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old thread (as of now) has over $200$ answers, and it is really hard to scroll through the mass of old and/or possibly outdated answers--voting is a mess, too.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

Please consider reopening:

Permutations with restrictions on item positions

The OP got what was needed from a Comment I left, and proceeded to ask this related Question. Once clarified the original Question was about efficiently counting certain restricted permutations, and the follow-up concerned efficiently generating them.

I find the original Q interesting in that the theorem of Valiant (1979) gives a "negative" answer and deserves to be more widely visible on Math.SE.

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Reopened

Request for reopening: Open Problems in Semigroup Theory.

This question received 5 upvotes, a high score for the tag "semigroups", indicating some interest of the community, but it was quickly put on hold as too broad. Only one open problem was mentioned in a comment.

The question is certainly too broad if group theory is not excluded, but otherwise I am not so sure it is that broad. Anyway, since the OP is referring to Howie's book, one may restrict the question to semigroups in algebra, in a wide sense${}^*$. I know of a few existing lists of open problems on semigroups, but it would be useful to gather them in a big-list. All in all, I see this question as a frequently asked question and I think it is worth to reopen it, perhaps in a slightly different form.

$^{* \text{ For instance many questions on finite semigroups are closely related to profinite semigroups, which are topological semigroups.}}$

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Reopened

A lot of context has been added to Binomial expansion going wrong!. I think it is a good question displaying a common pitfall when using power series expansions (and not only because I wrote an answer). I think it should be reopened, but I can't vote to open without unilaterally reopening the question.

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Reopened

This question started out with a few close votes before the author showed their work, and ended up being closed. Now that work has been shown it probably deserves to be reopenend (esp since it is an instructive example of use of the mod Distributive Law - which deserves to be much better known).

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    $\begingroup$ The question was not yet through the reopen review, which would likely have led to the reopening by itself. $\endgroup$
    – Daniel Fischer
    Jul 26, 2017 at 18:01
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Undeleted

Please consider undeleting this post. Both the question and the answer seems quite interesting and is of high qualities.

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    $\begingroup$ The Question was self-deleted on Aug. 14, a few days after it had been answered (and clarified in Comments on the Answer). I've upvoted the Answer so that self-deletion is no longer an easy option. $\endgroup$
    – hardmath
    Aug 16, 2017 at 13:56
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Reopened

This question: What does it mean system of contractions $\textbf{with overlaps}$? was closed as unclear. While I do think the original version was lacking context, it was only unclear if you don't know what an IFS with overlaps is. I've since edited the question to add context and recommend it here for reopening. It does have 3 reopen votes but has been stuck there for a couple of days.

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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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Reopened

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

Poisson process with dead time

Earlier versions of the question were unclear and confusing. I edited the question heavily, removed extraneous text about Monte Carlo simulations, and also added a diagram for clarity. As it stands now, I think the question is quite clear. It also has an answer.

I would therefore request this question be reopened, or the close voters give more specific feedback about what needs improvement so I can improve it.

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  • $\begingroup$ For some context, the answer accepted after reopening reads, in full: "I would have thought $T_n−T_{n−1}$ would be i.i.d. between the the two methods and over time. My simulation in R seems to confirm this." Sorry but where are the mathematics? $\endgroup$
    – Did
    Oct 4, 2017 at 10:17
  • $\begingroup$ I filled in the mathematics in my answer. Henry's "comment answer" got me on the right track. $\endgroup$
    – Atul Ingle
    Oct 4, 2017 at 14:47
  • $\begingroup$ You are saying there is an answer to the question you asked but that it is not the answer you accepted? So sophisticated... $\endgroup$
    – Did
    Oct 4, 2017 at 15:11
  • $\begingroup$ As I said, the "comment answer" did answer the question for me, so I accepted it, and I filled in the details. Perhaps I should accept my own answer instead? Or edit the comment answer to include my details? $\endgroup$
    – Atul Ingle
    Oct 4, 2017 at 15:23
  • $\begingroup$ Well, at this level of departure from logic, I would not dare to give any advice... :-) $\endgroup$
    – Did
    Oct 4, 2017 at 16:44
  • $\begingroup$ Since you have >1000x more reputation than me here I do really value your advice. It sounds like you are still bitter about name-calling, so I'd like to apologize again. $\endgroup$
    – Atul Ingle
    Oct 4, 2017 at 16:58
  • $\begingroup$ No idea what you are talking about... but please let us leave it at that. $\endgroup$
    – Did
    Oct 4, 2017 at 17:10
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Reopened, Reclosed, Deleted

I have edited this question, deleting the poster's opening statement about his hatred of statistics, which is off topic. It was closed as "primarily opinion-based". But the proper remedy was just to delete the initial opinion, and I have done that. The question remains intact after deletion of the off-topic opinion.

