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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.

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Reopened, reclosed or

Undeleted

at the start of the answer when a change of status occurs. (This also makes it easier to browse through the list by creating a visual difference for posts that still require action.)

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Notice that the first edit after the question was put on-hold pushes the question into reopen review queue, if the edit was done withing 5 days of closure. So does a reopen vote. It is reasonable to wait until the review is finished before posting here. (If the review has already been finished, it is shown on the timeline of the question.)


(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old thread (as of now) has over $200$ answers, and it is really hard to scroll through the mass of old and/or possibly outdated answers--voting is a mess, too.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.)

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Reopened, then marked as duplicate

Please consider reopening this question:

  • The Latex has been fixed,

  • the OP has given a good try, and

  • it has a decent answer.

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Reopened

Conditions for the convergence of two sorted vectors of samples

I think it was clear enough what was being said even though at least two things (one of which I mentioned above) were not as well expressed as they could have been. In its current state, the question is clear.

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Reopened

Please consider reopening this question. For now it is closed as a duplicate of this question.

The closed question is a special case of the linked duplicate. Unfortunately, both the question and the answer in the linked duplicate use languages from $C^*$-algebra which is not needed at all to answer the closed question. Consider that the closed question are much more elementary and can be answered using knowledge covered in a first graduate course in real analysis, it should be reopened (or at least, not closed as a duplicate of that linked question. I have the feeling that this question has been asked before).

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Undeleted

I added an answer to this (4 day old) question and right after this OP deleted the question.

I think it's a decent answer and hopefully this is not an acceptable practice.

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    $\begingroup$ Not really acceptable practice. $\endgroup$ Nov 9 '15 at 19:40
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Reopened

This question was originally trivial (I was one of the close-voters), but the OP subsequently clarified that they wanted geodesic distance on the sphere rather than Euclidean distance, and I edited that clarification into the question.

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    $\begingroup$ I don't understand what "representing these points in 4 dimensions" means, and I don't understand what $d_{new}$ is supposed to be. I think the question still needs clarification. $\endgroup$ Nov 15 '15 at 22:37
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    $\begingroup$ Is version 6 sufficiently clear? ​ ​ $\endgroup$
    – user57159
    Nov 15 '15 at 23:06
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Reopened

Is the recurrent sequence $a_{n+1}=\arcsin (a_n)$ converging? was closed for missing context. However, the question was answered by the OP, which shows their attempt and where they are having a problem. Although this information might better be added to the question, I think that this provides more than sufficient context and that this question should be reopened.

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Reopened

Is $\#^k \Bbb{RP}^2 \times I$, $\Bbb{RP}^2$-irreducible?

This was closed as unclear, but there is nothing unclear about it. Maybe it used language unclear to the non-specialist. I edited the title and body a little bit to clear up any confusion that seemed possible.

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Deleted

Request for reopening: https://math.stackexchange.com/questions/1595644/how-do-i-integrate-square-root-of-a-rational-function

The question was closed as too broad, and has since been clarified to ask only for when there is an elementary antiderivative. I don't know much about how to answer the question, but it seems like this is a focused enough question to be asked here.

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Reopened, Closed

The question Quadrilateral with maximum area inside a semicircle is in my opinion an interesting question. If the main reason for closing with missing context reason was the lack of work shown by the OP, then it is worth pointing out that the OP posted their own attempt as an answer. So I think that this question might be reopened.

(I am aware that today's post by the same OP had some comments which were a bit unpleasant. But maybe we can judge the posts by their own merit and not based on the OP. Moreover, the reaction of the OP might have been caused by the reason that as an inexperienced user they are not that familiar with how the site works - especially putting on hold and reopening. So they might have felt that putting the question on hold was in some sense unfair.)

