curves and modular forms.

But then Andrew Wiles said that he never uses a computer, he only uses pen and paper and I also heard of other mathematicians that don't use computers.

Do they not use computers because there are problems only a mathematicians can solve or are there other motives? Wouldn't the proof have taken much less than 7+ years if he used a computer?

I have voted to undelete. I am posting this request here as it seems a natural extension of the intention of this post. Also, if there are issues with the question, this may be the place to address them, which may also prove useful on its own.

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@Thomas That's fine. In this case, I believe the deletion happened because of the downvote, which I consider unwarranted. Regardless, I think (as I said) that the question can be useful to many, not just to the OP, which should (surely?) be our main consideration. –  Andres Caicedo Aug 11 '13 at 19:23
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I agree that the question with answers could be useful. But, on principal grounds, I believe that it is wrong to undelete a question that the OP him/herself deleted. I think a better approach is for someone else to repost the question. I don't think that we should assume we know why someone decides to delete his/her own question. –  Thomas Aug 12 '13 at 2:42
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Ok, I didn't know that. In that case I see no problem. –  Thomas Aug 12 '13 at 12:45

I would like to request that the following question be re-opened.

Any two sets $Y$, $Z$ have the same cardinality $\iff$ there are injective functions $f: Y \rightarrow Z$ and $g: Z \rightarrow Y$.

I has been closed as a duplicate. However, it is a "check my proof" question and so cannot be a duplicate. Yes, the underlying maths problems are the same, but the actual questions being asked are not.

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It might not technically be a duplicate, but it already has an accepted answer, and anyone else searching for something related to this question will probably be better off redirected to the other question. –  Tobias Kildetoft Sep 23 '13 at 11:02
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@TobiasKildetoft True, but that isn't grounds for closing. That is grounds for linking to the other question in the comments. –  user1729 Sep 23 '13 at 11:08
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While that might not be grounds for closing, I think it makes a decent case for not reopening, as this way the other question is placed at greater prominence for any later visitor. –  Tobias Kildetoft Sep 23 '13 at 11:11

[REOPENED]

I would like this question reopened. Closing it because the OP didn't state anything beyond the bare question is a little harsh, and the question is interesting.

EDIT: Paul Siegel's answer has been accepted with 3 upvotes. That seems to indicate that reopening is appropriate.

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@Did: because closing it seems like a heavy-handed way to get the OP to respond. The detailed comment that was left was more than sufficient. –  Grumpy Parsnip Nov 3 '13 at 22:22
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@GrumpyParsnip Two weeks later... and in view of the subsequent (or lack of) events, do you still hold the same view? –  Did Nov 17 '13 at 14:47
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@Did: I still think it deserves to be reopened, even if it has been abandoned by the OP. It's the question itself that is interesting, not the person behind it. –  Grumpy Parsnip Nov 17 '13 at 19:52

[REOPENED]

In my relatively expert opinion this question is perfectly clear. Yet it was put on hold as unclear what you are asking. The OP also has put a rather non-trivial amount of effort into understanding the question given that they say to have implemented the usual Viterbi decoding algorithm. Admittedly the OP has built a bit of a history in asking coding theoretical questions that actually are unclear and/or show very little effort. This question is not one of those. I will answer it in a comment (it has a very quick answer), and let anyone else take a first bite at answering. But keeping this on hold sends IMHO a wrong signal.

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This question about radian measures was recently marked as a duplicate of this question about $\pi$ being the same for all circles. The former question asks 2 concrete questions in its body, and I feel that at most one of them is covered by the latter question. I think it would be better if the question about $\pi$ were mentioned in a comment, rather than designated a duplicate.

Note: I don't have 3000 rep, so when counting reopen votes, don't count mine.

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[REOPENED]

Please reopen Are these sets with orbits subgroups of $S_A$ = set of all permutations of A? - Fraleigh p. 86 8.41-8.43.

My questions are marked with numbers (1.) to (3.) and I already updated the question. Therefore I don't see what's 'unclear what you're asking'?

The people who closed did not comment on what's 'unclear'.