The content of the question is substantial and serious. Now that the off-topic personal remark at the beginning is gone, I propose that this be re-opened.

Why should statistics be considered mathematics?

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Undeleted

I would like to request undeletion of Proof that $d^2 \phi = 2\pi \delta^{(2)}(r)dx\wedge dy$ in polar coordinates around a puncture?. I saw this thread recently in meta suggesting it is frowned upon to delete a recently answered question. In my case, I was in the middle of a lengthy edit in response to a request for clarification, and feel to have the rug pulled out from under me. Thanks.

I still have the tab open in my browser, although my response is hidden under an overlay. Is there any way to recover my work in progress?

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  • $\begingroup$ The question is probably deleted because there is a duplicate. The question should be closed anyway. $\endgroup$
    – user99914
    Nov 18, 2017 at 6:48
  • $\begingroup$ @John, it was deleted by the person who posted it, with no reason given. $\endgroup$
    – Gerry Myerson
    Nov 18, 2017 at 12:06
  • $\begingroup$ @JohnMa So do you think it is not worth the effort to recover an answer to a question that is worthy of being closed? $\endgroup$
    – ziggurism
    Nov 18, 2017 at 13:41
  • $\begingroup$ @gerry with Ted's comment that probably the reason. $\endgroup$
    – user99914
    Nov 18, 2017 at 15:31
  • $\begingroup$ @ziggurism it depends. Your answer is of high quality but there's already an almost identical one here. $\endgroup$
    – user99914
    Nov 18, 2017 at 15:37
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Reopened

Please undo the following duplicate link by reopening this question. The OP misread the duplicate and voted to close their question.

This question asks if the complex orthogonal group $$\{A \in M_{n\times n}(\mathbb C) : A^tA = I\}$$ has two connected component. The duplicate, however, is about real orthogonal groups. Worst still, the duplicate is also closed as a duplicate of another one, which shows that $GL(n, \mathbb R)$ has two components. Of course all the method employed there will not be useful in solving the original question, since $GL(n, \mathbb C)$ is connected.

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Please undelete Exterior product with coefficients from finite fields. as I had an answer near complete when it was deleted.

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    $\begingroup$ OP self-deleted the question after having been told it is unclear. It is not impossible, they'll come back with a clarified question. Until then I do not see the merit of undeleting as unclear as that one, especially when even OP somehow acknowledge it is unclear. $\endgroup$
    – quid Mod
    Nov 27, 2017 at 15:00
  • $\begingroup$ @quid So shall I delete my nearly completed answer? Start anew if the user decides to repost the question with clearer phrasing? $\endgroup$
    – ziggurism
    Nov 27, 2017 at 15:43
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    $\begingroup$ I assume you could store the text somewhere (locally); if nothing else as a draft for an email or something like that. If no question is forthcoming in the near future you could use the work to create a self-answered Q&A pair. $\endgroup$
    – quid Mod
    Nov 27, 2017 at 17:55
  • $\begingroup$ @quid how could we tell if/when the user reposts the question? $\endgroup$
    – ziggurism
    Nov 27, 2017 at 17:57
  • $\begingroup$ You can follow the activity of any user on their user page; in the present case here math.stackexchange.com/users/493344 You could also search for "exterior product" on the site; which is sufficiently distinctive to be helpful. I would not wait overly long, maybe a day or two. $\endgroup$
    – quid Mod
    Nov 27, 2017 at 18:12
  • $\begingroup$ Are you still requesting undeletion, zig? $\endgroup$
    – Gerry Myerson
    Nov 30, 2017 at 21:52
  • $\begingroup$ @GerryMyerson to be honest, my unfinished answer is not quite in a state to be submitted. I hope to polish it up soon $\endgroup$
    – ziggurism
    Dec 2, 2017 at 0:39
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Reopened

The question Need Examples to Understand Choice Function and Choice Structure is in no way a duplicate for the following question: (P(X),CR) may be a choice structure even if R is not a rational relation In the former I have asked people to provide me some examples to understand the definition of Choice function and Choice structure. In the other question, I have asked for an example to show that a relation underlying a choice structure should not necessarily be a rational relation. The only relation between these two questions is the definition included in the latter question to clarify the question. I though including the definitions will help people to understand my question! Would you please reopen my question or at least tell me how I can clarify the distinction between my question and request StackExchange to remove the hold on my account?

