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The purpose of this thread is to help focus the attention of the community on posts that may require reopen and undeletion votes. A request should be posted as an answer below (one request per answer).

Some guidelines:

  • Please be polite, and respect the many different viewpoints in our diverse community. This goes for the person making the request as well as those commenting on it.

  • There is a reopen queue. Please wait until a post has gone through this queue, before posting here. Notice that the first edit after the question was put on-hold pushes the question into reopen review queue, if the edit was done within 5 days of closure. So does a reopen vote. (If the review has already been finished, it is shown on the timeline of the question.) In doubt, wait 24 hours after the last substantive action.

  • To inform readers of the current (and past) states of the targeted post, please add once the request resulted in some action the information Reopened or Undeleted at the start. (If it the action is undone, add this too, like Reopened, reclosed.)

  • Do not only post a request, like "request reopening of ". Instead make a case for your concern. Yet keep in mind that it can be easier to get your request handled if you try to frame in a away that takes the feedback the post received into account in a positive way rather then seeking confrontation. Also, try to improve the post before posting here.

  • In case of "small" requests, like one missing vote, it can make sense to ask in chat instead of posting here. The room CRUDE is a reasonable place for such requests. The same guidelines apply there.


Earlier versions of this thread that served as a model:

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165 Answers 165

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Undeleted, reopened

Please consider undeleting this old post: Show that a function $f:P(X)\to P(X)$ preserving the subset relation has a fixed point

It was created on Sep 17 '16, has two good answers.

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    $\begingroup$ It seems to be a special case of this question, and Aloizio's anwer in fact answers the general question. So if the goal is to preserve answers, the logical thing to do would be to move them there. $\endgroup$ – Arnaud D. Jun 28 '19 at 13:48
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    $\begingroup$ It seems to me the Question at issue here has the more general formulation (it does not require an "increasing" function). In any case I've voted to reopen as the Question at issue meets my threshold for context (and it otherwise also on-topic). $\endgroup$ – hardmath Jun 30 '19 at 19:45
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    $\begingroup$ I disagree with your assertion that the question has two good answers. It has one good answer, and a hint. The question itself is not good at all. I am content to see the question left undeleted, but I see no reason to reopen it. Arnaud D.'s suggestion is, I think, preferable. $\endgroup$ – Xander Henderson Jun 30 '19 at 23:51
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    $\begingroup$ "I am content to see the question left undeleted." Well, that is what my proposal is. I agree that the question itself was not good enough and thus I did not ask for reopening. @Martin Sleziak has added useful information to the post. (Thanks!) Also, Aloizio's good answer mentions at the beginning "the nice answer by @Brian". $\endgroup$ – Jack Jul 1 '19 at 1:47
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Undeleted, deleted, undeleted, reopened

Please consider undeleting this post: What is the formula of this angle?

There is a well written and well-received answer to this interesting question.

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Undeleted, deleted, undeleted

Please undelete and consider reopening this post: Show this equation has every positive integer as solutions

OP did give his thoughts. NOT a no-context question. And there are two good answers.

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Undeleted, deleted, undeleted

Please consider undeleting this post: How to evaluate $\lim_{x \to 3} (x^2-5x+4)^{x-3}$, which is clearly not a zero-effort question.

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    $\begingroup$ You say that the question should be opened because it is "clearly not a zero-effort question." This seems to imply that the question was closed because it "lacks effort". However, "lacks effort" is not one of the close reasons used on MSE. Indeed, I see that question was closed for "lacking context". For example, a description of what theory the asker is familiar with and what course they are taking. Are they expecting a computation involving $\varepsilon$s and $\delta$s? L'Hospital's rule? What? The question should be deleted. $\endgroup$ – Xander Henderson Aug 15 '19 at 12:31
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    $\begingroup$ Moreover, examining the comments attached to that question, it is not even clear where the function being discussed is meant to be defined. Is it a real function with a very "hole-y" domain? or a complex function, in which case one needs to consider branches of the logarithm? It is not a good question. $\endgroup$ – Xander Henderson Aug 15 '19 at 12:34
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Deleted, undeleted, deleted, undeleted, deleted, undeleted, reopened, closed as duplicate, deleted, undeleted

What OP wrote under the problem statement clearly show his/her thoughts:

How can I do this if I don't know how $a_n$ is defined? I can use the given limit to get the range of $a_n$ in terms of $L$, but I lack the direction to complete the proof.

