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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.

To inform readers of the current (and past) states of the targeted post, please prepend tags such as:

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.)

Beware that "short" requests such as "request reopening of " 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).

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old thread (as of now) has over $200$ answers, and it is really hard to scroll through the mass of old and/or possibly outdated answers--voting is a mess, too.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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227 Answers 227

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Undeleted

I'd like to see Volume enclosed by $(x^2+y^2+z^2)^2=x$ undeleted.

The question was deleted very soon after a comment by myself and a reasonable answer by Christian Blatter - though, that answer did offer a solution using an alternative approach. It certainly appears to be a frustrating case where an experienced user creates a temporary account to ask a question only to delete it as soon as they feel they understand it. The comments lead me to think that they don't actually understand it and I'd like to add an answer illustrating the correct approach.

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  • $\begingroup$ @DanielFischer Thank you! $\endgroup$ – Mark McClure May 28 '16 at 23:23
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Reopened

Please reopen

Prove that there are not two matrices 2x2 such that: $AB-BA=I_2$

The question was closed as a duplicate of the more general question:

$AB-BA=I$ having no solutions

The point is that for the $2\times 2$ one can simply write two general matrices $A$ and $B$ with entries and show that there are no solution to this. This method wouldn't work (at least not very well) for the general case. As such there is value in having the $2\times 2$ case as a separate question.

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    $\begingroup$ I believe the etiquette is to hold back on posting here until the post went through the queue. But other than that I can see the point in this case more than in some other recent examples. It's at least not impossible somebody would be asked the former without knowing about the trace. $\endgroup$ – quid Jun 23 '16 at 20:09
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Reopened

A high school sophomore asked, in initially a sophomoric way, about proving the Fundamental Theorem of Algebra (FTA):

https://math.stackexchange.com/questions/1848874/the-proof-that-the-number-of-zeroes-of-a-polynomial-the-degree-of-the-polynomial

After receiving the usual feedback (downvotes, closure), the OP edited the Question into fair shape. After an exchange of Comments I further tweaked the wording to focus on aspects of stating and proving the FTA accessible at a high school level. I think this is a topic which would add valuable content to Math.SE, since the FTA is often introduced without proof to high school students. This must seem puzzling to those who pause to think why.

Since the Question has now completed its turn in the Reopen Review Queue, I'm appealing to Meta Readers for a couple of additional reopen votes.

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Reopened

This question on Lagrange multipliers was closed for being "unclear."

The actual question seems clear to me: When solving the system of equations for $\lambda$ in the typical Lagrange multiplier calculation, is it OK to reduce the equations to make solving easier? I admit it would be nice if the OP would say something about why they would think that might not be OK, but I still don't really see what's unclear.

There are currently 3 votes to reopen (one of them mine), and it seems to have gone through the review queue fully.

My apologies if I misunderstand "the natural course" of reopening, but I assume that's more or less done, having completed the review queue, albeit somewhat recently.

Edit: Thanks to quid, I do see the logic of the original closure; no qualms there.

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    $\begingroup$ You understood it right (the meta process I mean). On the question, well, yes I agree it is a borderline but seems about alright now (I cast a vote). But in fairness it was math.stackexchange.com/revisions/1862725/1 that was closed, which is both less clear and less polished than the current one. Indeed, it seems the closure induced an edit by OP that did improve the question. The process works as intended. $\endgroup$ – quid Jul 18 '16 at 22:17
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    $\begingroup$ @quid That's a good point -- I looked over the revision history, thinking "Oh, one line, it was basically a formality on their part." But I didn't consider the original version in isolation, it was less clear than I'd thought. I do understand the closure now. $\endgroup$ – pjs36 Jul 18 '16 at 22:20
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Undeleted

Please vote for undelete this answer. It's a full answer written by a well-known user and it's very useful. The request of the OP for a hint has expired, in my opinion. Thanks.

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  • $\begingroup$ It seems to me that this conversation reached an end of sorts. If you need to revisit it do it in the chat. $\endgroup$ – Jyrki Lahtonen Aug 26 '16 at 21:20
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Undeleted

Please vote to undelete How to solve this equations in modular arithmetic? $6247x \equiv 1139 \pmod{9461}$

The question might have come from http://www.cse.cuhk.edu.hk/~andrejb/engg2440/hw/16H03.pdf which specifically says that any use of online material must be properly cited and credited. Deleting the question after receiving an answer seems in bad taste.