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Please reopen What's $\mathbb{Z_2 \times Z_4}, \mathbb{Z_3 \times Z_8} $ isomorphic to - Fraleigh p. 112 Exercises 11.32e, f, g.

I removed the darkred color but can add it back. Please notify me which is better. My questions are marked with numbers (1.) to (3.). Hence any color problems should now be settled.

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Please reopen Algebraically compute $\lim_{x\to0}\frac{\sin x}{x}$ which was closed as a duplicate of How to prove that $\lim_{x\to0}\frac{\sin x}{x}=1$?. The latter explicitly asks for a geometric solution, while the former explicitly asks for a solution that uses as little geometry as possible. My own reopen vote has already expired, as have two others.

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I think that the

1+1+1+1 ...= -1/2

question, which at present is a merger of an older 1 upvote/1 (not so highly voted) answer Math SE question and a two days ago migrated from from Physics 5 upvotes/highly upvoted answers question, should be reopend as there is IMHO nothing unclear about the OP asking for an (intuitively understandable) proof that $ 1 + 1 + 1 + 1 \cdots = -\frac{1}{2}$.

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Can someone please open Colored Picture for Equivalence Classes, Relations, Partitions, ..

My questions are numbered in bold. What's purported unclear?

Sequel — Fev-6-2014 — I edited. The question has more original text now.

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Pretty much everything is unclear. What are all those boxes? What are all those circles? what are all those squiggly things beneath the circles? What's your, and what comes from the textbook? What is the textbook trying to do, and what are you trying to do, and what exactly are you asking us to do? –  Gerry Myerson Feb 6 at 8:36
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Good, it's better now. –  Gerry Myerson Feb 7 at 10:10

Please reopen If every convergence subsequence has the same limit, then bounded sequence converges to it. (S.A. pp 58 2.5.4) please.

My question is long but I put my questions in the gray shaded boxes. Questions are numbered too and spaced. Ergo it should not be 'unclear what you're asking.' None of the closers commented?

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Did you see where it said, "Beware that "short" requests such as "request reopening of <link>" may be automatically converted to comments by the SE software, so you will need to say more, such as why you think that the question should be reopened. "Please do not use this thread to engage in debates on contentious matters (e.g. reasons for closure)." –  Gerry Myerson Mar 1 at 23:20
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@T.Bongers And, worse, 10K users cannot vote to undelete since a Community bot deletion counts as moderation deletions. Bots should not be considered smarter than users regarding such. –  Bill Dubuque Mar 17 at 3:58

The 2012 question Prime counting inequality was put "on hold" two days ago, with the reason "unclear what you are asking".

I think the question is clear: what is the largest $c$ such that $\pi(x)>cx/\ln x$ for all integers $x$ [in the range where function $\pi$ makes sense]? It admits an answer: $c=\ln 2/2$. Not a very exciting answer, but not entirely trivial either. I see no reason at all for this question to be closed.

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Ah, but you are misquoting the question: it was asked for all integers $n$, not $x$. This led to a commenter asking whether $n$ was supposed to be $x$, a question which the author did not answer. So, technically, the question is unclear. But on the whole I don't see where closing it does any good. –  Gerry Myerson Mar 3 at 6:40

[REOPENED]

I think this post should be reopened: Evaluate $\int\frac{\sqrt {25 - x^2}}{ x^4}$ (It already has 4 reopen votes, so only one vote is missing.)

It was closed as a duplicate of another post, but they are definitely not duplicates. (It seems that the problem might have been that the OP did not know how to make a post using TeX/MathJax.)

If course, if there is indeed a duplicate somewhere on the site or if you think that it should be closed for another reason, I do not object to closing the question. But at the moment it is closed for incorrect reasons.

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[REOPENED]

Counting Shaded Squares was closed as "missing context or other details". It is a perfectly clear and straightforward question about combinatorics: how many ways are there to color exactly two squares in an $n×n$ array. Marko Riedel and I each had no trouble answering it.

(I voted to close, not as "missing context" but as a duplicate of how many unique patterns exist for a NxN grid . But on review I see that the two questions are different.)

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