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  • $\begingroup$ Wouldn't an example of a choice structure in which $R$ is not a rational relation also be an example of a choice structure? $\endgroup$
    – Carl Mummert
    Sep 20, 2015 at 19:57
  • $\begingroup$ @Carl Mummert, No, because in the former question, my problem was understanding the fundamental concepts. If you take a look at the content of the question, I have completely explained that. Providing the example for a choice structure based on a non-rational relation would not help me understand those concepts. The latter question is actually a proof, while the former one only need explanation and examples. $\endgroup$
    – user2521204
    Sep 20, 2015 at 20:03
  • $\begingroup$ I would recommend doing one thing at a time. If you want to understand the fundamental concepts, ask about them. If you want a non-rational structure, ask about that. But don't ask both at once. Also, it would have helped if you had explained the microeconomics context of these questions - they are not really about set theory, but few here are experts in economics, so people may not realize that is what you are asking about. $\endgroup$
    – Carl Mummert
    Sep 20, 2015 at 20:05
  • $\begingroup$ It is probably worth mentioning that these questions were also discussed in a separate meta thread: meta.math.stackexchange.com/questions/21473/… $\endgroup$
    – Martin Sleziak
    Sep 21, 2015 at 9:44
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Deleted (by OP), Undeleted

Matlab and the 3 Step Adams-Bashforth Method

I just noticed, that this question got deleted by the querist. It took me some time to answer this question properly, as it also contains some implementation in Matlab. IMO this question is not the best example for a question that should be asked on MSE. Still, this contribution might be helpful for other users struggling with similar issues.

If the community sees that differently, then the deletion is fine. But the OP should have never deleted that question in the first place, as he got help solving his problem, but did return nothing..

I suggest to change the title to "Why does the Adams-Bashforth Method converge for this ODE?" Better ideas?

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Undeleted

Another day, another post deleted while I was halfway through a lengthy answer. This time it is Doubt on Do Carmo proof of Gauss Formula (Isometric immersion). It's probably a self-deletion by the original poster, but I spent some time on it, so if it's reasonable please undelete.

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[Undeleted]

Please consider undeleting Aleph and omega - were these deliberately chosen to represent infinities because of the connection between God (the alpha and the omega) and infinity?

As I mentioned in the old thread, this is a reasonable reference request on whether Georg Cantor's (well documented) views on religion influenced his choice of notation. I think there are common misconceptions here, and this question may be a good place to at least start addressing them.

If undeleted, I think if nothing else we could collect the relevant comments under the question as an answer, although of course something more detailed would be even better.

(It would perhaps have made sense to edit the request in the old thread rather than posting this one here, but this is not possible since that thread was since closed as a duplicate of this one.)

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  • $\begingroup$ Now undeleted, but notice that while it was deleted it said, "deleted by Community♦ Mar 8 '14 at 13:28 (RemoveDeadQuestions) This question was automatically deleted." So I assume it's in danger of being automatically deleted again, unless, what, it gets an upvoted answer? I can never remember exactly what it takes to make a question safe from autodelete. $\endgroup$
    – Gerry Myerson
    Jan 11, 2018 at 9:50
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    $\begingroup$ Yes, thank you Gerry. I'll try to post an answer later today if no one else does first. $\endgroup$
    – Andrés E. Caicedo
    Jan 11, 2018 at 12:41
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Reopened

This question has been inexplicably classified as "unclear what you're asking".

Convexity of the natural parameter space of an exponential family of probability distributions is a standard result introduced in standard theory-of-statistics courses.

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Undeleted

This answer has just been deleted, and I was one of those who recommended its deletion through the review queue (as can be seen in the review history) because it was identical to the question, and thus seemed completely useless; but I realized afterwards that the question had been edited to match the answer after it was posted. I have rolled back to avoid other reviewers to be confused (see also the revision history of the question), but I suspect that the other reviewers voted for the same reason, as almost all the deletion votes have been given after the question was edited.