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Undeleted

The question How find the function if such $f(P(x)+Q(f(y))+Q(y)P(f(x)))=Q(y)+P(f(x))+P(x)Q(f(y))$ got recently deleted, despite having a score of 3 upvotes and no downvotes, and an answer with the same score. That answer was written by me, and this deletion made me lose 55 points in a single blow. While I understand the necessity of cleaning the site up, I strongly question this specific deletion decision; I cannot find any fault with the question. Could the community take a look at it and voice its opinion, please? Thank you.

(Given that we are talking about a deleted post, only users with at least 10000 reputation points will be able to see it, the others will be directed towards the standard "not found" page.)

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Autodeleted

I would like to ask whether you would consider reopening Show that the set $\{(x_1,x_2,x_3,x_4)|x_1x_2x_3x_4=0\}$ a linear subspace of $\mathbb{R}^4$?

At the moment it is closed with the close reason missing context. The OP reacted in comments to some hints. After that I have edited his attempts into the post. (It would have been better if the OP edited the post, but perhaps they will do so next time.) After the edits, I do not think that missing context is still a valid reason. (I would have no problem with closing the question as a duplicate. The closest I was able to find was this one.)

I do not claim that it is great question. But reopening would help reinforce the message to the OP (and other new users): "Editing your posts to include what you were able to do after some hints in comments might lead to reopening."

The question already went through reopen review queue. And I have also suggested reopening this question in chat before asking here, but without success.

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Reopened

I propose reopening this question. It's basically a request for references, so I don't think the "show your work" objection applies.

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Reopened

What is the intuitive explanation for the CDF of any random variable to follow uniform distribution (0,1)?

It seems probable that the reason this was put on hold is that initially the question was phrased in such a confused way that many couldn't tell what was meant. That has been remedied and so far three people have voted to reopen it.

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Reopened

Based on Andrej Bauer's comments, it seems that the class of spaces discussed in "Base having open compact members" is actually interesting and useful in some contexts.

I agree that the formulation of the question is not great (I did at least some minimalistic attempt to add some context, although probably I'd have to know more about this type of space to improve the question more). Still, I would suggest to reopen this question, so that the stuff which is currently only posted in comments can be better formatted and posted as an answer. (And who knows, maybe after the reopening we will here also from other users who can add something interesting.)

As you can see on the timeline, the question already has been through reopen review queue. I have also asked in the c.r.u.d.e chat room, but without getting any feedback so far.

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Undeleted, deleted, undeleted,

I would like to request this question be undeleted.

Shortly after I wrote an answer, the asker commented on the answer, and then deleted the question without explanation. I voted to undelete and also flagged the question, and then a moderator (Jyrki Lahtonen) undeleted the question and explained to the asker why it is rude to delete an answered question. But then the asker deleted the question again without explanation, and then asked a follow-up question separately.

If there is a duplicate question somewhere, then I am fine with closing the question as a duplicate (rather than deleting it), but I was unable to find a duplicate.

Also, if I have handled this situation poorly (e.g. answering a question instead of giving hints, or insisting on undeleting a question), please let me know and I will try to correct my behavior.

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    $\begingroup$ Very odd situation here. OP tried very hard to bury the question on m.se, deleting it twice, then when that didn't work vandalizing it twice, and was sternly rebuked by a moderator. And then a moderator went ahead and deleted it. What's going on? $\endgroup$ – Gerry Myerson Nov 4 '17 at 22:09
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    $\begingroup$ @GerryMyerson the question was not deleted by a moderator. It was auto deleted by the community user. This happened because negatively scored posts of deleted users are deleted. Arguably, it is not a good idea to delete negatively scored questions that have an positively received answer in this way. If you want this changed you may consider to upvote this feature request. // I undeleted again; but I am not sure if autodel will delete yet again, let's see. $\endgroup$ – quid Nov 5 '17 at 1:04
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    $\begingroup$ @quid, thanks for enlightening me, and thanks for undeleting. I have now upvoted the feature request, but I note that that post goes back four years, with no activity on it for nearly two years now. Does that mean no one higher up has ever looked at it? $\endgroup$ – Gerry Myerson Nov 5 '17 at 2:24
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    $\begingroup$ @GerryMyerson I am quite sure that somebody looked at it but they decided not to do it (at this point). That is, it is not inertia or neglect but an active decision. The reason I am quite sure is that there was some change related to this more recently. The behavior was changed for metas (per site and the main one) and they added a mod-tool to track such posts more easily. Then it ought to have been trivial, and likely even more simple, to change it also on mains. $\endgroup$ – quid Nov 5 '17 at 12:32
  • $\begingroup$ Thanks again, @quid. I'd rather that when people make active decisions they also make them in public, with their justification. $\endgroup$ – Gerry Myerson Nov 5 '17 at 21:53
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    $\begingroup$ @GerryMyerson I agree that sometimes the communication related to feature request could be more active. The following thread that was started some time ago helps a bit meta.stackexchange.com/questions/291031 $\endgroup$ – quid Nov 5 '17 at 22:03
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Undeleted