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    $\begingroup$ This is a post that is written in a very low-quality manner by an OP who is still active. Rather than re-opening the question before it is fixed, we should wait for the OP to improve it. I don't know anything about the comments by the OP, but if they wish to post a question and immediately answer it, the question itself needs to be composed as a high-quality post, including good motivation of the problem $\endgroup$ Feb 19 '16 at 15:24
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Matrix equation with transpose

This is a question about a linear system where the unknown is a matrix. As someone who has some experience with systems like this, the question as written was perfectly clear and interesting to me.

I assume that people who voted to close tried to understand the question in the normal linear algebra context where the unknown is a vector, got confused, and therefore assumed the question was unclear.

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  • $\begingroup$ It seems that Gerry has made an edit to that question to clarify the notation. However it might not be good enough. I am happy to reopened it if someone (the OP would be in the best position to do that) make an authoriative definition of what $X-2$ is. $\endgroup$
    – user99914
    Mar 14 '16 at 6:17
  • $\begingroup$ Err, although well-intentioned, I think the edit made the question worse. First, the exact size of the matrix is irrelevant and over-specific. Second, whether '2' represents two times identity or the matrix full of twos (or any other matrix, actually) is also irrelevant since it doesn't change the structure of the problem or the solution method; that matrix only enters the problem as right hand side data. $\endgroup$
    – Nick Alger
    Mar 14 '16 at 9:40
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    $\begingroup$ I see you have edited the question, Nick, which is a good thing. I take it that it's not important whether the matrices are invertible? $\endgroup$ Mar 14 '16 at 11:27
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    $\begingroup$ @GerryMyerson I think the most general requirement is that $I \otimes A^T - C^T \otimes B$ has full column rank. If the input matrices are square and invertible that would be sufficient, of course, but much less interesting. In optimal control problems you sometimes get matrix equations similar to this where the matrices are not square. $\endgroup$
    – Nick Alger
    Mar 14 '16 at 11:39
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Reopened

Please vote for the reopening of this question: Give an example of a commutative von Neumann regular ring which is not a product of fields.

The OP has provided some context, and want to see what's going on whether a part of the hypothesis is dropped, which I find a legitimate question. Moreover, as it is now the question has a wrong answer, the answerer making a confusion between regular and Von Neumann Regular rings. But under the question there is a comment providing a good answer, and I think we have to give the commenter the opportunity to post it as an answer.

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Reopened, reclosed

I think Two i.i.d random variables inequality should be reopened. The OP has given their approach to the first part of the question, and would like help with the second.

This question has received a decent amount of interest in comments since its closing. I think its answers would be of interest.

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    $\begingroup$ It has only been closed for 9 hours. Why not give the OP some time to improve the question? I see that, unfortunately, nobody has used the (undeleted) comments to encourage the OP to do that. $\endgroup$ Mar 10 '16 at 12:16
  • $\begingroup$ The fact that this was an interview question had been given in the title, but it was removed after closure. The OP tells what they used to solve the first part; however, it seems that they have no idea how to approach the second part. If that is so, then I guess they can't add much and they are out of luck here. I have added some comments on how to proceed. We'll see if that helps. $\endgroup$
    – robjohn Mod
    Mar 10 '16 at 13:59
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Re-opened

Request to reopen the question If $p + q = 1$ prove that for any natural $n, m$ following is true: $(1 - p^n)^m + (1 - q^m)^n \ge 1$ since:

  • User described his effort
  • The question is interesting by itself
  • It has a decent answer already
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Re-opened

Proving without Zorn's Lemma additive group of the reals is isomorphic to the additive group of the complex

This Question is closed as duplicate of Is it true that $\mathbb{R}$ and $\mathbb{R}^2$ are isomorphic as abelian groups? Although intrinsically related, the newer Question asks if (for the sake of simplicity) one might avoid using Zorn's lemma (to exhibit bases over the rationals of equal cardinality) as the earlier Question discusses.

Since the point of the Question was from the beginning a follow up to the proof offered in the duplicate, I'd like to see this reopened. It already has a cogent (and accepted) answer, but possibly there is more to be said.