I think it's unfair to the answerer that the answer was deleted due to a subsequent change of the question. I thus suggest that this answer get undeleted.

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    $\begingroup$ I undeleted. Even though the answer is not so great, but I see the point you want to make. $\endgroup$
    – quid Mod
    Apr 10, 2018 at 15:12
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Undeleted

The question Is the function from the coefficients of a monic cubic polynomial to its least root continuous? received an answer (from an experienced user) in half an hour after creation. However, OP self-deleted his question shortly 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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    $\begingroup$ It seems that we edited nearly at the same time and our edits clashed. I have rolled back my edit. $\endgroup$
    – Martin Sleziak
    May 2, 2018 at 15:08
  • $\begingroup$ @MartinSleziak Thanks for your time for editing this. Pasting the URL of the post used to show the title link. It seems that it no long works. I didn't know why there's no preview while I was writing this. $\endgroup$
    – GNUSupporter 8964民主女神 地下教會
    May 2, 2018 at 15:12
  • $\begingroup$ I have observed on occasion an inconsistent behavior of link expansion between the preview and the post. I am not really sure when it works and when it does not. Here is a test in the sandbox and link to the source. $\endgroup$
    – Martin Sleziak
    May 2, 2018 at 15:20
  • $\begingroup$ @MartinSleziak Thanks for your answer. I've corrected this. $\endgroup$
    – GNUSupporter 8964民主女神 地下教會
    May 2, 2018 at 15:29
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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, 2018 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, 2018 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, 2018 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, 2018 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, 2018 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, 2018 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, 2018 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, 2018 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. $\endgroup$
    – amWhy
    Mar 7, 2018 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 Mod
    Mar 7, 2018 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 Mod
    Mar 8, 2018 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 Mod
    Mar 8, 2018 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, 2018 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 Mod
    Mar 11, 2018 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, 2018 at 21:24
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Reopened

This question was initially primarily about implementing Newton's method in Matlab, and was closed as off-topic. But the specific polynomial exhibits some properties that makes it interesting from a mathematical point of view. The question has been reworded to be primarily about math, and the existing answer addresses the math question. I'd like for it to be reopened.

By the way, the question has been on the starboard for four days at Reopen? Undelete? Close? Delete?. I think most people interested in this meta thread would also be interested in that chat room.

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Reopened

This question was closed as "unclear what you're asking." I edited it into standard mathematical notation so that the question is unambiguous. The question does not appear otherwise off-topic (I'll certainly accept having found the trivial solutions as context). The question interests me and I would like it reopened.

(I am unsure of why the review queue responded so negatively to the question)

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    $\begingroup$ I said leave closed because it just said I found this and this without showing any work and then requested how to do it. To me, that is like saying what is some integral I found it to be x. We know they can probably use Mathematica to get the value so what have they really done? $\endgroup$
    – dustin
    Mar 8, 2015 at 18:06
  • $\begingroup$ @dustin Thanks for your reply; it is good to get a response from someone who reviewed it. (I also realized, looking at the review dates, you would almost certainly have reviewed this revision, which has severe formatting issues too) $\endgroup$
    – Milo Brandt
    Mar 8, 2015 at 18:19
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    $\begingroup$ I can't remember which version I saw. I just knew why I wouldn't choose reopen. If it said I did this with work and obtained this, I would have said okay. $\endgroup$
    – dustin
    Mar 8, 2015 at 18:21
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Reopened

Topology of sets - prove or disprove the following statements was originally posted without any context or signs of effort. After others complained in the comments, OP actually fixed the question, explaining their thoughts on the problems, all of which were well-considered. This is a good outcome, all too rare, which we should encourage!

But then the question was closed anyway, belying the commenters who asked for clarification.

Let's reopen.