This question convergence in operator norm received an answer (from an experienced user) in an hour after creation. However, OP self-deleted his question three hours after that. According to one of our site mods, this is an "abusive behaviour". Therefore, I propose to undelete this question.

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Undeleted - Redeleted - Undeleted - Deleted - Undeleted - Reopened - Closed as duplicate

How to find? $\sum_{n=1}^{\infty}\frac{H_{n+1}}{n(n+1)}$ was deleted. It is true that it has no context, other than the poster used Mathematica to get the result is $2$, but the question is a good example of changing the order of summation and telescoping series.

I think that this question, unless it is a duplicate, should be undeleted. The question is mathematically good, even though the context is a bit thin.

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    $\begingroup$ Your giving a hint instead of a full answer is good in my opinion, but you are a moderator, and should not be encouraging poor question generators. I'm sure you can see all the poor questions generated by this one user even till recently. If you like this question very much, you could easily post a new Q&A pair of your own with high quality, instead of leaving junk posted by others around. After all, the SE system does explicitly encourage (on the Ask-A-Question page) you to share your knowledge in the Q&A format if you are inclined to. No? =) $\endgroup$ – user21820 Mar 6 '18 at 15:24
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    $\begingroup$ @user21820 Alternatively, your opinion of what merits deletion is not widely shared amongst reputable users. Hardly surprising, given the scope of the site. $\endgroup$ – Mark McClure Mar 6 '18 at 16:18
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    $\begingroup$ @MarkMcClure: We've been over this before. The proportion of users that actually care about the site quality is close to zero. And I am not interested in the opinions of users that do not care. Reputation or reputability is of little importance here. I have already provided a clear and viable approach for preserving good content, which nullifies any concern that deletion is harmful to good content. Incidentally, I don't think that the answer in question here is so awe-inspiring anyway... You'd have a better argument for lousy questions like this. $\endgroup$ – user21820 Mar 6 '18 at 16:29
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    $\begingroup$ This is a textbook example of a PSQ, which does not seem to require "exceptional handling" as this meta thread is aimed to provide. The most interesting aspect of the question is that the OP implied they have access to Mathematica 2. The OP had no lack of time to improve the post. $\endgroup$ – Carl Mummert Mar 6 '18 at 17:58
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    $\begingroup$ @user21820 suggested reposting a better version. I have additional suggestion: We could use the existing question (visible for 10k+ users) as a place where some kind of draft of this better question can be made. (And the changes can be then reverted if/when the new question is posted.) I have added at least links to somewhat related questions. If there are further suggestion what else can be added so that the question no longer lacks context, they are more than welcome. (Ideally something which explains why the question should be considered interesting/useful.) $\endgroup$ – Martin Sleziak Mar 6 '18 at 18:31
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    $\begingroup$ @robjohn: that's completely possible - but I posted a comment about it on Feb 25 and nothing was edited before the question was deleted today, suggesting that the OP was not actively participating in the question. That also speaks in favor of deletion, IMO, when the OP does not respond to comments about the question, which looked like this: math.stackexchange.com/revisions/2665553/1 $\endgroup$ – Carl Mummert Mar 6 '18 at 19:33
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    $\begingroup$ @Martin Sleziak: In my opinion, I don't think that the links to other questions make this one better. I am interested in knowing where the OP found this series, why it is of interest, etc. - real, meaningful context, not filler context just to avoid being a PSQ. There are numerous minor variations of every series, and they can't all be interesting just because they are a variation of some other series. $\endgroup$ – Carl Mummert Mar 6 '18 at 19:38
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    $\begingroup$ @robjohn If I may suggest (and you can see both from the comments above and the discussion in chat that several users find this a reasonable solution), maybe posting a new question would be way to go. And since you are the user who started this thread, you seems to be natural candidate for the poster of the question. (It goes without saying that you're free to use any material comes from my edits - I have explicitly stated a few comments above that my edits are done with the intention to created a draft for the new question.) $\endgroup$ – Martin Sleziak Mar 7 '18 at 17:13
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    $\begingroup$ You're welcome to ask a new question which you so desperately want to answer, too, @robjohn. The question was asked by a user currently suspended precisely because of their prolonged history of asking low-quality PSQs over the years (yes, years). I would support a newly asked question, that better models a good question, which you can copy and paste your earlier answer into. Please take the initiative to do so: custom make a model question that your answer answers. Sometimes the quality of answers, which in this case, isn't outstanding, doesn't justify keeping a poorly asked question. $\endgroup$ – Namaste Mar 7 '18 at 17:13
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    $\begingroup$ @amWhy: I am in no "desperation" to answer a question. There have been a large number of questions I have answered, but then decided not to post my answer because of the quality of the question, or because another answer was too close. This question seemed to be from someone trying the sum in Mathematica and wondering how it got the answer $2$. I provided a hint since the "context" was thin. I just did not see a reason to delete the question. $\endgroup$ – robjohn Mar 7 '18 at 17:47
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    $\begingroup$ @amWhy: I commented why I thought it was a good question: it involves a good combination of ideas like telescoping series and change of order of summation. The way the question was posed was not up to the standards imposed by the community, but that does not make the question a bad question. By the way, I am amazed that you can count "an even larger number". How did you count how many questions I answered but did not post? $\endgroup$ – robjohn Mar 8 '18 at 2:30
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    $\begingroup$ @amWhy: I don't want to stoop to ad hominem attacks. Please stay on topic for this comment thread and leave me to volunteer my time as I see fit. $\endgroup$ – robjohn Mar 8 '18 at 2:43
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    $\begingroup$ This is ridiculously abusive. The post content is fine, it's mathematically interesting, and generated three good answers. Moreover, it's clear from amWhy's comment(s) there that the deletion is at least partially motivated by personal animosity against the poster. The question wouldn't be deleted now if it wasn't for the repetitive deletion votes of a (very!) small group of users repeatedly targeting the thread. $\endgroup$ – user296602 Mar 11 '18 at 1:23
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    $\begingroup$ @amWhy: "an even larger number of questions" than what? This seems to refer my statement that "There have been a large number of questions I have answered, but then decided not to post my answer because of the quality of the question, or because another answer was too close" since that is the only "large number of questions" I mentioned. I was not attempting to misrepresent your words, just questioning them. $\endgroup$ – robjohn Mar 11 '18 at 9:44
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    $\begingroup$ Probably it is worth mentioning at least here that I posted a new version of the question. (I did link to meta post neither in the question nor in the comments under the question - I thought it is not necessary to attract additional attention to this discussion.) $\endgroup$ – Martin Sleziak Mar 11 '18 at 21:24
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[Undeleted]