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    $\begingroup$ Should that question perhaps also be tagged (axiom-of-choice)? ("Without Zorn's Lemma" can be also interpreted as "without AC". Although it is not clear whether this is what the OP intended.) $\endgroup$ Jan 19 '17 at 17:40
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Undeleted

There is a great deal of useful information in an answer to Collections of undergraduate research projects

I would like to see the question undeleted, to save that information.

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Undeleted

This deleted Question was deleted after being closed as it had shown no effort.

However, after discussing it with the user @WordShallow here, I added the solution he had attempted into the question.

I request that this question be undeleted.

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Reopened.

I would like this question to be reopened Prove or disprove that a graph made by $n$ straight lines is Hamiltonian. I have made corrections to the question and have mentioned my efforts which was the reason for closing this. Some other users were also questioning for this to be reopened.

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Reclosed as dupe

Request for reopening: Let $A=3^{105} + 4^{105}$. Show that $7\mid A$.

It has been closed as duplicate of Proof of $a^n+b^n$ divisible by a+b when n is odd, which is currently closed as well.

If OP had recognized that his question, in short "Show that if $A=3^{105} + 4^{105}$, then $7\mid A$", was a particular case of the question "Proof of $a^n+b^n$ divisible by a+b when n is odd", then indeed it would be appropriated to consider it a duplicate. But what if he or other future user had not?

Since I tend to consider the latter question as an abstract duplicate question, rather than a truly duplicate one, I am in favor of reopening the question Let $A=3^{105} + 4^{105}$. Show that $7\mid A$.

ADDED. All the four existing answers to the question Proof of $a^n+b^n$ divisible by a+b when n is odd solve the problem using algebra-precalculus concepts, while the OP of A problem dealing with number theory tryied to solve it using congruences.

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    $\begingroup$ The general opinion is that specific cases can be closed when an abstract duplicate exists; see Coping with abstract duplicate questions and List of Generalizations of Common Questions. $\endgroup$
    – epimorphic
    May 3 '17 at 16:50
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    $\begingroup$ @epimorphic There is also this opinion: "One of the reasons we get the same question over and over is people who are learning new concepts cannot tell if two problems are essentially the same.", which I agree with. $\endgroup$ May 3 '17 at 17:06
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    $\begingroup$ That's an attempt to explain why such questions pop up over and over again. I don't see futurebird advocating that lone specific cases (which is what's in contention here) should be kept open. $\endgroup$
    – epimorphic
    May 3 '17 at 17:27
  • $\begingroup$ @epimorphic Let's see if OP has any reaction. One of the things that occurs here is that both questions, the specific one and the more general one are closed and the latter is not listed in the List of Generalizations of Common Questions. $\endgroup$ May 3 '17 at 17:53
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    $\begingroup$ Another fact that weighs in favor of reopening (it has gotten a couple more close votes since being reopened!) is that the OP made an unsuccessful attempt to solve/prove by factoring. I'm sympathetic with attempts to show that the gist of such attempts can be made successful with rework. $\endgroup$
    – hardmath
    May 6 '17 at 14:13
  • $\begingroup$ @Bill I am confused by your edit. It appears you just "marked as duplicate" the question. Is this an oversight or did I miss some subtlety. $\endgroup$
    – quid Mod
    Jun 17 '17 at 10:53
  • $\begingroup$ @quid The subtlety is the real-world - I was pulled away from the computer before I had a chance to return to meta and update this question. Now done. $\endgroup$ Jun 17 '17 at 13:10
  • $\begingroup$ Generally I prefer not to close specific cases when they are amenable to special techniques that prove pedagogically useful. But that is not the case here. $\endgroup$ Jun 17 '17 at 13:19
  • $\begingroup$ @Bill Thanks for the reply and the edit. I did not realize at first that your first edit predates your closure by small but still non-negligible time (as opposed to the two being essentially done at the same time). $\endgroup$
    – quid Mod
    Jun 17 '17 at 13:22
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    $\begingroup$ @quid Yes, I edited the meta question to fix the syntax, then decided to have a closer look at the question. Eventually I agreed with Lab's decision that it is a dupe, so reclosed it about 18 minutes later (I wasted some time looking for better dupes and was sidetracked by other tasks in the interim). $\endgroup$ Jun 17 '17 at 13:30
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[Deleted, 14 May 2017.]