This is not a zero effort question. Please consider undeleting this post:

Given positive sequence $a_n$ where $\lim _{n\to \infty} a_n = L, L >0$, prove that $\lim_{n \to \infty} 1/a_n = 1/L$

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    $\begingroup$ Did you check if it's a duplicate? It's a very standard question. $\endgroup$ – quid Apr 26 '19 at 14:32
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    $\begingroup$ @GerryMyerson a question might be eligible for closure for multiple reasons. It's maybe not a PSQ in it's purest form, but it's still not a good question. I'd say if somebody wants to restore a question it's on them to make sure no reason applies not just the one that was chosen. Further, it would have been on the asker to make some good faith effort to find a dupe before answering (and then complaining that their answer was removed). If a dupe of this question is not findable what does this tell about the level of organization in those tags. The priorities are not good. $\endgroup$ – quid Apr 27 '19 at 7:12
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    $\begingroup$ @GerryMyerson here math.stackexchange.com/questions/1171733/… it was pretty trivial to find. $\endgroup$ – quid Apr 27 '19 at 7:22
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    $\begingroup$ Well, in any case they complain. We can replace the "that" by "after". On the rest, again the onus is on them. They ought to explain that there is no dupe (supposedly). Further, it is generally considered as good form to mention if one has answered oneself. Especially if it is the only answer. @GerryMyerson $\endgroup$ – quid Apr 27 '19 at 7:26
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    $\begingroup$ @GerryMyerson not sure about that. I might have had the luck to use the better strategy for that example, searching for words via Google on this site not for formulas for example 'convergence of inverse of convergent sequence' works well. $\endgroup$ – quid Apr 27 '19 at 7:31
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    $\begingroup$ @GerryMyerson (and quid) I did try to find a duplicate and I indeed noticed that limit of reciprocal of a sequence away from zero should be a rather standard exercise in calculus/analysis and should have been asked before. However, I think "context" counts as an integral part of a post, and that is why we insist on "contexts", don't we? [cont.] $\endgroup$ – Jack Apr 27 '19 at 13:14
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    $\begingroup$ [cont.] I am not able to find any same/similar confusion ("How can I do this if I don't know how $a_n$ is defined?") in any known post, for instance in the linked post in one of quid's previous comments and thus I do not think this post should be considered as an "absolute" duplicate that one should delete it. $\endgroup$ – Jack Apr 27 '19 at 13:14
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    $\begingroup$ I agree with Jack's comments above, I don't think this post should be closed if a duplicate is not found where the same confusion is addressed. To the best of my knowledge, this confusion has not been addressed before, so this post should be undeleted and reopened. $\endgroup$ – Brahadeesh Apr 28 '19 at 5:43
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    $\begingroup$ @GerryMyerson Basically a duplicate, except it doesn't require $a_n$ to be a positive sequence, was asked fairly recently at Sequence Limit Reciprocal Law Proof, where I provided an accepted answer. As for finding it, note it originally only used the tag "elementary-number-theory", but I changed it later to "limits", so searching for it initially might not have turned it up. However, I can't see the original deleted question content, but the page it brings up shows $2$ other somewhat related ... $\endgroup$ – John Omielan May 2 '19 at 8:08
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    $\begingroup$ (cont) questions: Given: $\lim\limits_{n\to\infty}a_n=0$ prove $\lim\limits_{n\to\infty}\frac{1}{a_n}=\infty$ and How to formally prove that if $\lim \limits_{n \to \infty}a_n=\infty$, then $\lim \limits_{n \to \infty}\frac{1}{a_n}=0$.. Although they're somewhat different, and I haven't checked them carefully, I suspect their techniques are transferable. $\endgroup$ – John Omielan May 2 '19 at 8:09
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    $\begingroup$ The status of the question had not changed at all. What is the justification for this edit? $\endgroup$ – quid May 31 '19 at 16:57
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    $\begingroup$ On the comments above, which I only see know, due to lack of a notification, that's not a particular confusion. Any generic proof will necessarily do just fine to address this. I mean the proof you gave is completely generic. What do you think you did to address whatever specific confusion? $\endgroup$ – quid May 31 '19 at 17:04
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    $\begingroup$ I agree with Brahadeesh's comment and I've undeleted the question. $\endgroup$ – GNUSupporter 8964民主女神 地下教會 Jun 14 '19 at 9:21
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    $\begingroup$ @quid The quoted question in Jack's last comment answers your first question; point (3) in Jack's answer addresses your second one. $\endgroup$ – GNUSupporter 8964民主女神 地下教會 Jun 16 '19 at 3:17
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    $\begingroup$ @GNU As detailed this ask nothing more than that the author understands that they might be able to do it or at least how to strat for a specific sequence like say $2n/(n+1)$. Any generic proof will do just fine, and what about the third point? Again any proof will have to address this somehow (in one based on continuity it might be hidden). Certainly it's findable in other answers. It's just a contrived argument to save this post, and frankly I am appalled by this. Explain how this answer math.stackexchange.com/a/1171755 is not sufficient or math.stackexchange.com/q/54754? $\endgroup$ – quid Jun 16 '19 at 8:51
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Please consider undeleting and reopening this elementary question: On proving $\sum_{k=1}^{2^n+1} \frac{1}{k} > \frac{n}{2}$