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    $\begingroup$ I think only the last vote came after the edit, which appears to be an anomaly. (By the way, it is possible the review queue would have taken care of this. You could wait a while after voting to reopen to see if there's a reason to still post here.) $\endgroup$
    – Jonas Meyer
    Mar 17, 2015 at 15:20
  • $\begingroup$ Just so that everyone knows, there are online OCR tools that convert images to text. You still need to correct the formulas, but it significantly reduces the time needed to do it. In this case, it took me about a minute (plus a few seconds to add linebreaks in the last paragraph). Compare the two scenarios: a question formatted like this one was arrives in the reopen RQ, vs. a question formatted like this one is now arrives in the RQ... $\endgroup$
    – Najib Idrissi
    Mar 17, 2015 at 15:32
  • $\begingroup$ @Najib I would very much appreciate pointers to some of those tools; I spend a lot of time inlining scans and screenshots. $\endgroup$
    – MJD
    Mar 17, 2015 at 15:40
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    $\begingroup$ I used onlineocr.net in this case, I only had to correct the formulas. I don't know how well it works for larger bodies of text. $\endgroup$
    – Najib Idrissi
    Mar 17, 2015 at 15:42
0
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Undeleted, Redeleted

What was the point of deleting https://math.stackexchange.com/questions/106624/finding-an-f-automorphism-having-specified-the-images-of-two-elements ? Is the website really better without the mathematical content in the answer to that question, than with it?

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    $\begingroup$ The question is unclear and missing context. It would be preferable, IMO, to start from scratch and post a new question with explicit hypotheses, since OP isn't around to clarify anymore. $\endgroup$
    – Najib Idrissi
    Apr 10, 2015 at 7:21
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    $\begingroup$ The thread is quite confusing. I am not sure how I would vote myself. But, arguably, somebody interested in the subject is better served finding/reading something other than those posts. $\endgroup$
    – quid Mod
    Apr 10, 2015 at 11:06
  • $\begingroup$ Curious. It was autodeleted shortly after my vote undeleted it. $\endgroup$
    – Matt Samuel
    Sep 5, 2015 at 3:02
  • $\begingroup$ Yes, that's very strange. I didn't think the Community user acted that fast, @Matt. $\endgroup$
    – Gerry Myerson
    Sep 5, 2015 at 6:23
0
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Deleted

Recently a reference/book request question was put on hold as too broad. However, we have allowed and currently still allow these questions since older and newer questions aren't being put on hold. For this reason, I request books on math bio be reopened unless we are now closing all other book/ reference post as well.

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    $\begingroup$ The question is unreasonably broad. OP does not even say anything at all about their general background in math. $\endgroup$
    – quid Mod
    Mar 8, 2015 at 18:35
  • $\begingroup$ @Quid that is exactly how almost all the reference request are. $\endgroup$
    – dustin
    Mar 8, 2015 at 18:36
  • 4
    $\begingroup$ Then we should close them too. However, let me also add that the math background thing is less of an issue when somebody asks about, say, an intro to abstract algebra. But for this question it is really key to know of OP knows statsitics, (P)DE, dynamical systems, etc. or not. $\endgroup$
    – quid Mod
    Mar 8, 2015 at 18:38
  • $\begingroup$ For example that one, while really not great, is a lot more specific math.stackexchange.com/questions/1180720/… in that it mentions a beginning physics background. This one is better too math.stackexchange.com/questions/1180124/… $\endgroup$
    – quid Mod
    Mar 8, 2015 at 18:40
  • $\begingroup$ @quid when i am not on my phone i will link well received overly broad requests. $\endgroup$
    – dustin
    Mar 8, 2015 at 18:42
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    $\begingroup$ Great. I will keep the close-votes handy. $\endgroup$
    – quid Mod
    Mar 8, 2015 at 18:43
  • $\begingroup$ @quid the complex one you linked should probably closed as dupe since there are a bunch on it. $\endgroup$
    – dustin
    Mar 8, 2015 at 18:44
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    $\begingroup$ I don't know. Very possible. My point was only it gives a lot more details and background. $\endgroup$
    – quid Mod
    Mar 8, 2015 at 18:45
0
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Reopened

Relation between number of unique values in Gramian Matrix (G) and the matrices that created it got closed in spite of the clarifications made in the comments. The clarifications have now been incorporated in the post, and further improvements made. It should be reopened.

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Reopened

Did marked my question a duplicate. Why is it a duplicate?

I think e is answering a question using a fact asked about in another question. How does that make my question a duplicate?

Before the marking, I got an answer and accepted it. That there was this really long answer that was unrelated to what was suggested by Did seems to suggest my question was not a duplicate.