I have voted to reopen this question because comments under the question show that the poster has exerted some effort and does understand the question. Moreover, answers can be calibrated to the level of effort or the particular aspect that the poster wonders about.

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  • $\begingroup$ Deleting the comments. Nothing to see here. Move on. $\endgroup$ May 11 '17 at 19:57
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Reopened

This Question was initially put on-hold for missing context:

Calculating determinant $a_{ij}=x_{i}y_{j}$ if $i=j$ and $1+x_{i}y_{j}$ if $i\neq j$

Soon after the OP explained their approach/progress in a Comment, which I have added into the body of the Question.

Please consider reopening. Although closely related to other Questions involving rank one updates of diagonal matrices (see abstract duplicate linked there), this Question has some novelty because it amounts to rank two update of a diagonal matrix.

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[Reopened] [and ready for more appropriate reclosure]

Please consider reopening $2^n + 1$ is prime $ \implies n$ is a power of $2$, possibly so that it can be linked (one way or the other) to a more appropriate duplicate. Given the "proof verification" nature of the Question, I could also see it being considered not a duplicate.

It is obviously about Fermat primes, but it was closed as a duplicate of this older Question about Mersenne primes.

The OP was actually having difficulty relating the proposition to "contradiction of the contrapositive," and Robert Israel gave a good Answer to clear that up.

A valid older duplicate might be:

Fermat primes relation to $2^n+1$

If $2^n+1$ is prime how to show that $n$ must be a power of $2$

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Reopened, deleted by Community ♦, then undeleted

I would like to re-open Real Analysis: Uniform Continuity [on hold] (question deleted) since the author has shown his work after my comment. There's no reason to block this question now.

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    $\begingroup$ Did you have an inclination to answer this reopened Question? $\endgroup$
    – hardmath
    Feb 27 '17 at 14:43
  • $\begingroup$ Yes, I had, but we should judge solely on the question body itself---OP has a doubt but he had asked in a wrong way. After he corrected this (by adding more context), he should no longer be stopped. $\endgroup$ Feb 27 '17 at 15:09
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    $\begingroup$ My point is the Question is reopened, as you asked, so now an Answer can be posted. $\endgroup$
    – hardmath
    Feb 27 '17 at 15:16
  • $\begingroup$ Thanks for reminding me that. Sorry that I misunderstand your point. I'll try it later if I've time. $\endgroup$ Feb 27 '17 at 15:17
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    $\begingroup$ One month after the question was reopened... $\endgroup$
    – Did
    Mar 5 '17 at 7:45
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    $\begingroup$ Please do not erase the link to the question. $\endgroup$
    – Did
    Apr 2 '17 at 22:26
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Deleted by Roomba

Asking for more opinions about this question.

The question nominally looks like a question about Matlab and its handling of infinity/NaN. It may turn out that the problems are, indeed, there. But in my somewhat expert opinion there are other possible explanations to the observed quirks in the behavior of the Belief Propagation algorithm.

Because is on-topic at our site, in my opinion this question could also be on-topic at Math.SE in spite of appearances to the contrary.

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  • $\begingroup$ I am dropping the matter - closed+deleted it is :-). Thank you for your votes/opinions. Undeleting this request in case it helps users with <10k to gauge the success rate of similar future requests :-) $\endgroup$ Oct 1 '17 at 16:01
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Reopened, Reclosed

So, OP edited "context" into this question and two hours later it got put on hold anyway.

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Undeleted

Could this question be undeleted, since it related to several other questions I had (see "Linked Questions" section)...

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Undeleted

Vector fields pointing from one fiber to another - Notation was deleted while I was substantially editing an answer to address followup questions by the original poster. Please undelete.