OP had his/her thoughts on the problem and also added an attempt for verification in the post after interaction in comments with others. This question has also been edited.

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  • $\begingroup$ Did you check to see whether it's a duplicate? Looks to me like the kind of question that would have been asked before. $\endgroup$ – Gerry Myerson Aug 23 '19 at 12:20
  • $\begingroup$ @GerryMyerson: I can only find a similar one. Since I have a very narrow definition of duplicate, I don't consider it as a duplicate to the mentioned one. Others may probably find closer one. One may close it as a duplicate if one does find one. But I would object the strong action of deletion of this post. $\endgroup$ – Jack Aug 23 '19 at 18:02
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    $\begingroup$ Asker account deleted, answer a brief hint, the argument definitely on the site numerous times, the specific variant a bit artificial. Why in the world should we keep that? $\endgroup$ – quid Aug 23 '19 at 21:08
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    $\begingroup$ @quid Why the account deleted is a 'point to count' to support the deletion? $\endgroup$ – allesia_b Aug 23 '19 at 21:59
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    $\begingroup$ @Isa sometimes arguments are made along the lines of: "Maybe the post is not great, but OP made an effort and we should not discourage them via deletion." There are also glimpses of that in this meta post. Once the account is gone that caries rather less wait. Differently, it's not so much that it's a point for deletion, but it's a point less against deletion. $\endgroup$ – quid Aug 23 '19 at 22:08
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    $\begingroup$ @quid I don't get it. Your previous comment suggested like 4 'bad' things about the post, so why to keep that. $\endgroup$ – allesia_b Aug 23 '19 at 22:21
  • $\begingroup$ @Isa I asked why we would want to keep the post. A common argument given is that we keep it not to penalize the person that asked it. This argument does not really apply in this case. Another argument is that the answer is good; Again does not apply. Still another argument we keep it for others users. But it seems unlikely to be useful, as the argument in principle is on the site numerous times and the specific version is a bit strange in a non interesting way. Thus, I still am at a loss why anyone would bother to keep that particular post. Surely there are plenty of better deleted posts. $\endgroup$ – quid Aug 23 '19 at 22:30
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    $\begingroup$ @quid I may agree with you the day I had 10k :) with your mentioned points: answer a brief hint, the argument definitely on the site numerous times, the specific variant a bit artificial. Except your first point: account deleted. I don't see any possibility of me agreeing with that point. $\endgroup$ – allesia_b Aug 23 '19 at 23:02
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    $\begingroup$ Not exactly the same, but the question seems rather close to Prove this inequality: $\frac n2 \le \frac{1}{1}+\frac{1}{2}+\frac{1}{3}+...+\frac1{2^n - 1} \le n$. However, it's not suitable for merging - the bounds for sum are different, the other post has an answer rather similar to the one already posted. $\endgroup$ – Martin Sleziak Aug 29 '19 at 6:58
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Undeleted, deleted, undeleted, deleted, undeleted

I nominate Optimization problem for routes for undeletion because the question asker self-deleted his question shortly after receiving an answer.