I suggest we undelete Most natural intro to Complex Numbers, a question that has a net of 7 upvotes, 3 favorites, and several answers with 5 or more net upvotes (including, I add in the interests of full disclosure, one of mine).

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    $\begingroup$ This is tangential but I feel like mentioning it: favorites in my mind is not an indicator of quality. More often than not I favorite a question as something is wrong with it and I want to keep monitoring the issue. $\endgroup$ – quid Aug 12 '15 at 18:17
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Reopened, Reclosed as a duplicate

A matrix is positive if and only if it is Hermitian and its eigenvalues are positive

For real matrices the statement in the question is incorrect. However, the question is actually quite interesting for the case of complex matrices. According to a recent edit, the author of the question is interested in this complex case.

This question is highly relevant to anyone out there on the internet who is contemplating:

  1. the various definitions of positive definiteness and how they relate to each other, and

  2. how a matrix could be positive definite over the reals, but the same matrix is not positive definite over the complex numbers.

This is the sort of question that math.stackexchange needs to have a canonical answer to, for the benefit of the internet community.


Edit: This question is not a duplicate of the linked one given by Najib Idrissi in the comments. I think many people who originally voted to close misread it as such. The other question seeks to prove the following two assertions:

  1. $\text{Hermitian} + \text{positive eigenvalues} \implies \text{positive definite}$, and

  2. $\text{Hermitian} + \text{positive definite} \implies \text{positive eigenvalues}.$

This is a trivial application of the Spectral theorem, and a question commonly asked on homeworks in undergraduate linear algebra classes.