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    $\begingroup$ Posts in this thread should not be deleted if they lead to any kind of action, to keep the record visible. $\endgroup$
    – Daniel Fischer
    Jul 25, 2015 at 16:39
  • $\begingroup$ For some context. $\endgroup$
    – Did
    Nov 17, 2015 at 9:37
0
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Reopened

Please reopen Finding Nash equilibrium in game with random event at event tree?. After the first vote to close as duplicate was cast, the OP pointed out that this question is about a different game. This is immediately confirmed by a look at the image linked to in the post (which the OP can't include in the post due to lack of reputation). The question was nevertheless closed as a duplicate.

This is particularly unfortunate since it was good style on the part of the OP to ask a new question about this new game instead of continuing to ask me new questions in the comment thread under my answer to the old question. Closing this new question disincentivizes this constructive behaviour.

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    $\begingroup$ I saw that question in the review queue, but it lacks any context for why the (new) game in question is of interest, so it would still qualify for closing under that reason, even if it is not a duplicate. $\endgroup$
    – Carl Mummert
    Aug 13, 2015 at 11:58
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Undeleted, Reopened, Reclosed, Reopened

Please undelete this question because:

  • Qiaochu Yuan's answer is of (very) high quality. It is relevant in many senses and canonical for this kind of problems (which are not so rare on the site).
  • The two answers + OP totalise 33 upvotes and 3 downvotes. So apparently the question is appreciated by the community.
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    $\begingroup$ Slightly ironically you link to your answer not the question, while not mentioning at all that one of the two answers is yours. (It is not possible to use at-notify in the way you seem to want to use it. Even if you are aware of this it is not a good idea to use the syntax as it can confuse others into believing it in fact does work.) $\endgroup$
    – quid Mod
    Aug 4, 2015 at 17:00
  • 1
    $\begingroup$ @quid you are right, it is because I copy-pasted the url and accessed it from my profile. I have no problem to mention that one answer is mine but I don't think that the question should be reopened because of that. Nevertheless I still think it should be reopened for Quiaochu's answer. $\endgroup$
    – Surb
    Aug 4, 2015 at 17:02
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    $\begingroup$ @quid regarding the notification: ok I'll avoid that next time. $\endgroup$
    – Surb
    Aug 4, 2015 at 17:05
  • 3
    $\begingroup$ A problem I see with that Q&A is that while Qiaochu's answer should be a good answer to the question "Is every natural number interesting?" or something like this, it appears it is quite explicitly not what is asked for. Rather the question is about one specific number. (I have not voted on this myself either way. Certainly, there are worse things on this site.) // Thanks for following up on my other remarks. $\endgroup$
    – quid Mod
    Aug 4, 2015 at 17:12
  • 2
    $\begingroup$ @quid If the question doesn't get reopened, maybe it could be a not too bad idea to create question fitting the answer and copy-past the answer in this question (everything in CW obviously). $\endgroup$
    – Surb
    Aug 4, 2015 at 17:14
  • $\begingroup$ Yes this seems like a good idea (assuming something like this does not exists already, which I do not know). $\endgroup$
    – quid Mod
    Aug 4, 2015 at 17:16
  • 4
    $\begingroup$ I'm sorry but is the question no longer opinion-based? Did you do anything at all to improve the question? Note that the popularity of a question has nothing to do with its appropriateness for the website, and it's dishonest to simply add up the scores of the Q and each A, as someone can simply upvote all three -- does this mean that it's three times as popular? And if indeed this sort of problem is "not so rare", then I'm sure you can find a good example of question where Qiaochu Yuan's answer can be transferred. $\endgroup$
    – Najib Idrissi
    Aug 8, 2015 at 7:33
  • $\begingroup$ @NajibIdrissi That's a good point, I'll try to improve the question this weekend. $\endgroup$
    – Surb
    Aug 8, 2015 at 15:00
  • $\begingroup$ @NajibIdrissi So I tried to edit the question and hope it can be considered on-topic now. $\endgroup$
    – Surb
    Aug 10, 2015 at 13:06
0
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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, 2015 at 8:17
  • 2
    $\begingroup$ 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, 2015 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, 2015 at 18:31
  • 1
    $\begingroup$ 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, 2015 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, 2015 at 21:50
  • 2
    $\begingroup$ 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, 2015 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, 2015 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, 2015 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, 2015 at 8:38
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