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Reopened

I was among those who voted to close Integrating to find mass and centre of mass, but the OP has now supplied by Edit a good account of their attempt and some specifics of the confusion that needs to be cleared up by a good Answer.

Please consider voting to Reopen, as I have.

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Undeleted

Prove cf(α)≤|α| if α is a limit ordinal

Was deleted shortly after receiving an answer, and halfway through me typing mine. The question itself is certainly valid. So it might be the OP trying to hide asking online.

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Reopened

I propose that this question be reopened: Why is the fact that a quotient group is a group relevant? This question was closed as being a duplicate of the question Why the term and the concept of quotient group? However, the questions are completely different, as can be seen by the huge differences in the types of answers. One question asks "why is the idea of a quotient group like division?", while the other asks "why do I care that a quotient group is a group?"...

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Reopened, [Re-closed], Deleted (https://math.stackexchange.com/posts/2538958/revisions)

I propose that this question be reopened. I understand that there are already four votes to reopen. But since the question is about the free product of groups, I expect that many users will skip it, even some group theorists. Thus the number of users who are likely to see and evaluate it will probably be small reducing the likelihood of getting a fifth vote to reopen.

The question asks how to apply a Theorem, which the OP states explicitly, to prove a corollary, which is also stated explicitly. The proof of the corollary involves a simple substitution. It is the kind of thing that one may see quickly or completely miss. Thus there is little additional "context" which the OP could provide.

On the other hand, one can assume that since the OP is working in free products, he or she, will be able to understand the easy solution. So, additional context is not needed for that purpose.

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    $\begingroup$ The context could easily include the context where the OP encountered the problem (i.e. its source), the background and interest of the theorem, or why the OP is interested in proving the corollary in the first place. $\endgroup$ Dec 1 '17 at 23:53
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    $\begingroup$ @CarlMummert I agree, but how would it help in this case? And it would only serve to make a short question longer. $\endgroup$ Dec 2 '17 at 3:46
  • $\begingroup$ !Stephen Meskin: This has been discussed at length, but newer users may not know just how much length the discussion has taken. For example, see math.meta.stackexchange.com/q/9201/630 and links from there. $\endgroup$ Dec 2 '17 at 14:26
  • $\begingroup$ @CarlMummert That is an interesting string. I could only read a small fraction of it at this time. From what I read,, it seems like there are alternative views. The comment that got to me was by Alexander Gruber of Apr 26 '13. Not withstanding the pros and cons of the issue, I cannot tell whether a conclusion was reached and, if so where to find a list of such conclusions. Moreover, I think the statement describing the reason for closure can be misinterpreted. I have mentioned these issues in another post. See math.meta.stackexchange.com/q/27449/465208 But we are getting off topic. $\endgroup$ Dec 2 '17 at 17:49
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    $\begingroup$ !Stephen Meskin: the nature of this site is that a clear conclusion is rarely reached - the number of enforceable policies is extremely small. There are some people who are OK with PSQ posts, but many people are not. Whether a particular PSQ post is closed often depends on who looks at it. But in general the way for someone to avoid the closure like this one is just to write a more self-contained post, with some background, motivation, and sourcing. In this case the OP had the theorem and corollary already stated but did not say where they were from. $\endgroup$ Dec 2 '17 at 18:00
  • $\begingroup$ @CarlMummert I'm beginning to discern the nature of this site. It will take a while. I have edited the post in question with a reference to a prior post on a closely related question which except for a few unrelated personal statements is of the same format and level, but doesn't seem to have received even one downvote. or flag. Should we not have a bit of consistency? $\endgroup$ Dec 2 '17 at 18:18
  • $\begingroup$ @Carl, what is the function of the exclamation mark you are putting in front of Stephen's name? $\endgroup$ Dec 2 '17 at 22:26
  • $\begingroup$ @CarlMummert My edit didn't seem to take. The reference was math.stackexchange.com/q/346802/465208 I think this reference also gave some context to the question since the referenced question gave the source of the the Theorem. $\endgroup$ Dec 2 '17 at 22:57
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