On the other hand, the actual question seeks to prove a very different assertion:

  1. $\text{positive definite} + \text{field is complex} \implies \text{Hermitian}$.

This is more interesting question, and requires a clever trick to prove (as explained in my comment on the question). Indeed, if the field of vectors used to test the positive definiteness is only real, then the assertion is false, as the following real-positive-definite matrix demonstrates: $$\begin{bmatrix}\bar{a} & \bar{b}\end{bmatrix}\begin{bmatrix}1 & 2 \\ 0 & 1\end{bmatrix}\begin{bmatrix}a \\ b\end{bmatrix} = (a+b)^2 \ge 0, \quad a,b \in \mathbb{R},$$

whereas we can see that it is not complex-positive-definite by choosing $a=i, b=1$,

$$\begin{bmatrix}-i & 1\end{bmatrix}\begin{bmatrix}1 & 2 \\ 0 & 1\end{bmatrix}\begin{bmatrix}i \\ 1\end{bmatrix} = 2 - 2i \notin \mathbb{R}.$$

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    $\begingroup$ "For real matrices the statement in the question is incorrect." Is the adjective "Hermitian" commonly used (or indeed used at all) for real matrices? Anyway that question is unsurprisingly a duplicate, see eg math.stackexchange.com/questions/561461/… $\endgroup$ – Najib Idrissi Aug 17 '15 at 8:17
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    $\begingroup$ The question itself is a PSQ. If the goal of the OP was to have a canonical answer, the first step should be to write a canonical question, that includes adequate background and motivation. $\endgroup$ – Carl Mummert Aug 17 '15 at 14:43
  • $\begingroup$ @CarlMummert To me it looks like a precisely written question, not a PSQ. I really doubt it's from homework since it's is too hard for an undergrad linear algebra class, and a "left to the interested reader" type question that wouldn't be asked as homework in a graduate functional analysis class. Also, if you read the comments to the deleted answer, the author says his motivation is trying to understand the connection between two different definitions of positive definiteness. $\endgroup$ – Nick Alger Aug 17 '15 at 18:31
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    $\begingroup$ Everyone who followed the same curriculum as I did could have definitely solved this question in undergrad (by the end of the second year after high school). It only takes the spectral theorem for Hermitian matrices... "PSQ" means "Problem statement question", i.e. a question which consists of nothing more than a problem statement -- this has nothing to do with precision. And again, the question is a duplicate anyway and will be closed as such as soon as it's reopened: what's the point? $\endgroup$ – Najib Idrissi Aug 17 '15 at 20:30
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    $\begingroup$ @NajibIdrissi It is not a duplicate of the question you link, it is more interesting. See the edit to the meta post above for details. $\endgroup$ – Nick Alger Aug 17 '15 at 21:50
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    $\begingroup$ Well then, I edited the question to include that important piece of context. It's easy to miss on a first reading (I did, anyway), and I think it really adds to the value of the question, in addition to population the "Linked Questions" sidebar. $\endgroup$ – Najib Idrissi Aug 18 '15 at 7:10
  • $\begingroup$ To the two people who cast a reopen vote: did you go to the new duplicate target? It's not the same as before. $\endgroup$ – Najib Idrissi Aug 19 '15 at 8:15
  • $\begingroup$ @NajibIdrissi Nope, must have been someone else who voted. It is indeed a duplicate of the new target. $\endgroup$ – Nick Alger Aug 19 '15 at 8:27
  • $\begingroup$ Yes, I think so too. I think two people just voted to reopen as a knee-jerk reaction. $\endgroup$ – Najib Idrissi Aug 19 '15 at 8:38
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Reopened, reclosed as a duplicate of another question

I think that this question should be reopened, and the other question be closed as a duplicate.

Based on the quantity of answers and votes, it would be more useful for the other question to be closed as a duplicate, despite the timestamps.

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    $\begingroup$ I think reversals of duplicates are better asked to moderators directly, via a flag or maybe in the "Reopen?..." chat. * $\endgroup$ – quid Oct 11 '15 at 22:14
  • $\begingroup$ Moderators have the option of merging a question and a duplicate, so answers don't get lost – isn't that right? $\endgroup$ – Gerry Myerson Oct 12 '15 at 2:20
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    $\begingroup$ @GerryMyerson it is true that question can be merged, it can be tricky though, say, if notations differ or the question (that then will be lost) is referenced in a direct form. Yet then, also if a question is closed as a duplicate the answers are not usually lost (except somebody deletes the thread explicitly). $\endgroup$ – quid Oct 12 '15 at 5:10
  • $\begingroup$ I closed this question as a duplicate while going through the review queue. I did notice that the target was not nearly as highly voted. I did spot an even earlier (and higher voted) third question about this same theme. I did not want to go against the popular opinion at that time, so I just added a link. Now I redirected the duplication to that question instead. FWIW the oldest version stands currently at +47 votes as opposed to +11 here. $\endgroup$ – Jyrki Lahtonen Oct 12 '15 at 5:29
  • $\begingroup$ I voted to close the "other question" of a duplicate of the one @JyrkiLahtonen mentions now, too. $\endgroup$ – quid Oct 12 '15 at 6:49
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Undeleted

This answer posted by a deleted user is correct and it answers the question in an elementary way. The OP also wants to accept it (and asked me to undelete the answer, but unfortunately I can't).

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Reopened

Is $\mathbb{Z}[x,y,z]$ an $\mathbb{N}$-graded ring?

This very straightforward question was closed as lacking context. I don't really know what more context you could want from the question; I already added to the question information from the comments clarifying OP's motivation in asking the question.

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    $\begingroup$ I would want to know in which way OP tried to apply Nakayama's lemma. In a way it is not strictly necessary for the question as asked but that one is completely trivial and the more interesting thing could be to figure out what was actually going on. Moreover, it would also not hurt if a defintion of Z and N graded were included. One may speculate that had OP taken the time to write them down their question would have resolved itself. (At least the part that is now answered; the actual one is unanswered and unknown in any case) $\endgroup$ – quid Mar 12 '16 at 1:22
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Reopened

Why aren't all matrices diagonalisable?

This question was closed as "unclear", perhaps because the original title and first sentence are rather vague despite the remainder of the post posing a clear question. The question was clear enough for two people to give good answers to it (one accepted), and I changed the title to better match the question being asked.

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Reopened, closed as duplicate

prove that $A^2 = I $ implies that $A=I $ or $ A= -I $

It is clear what is being asked here. The fact that the proposition to be proved is not true doesn't make it unclear.

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Re-opened

For invertible matrices $A$ and $C$, prove or disprove that $(C^{-1}BAB^T + I)$ is invertible.

User had a conjecture about some matrices and asked for help proving or disproving it. Ultimately, it turned out to be false based on a simple counterexample, which was posted as an answer. As of this writing, the question had 2 upvotes and zero downvotes, and the answer had 4 upvotes and 0 downvotes.

In other words, the site was working exactly as intended. Then, days later, the question was randomly put on hold.

Now, this is not a groundbreaking or amazing question by any means, but it is also not a bad question either, and therefore it should be left open.

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    $\begingroup$ Something feels really weird about the asker being the one to close the question because they didn't include enough information - seems mostly to be an attempt by the asker to get rid of the question now. $\endgroup$ – user296602 Mar 28 '16 at 14:17
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    $\begingroup$ What is the value of conserving this question on the site? $\endgroup$ – Did Mar 28 '16 at 15:04
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    $\begingroup$ @Did It doesn't add much value, but it does add a little bit, and that is enough. This is one of the main philosophical tenants of the stackoverflow/stackexchange model. $\endgroup$ – Nick Alger Mar 28 '16 at 16:06
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    $\begingroup$ @NickAlger Any source for the "philosophical tenant" thing? (Please be specific.) $\endgroup$ – Did Mar 28 '16 at 16:09
  • $\begingroup$ @NickAlger: Should "tenant" be "tenet"? $\endgroup$ – hardmath Apr 9 '16 at 23:00
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Deleted by OP, but reopened while deleted, undeleted

[NB: I have now cast a vote to undelete.]

Please reopen undelete How to find all systems of distinct representatives using i.e. Hopcroft–Karp algorithm?

Recently a Question about finding all maximal matchings of a bipartite graph was closed (placed "on-hold") as off-topic in part because it was a cross-post of a StackOverflow post and in part because it concerned how an existing program that finds one maximal matching could be modified to find all of them.

So far so good, as I also voted to close.

Then the same user posted this new Question, How to find all systems of distinct representatives using i.e. Hopcroft–Karp algorithm? While substantively the topic is nearly identical, it asks about modifying that algorithm, not about modifying some code. The problem of finding all maximal matchings in bipartite graphs is studied in the literature, esp. by Takeaki Uno (see this 1997 paper and some later papers).

With this perhaps subtle change I think the new Question is on-topic for Math.SE, and I've voted to reopen. In general I think we should avoid "closing as duplicate" when the target has no (good) answers and the underlying problem is truly on-topic here.

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    $\begingroup$ There are currently four votes to reopen this deleted question. I'm tempted to cast the 5th reopen vote, just to satisfy my curiosity as to what happens when a deleted question is reopened. $\endgroup$ – Gerry Myerson Apr 5 '16 at 23:26
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    $\begingroup$ @GerryMyerson: Now we know what happens (when a deleted question is reopened). $\endgroup$ – hardmath Apr 8 '16 at 17:03
  • $\begingroup$ And now there are two votes to undelete. $\endgroup$ – Gerry Myerson Apr 8 '16 at 23:47
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Reopened

Please vote for the reopening of this question. The OP understood that he has to add some context to his question, and did it. Thanks.

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  • $\begingroup$ I voted to reopen. But to be honest, the rant-like paragraph in the last revision by the OP is somewhat putting me off. (Judging by your edit and your comment, you did not like it either.) $\endgroup$ – Martin Sleziak May 4 '16 at 10:47
  • $\begingroup$ @MartinSleziak We are definitely right, but I think we have to give the OP a chance to understood that this is the right way to ask a question. $\endgroup$ – user26857 May 4 '16 at 11:30
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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. $\endgroup$ – Carl Mummert 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 Mar 10 '16 at 13:59
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Reopened, reclosed, deleted, undeleted, reopened

I have edited the question it shouldn't cause any trouble now. Need one more reopen votes.

A space more fundamental than Euclidean space

Update: The question has been revised again.

Second update (from the editor of the post): I feel I must respond to the criticism (from Nick Alger and others) that I have changed the OP's intent. The title of the post was (and is) "A space more fundamental than Euclidean space" and, to my mind, the OP's intent was to ask what this space is, and how it is more fundamental. This is borne out by the text of the original post

I have heard that the space of the 0-vector is more fundamental then euclidean space, that euclidean space is more complicated. Could someone explain what is the space of the 0-vector? Google couldnt find anything. And how is it more fundamental?

The phrase "Weierstrass minimial surface" was added in revision 4, an hour later, presumably in the hopes that someone might recognize it and that that would help to answer the original question. Unfortunately, it probably made things worse, since there apparently is no such thing as a Weierstrass minimal surface.

Based on all this, my reading question is: I heard a physicist talk about a geometry more fundamental than Euclidean geometry. What is this geometry, and how is it more fundamental? I feel that my revision is in the spirit of the original question, and is more likely to get an answer, as it clarifies what the speaker meant.

The factors that made the original post difficult to answer, which are no fault of the OP, are that

  1. the question is based a conversation in a video, and the speaker is difficult to understand, resulting in some key phrases being mistranscribed. (My belief is that the speaker didn't actually say "Weierstrass minimimal surface".)

  2. the speaker misspoke at several points, saying "zero vector" when he meant "null vector". (One can verify this by looking at the speaker's publications on this topic.)

  3. the speaker may have have conflated the issue of a fundamental, underlying geometry with the issue of minimal surfaces, as one is wont to do when summarizing a large research area in brief extemporaneous remarks. (This is not my area, so I am uncertain on this point, but many of the speaker's papers talk about a fundamental geometry without mentioning minimal surfaces.)

The result of these confusions was that mathematicians reading the question saw the nonsense phrase "zero-vector space generated by Weierstrass minimal surfaces" (which is not in the video) and said to themselves "must be some of that gibberish that physicists are always spouting. Go ask one of them." (Disclosure: I am trained as a physicist.) In fact, after all the misunderstanding is stripped away, there is a mathematical question here, and I wanted to make that clear.

I'm sorry that the question now comes across as an impenetrable wall of text. One mistake I may have made was including the term "pure spinor", which comes up often in the publications, and relates to null vectors in, I think, a somewhat involved way. I'd be happy if someone can pare it down to something more reasonable.

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    $\begingroup$ Changes of state are recorded, yet not the initial one. $\endgroup$ – quid Jul 20 '16 at 10:02
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    $\begingroup$ Question seems fine now. After stumbling upon the current (edited) version of the question, I myself am interested in knowing the answer. $\endgroup$ – Nick Alger Jul 20 '16 at 10:32
  • $\begingroup$ @Will Thanks for the reply on the post; at the moment I cannot reply there, since the post is deleted. At this point, I do not consider this as good and voted to undelete. However; this disruption was imminent at the time or your edited, which is one reason why I had considered it as wiser to make a new post. $\endgroup$ – quid Jul 25 '16 at 10:47
  • $\begingroup$ @quid: Isn't that better to give a new start to the question (That's why I voted to delete the question)? $\endgroup$ – user99914 Jul 25 '16 at 11:12
  • $\begingroup$ @ArcticChar in principle yes, as I said in comments there. However to try to push this through in this way is ill-advised, in my opinion. $\endgroup$ – quid Jul 25 '16 at 11:18
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    $\begingroup$ I think that the idea of trying to salvage the question is well intentioned, but entirely rewriting it in place of the OP ( math.stackexchange.com/posts/1854049/revisions ) seems less in line with community norms than just starting a new question with the improved content. It really should be up to the OP to make significant improvements on their question, IMO. $\endgroup$ – Carl Mummert Jul 25 '16 at 13:30
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    $\begingroup$ Yeah I don't agree with the newest massive edits; while well-intentioned, they really override the original author's intent. Would the original author even understand his/her own question now? Previously it was a simple and direct question about the definition of a phrase and the intuition behind it. Now it is a rambling wall of text asking general questions about spinors. The key phrase "Weierstrauss minimal surface" doesn't even appear anymore (though it does appear partially), and the key object "zero-vectors" don't appear until one has already read through a page of text. $\endgroup$ – Nick Alger Jul 26 '16 at 13:34
  • $\begingroup$ @NickAlger: I have responded in the post. $\endgroup$ – Will Orrick Jul 26 '16 at 15:11
  • $\begingroup$ @WillOrrick one can ask this question at various levels of detail. The original question, in my mind, mainly asked for a rough categorization of the remark in the talk. You provided it, and IIRC OP even said that your remarks are an answer to the question. You think otherwise. But I believe that you in fact did answer the question to the extent that some cared about it (possibly including OP but I do not want to interpret them). The question now asked is a lot more specific. $\endgroup$ – quid Jul 26 '16 at 16:49
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Reopened

Counter-examples related to Slutsky's Theorem.

Nothing is unclear about this question. The fact that the well known Slutsky's theorem is mentioned makes it clear that the question requests counterexamples showing that one of the hypotheses cannot be weakened in a certain way. A comment says "are you dealing with probability spaces, general measure spaces, what are $X_n,Y_n,\ldots$". But the term "Slutsky's theorem" certainly leaves no doubts about that.

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Reopened

The following question was closed as a duplicate of a question that looks very similar, but really it is not.

For a subgroups $H_1$ and $H_2$ of $G$ and a normal subgroup $N$ of $G$: $H_1/N \cong H_2/N \Rightarrow H_1 \cong H_2$. Is it true?

This one assumes isomorphy, the dupe target actual equality. (Some answer there does answer this question, but is sketchy and not very assertive. The two other answers, including the accepted one, answer that question as asked that is something else entirely.)

ps. The even is recent but the question is through review one vote missing.

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Undeleted


Please vote to undelete this question (only one vote needed): Polynomials in one variable simple module here or not?. The answer is remarkable. (I think the users doing such things should be warned.)

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    $\begingroup$ Reopen? Did you mean, Undelete? Anyway, it has been undeleted. $\endgroup$ – Gerry Myerson Oct 7 '16 at 21:37
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Undeleted

Homogeneous equation solve $y''+9y=0$

Is yet another question that was deleted just 12 minutes after receiving an answer.

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  • $\begingroup$ "deleted just 12 minutes after receiving an answer" Is it a problem per se? (Regarding the question itself, I see some context in it hence, indeed, closure does not seem adequate, but this is another argument.) $\endgroup$ – Did Nov 7 '16 at 7:53
  • $\begingroup$ Did, it often indicates that some foul play is afoot. In this specific case, the OP had changed the parameters of the question (or rather their professor has), and deleted this copy to post a new one. It turned out okay, and there was nothing fishy going on. But this was posted before everything was clarified. $\endgroup$ – Asaf Karagila Nov 7 '16 at 7:56
  • $\begingroup$ Oh, I get it, the asker deleted their question... Thanks for your reply. $\endgroup$ – Did Nov 7 '16 at 7:59

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