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Deleted, undeleted, deleted, undeleted, reopened, closed as duplicate

Please reopen Calculating the summation$\sum_{n=1}^{\infty}\frac{H_{n+1}}{n(n+1)}$ because it's not an elementary question and OP has tried checking the proposition using Wolfram Alpha.

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    $\begingroup$ In my opinion it's very poor style to reignite this after month without mentioning any of the earlier activity in which you even were involved. Thus, I locked the post for now. $\endgroup$ – quid Apr 30 '19 at 11:43
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    $\begingroup$ I'm puzzled. A "content dispute" notice was lifted in March 2018. The content has not been altered since then. So it seems strange to lock it for "content dispute" now, @quid. [I guess I should mention that I was involved in earlier activity.] $\endgroup$ – Gerry Myerson Apr 30 '19 at 13:56
  • $\begingroup$ @GerryMyerson Are your really puzzled or are you just searching an argument? First, the content of the post was also not changed between the first locking and the first unlocking. Second, the lock was not lifted in the sense of somebody lifting it, it simply expired. It was not reinstated as there seemed no need back then about a year ago. Then half a year ago, there was one more round of undelete-delete but it fizzled out quickly. Now somebody wants to go back to this again, indeed without any further changes, simply picking up the del-undel again. To prevent this, I locked it. $\endgroup$ – quid Apr 30 '19 at 14:36
  • $\begingroup$ Third, it might be relevant to note that the lock was "lifted" while the post was deleted. Thus, if consistency is the concern of course I should lock it again now. I could also have it become undeleted and then personally delete it again. Last time around some where keen on locks being used. So, there you go. Of course that's not fine for you now either, presumably because it does not cater to your personal preferences. $\endgroup$ – quid Apr 30 '19 at 14:43
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    $\begingroup$ To sum up, I locked it to prevent the delete-undelete cycle being restarted. By now the question was deleted four times. Plus it is an odd situation, in that it first got siginificantly improved by a third party only for this then being undone If anything should happen likely a completely new version should be posted. This would be a reasonable thing to do if preserving content is the actual goal. Of course if playing un-del games should be the goal... thus locked. @GerryMyerson $\endgroup$ – quid Apr 30 '19 at 14:48
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    $\begingroup$ @quid, when I write that I am puzzled, please do me the courtesy of accepting that I really am puzzled. I'm puzzled not so much by the locking but by the phrase, "Content dispute", when there was no change in content. Had the locking notice said "Prevent delete/undelete cycle" I don't know that I would have made any comment. Does a moderator have the option of inserting custom-made locking notices, or is "Content dispute" the only phrase available? Also, I always assumed that when a moderator locked a question, the question stayed locked until a moderator decided it was time to unlock it.... $\endgroup$ – Gerry Myerson Apr 30 '19 at 22:46
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    $\begingroup$ ....I didn't know that locks expired of their own accord after a week. I've been here almost a decade, and I'm still learning how the place works. $\endgroup$ – Gerry Myerson Apr 30 '19 at 22:47
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    $\begingroup$ @GerryMyerson the list of reasons is fixed (we can ask for additions in principle but not spontaneously) "content dispute" is the one commonly used in this case, others are "off topic comments", "contest question", "cw answer", "historical significance". We can lock for an hour, for a day, for a week, or permanently. Usually one tries to avoid permanent locks. $\endgroup$ – quid Apr 30 '19 at 23:58
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    $\begingroup$ Probably it's worth mentioning that there already is one answer in this thread about the same question. $\endgroup$ – Martin Sleziak May 15 '19 at 7:34
  • $\begingroup$ @quid, thanks for the explanation about length of locks. It's not possible, is it, for non-moderators to see what length a moderator has chosen when that moderator locks a question? $\endgroup$ – Gerry Myerson May 20 '19 at 4:26
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    $\begingroup$ @GerryMyerson indeed I think that's not possible. $\endgroup$ – quid May 20 '19 at 8:39
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Undeleted, reopened

Please undelete and reopen this post: Evaluate this limit: $\lim_{n \rightarrow \infty} \frac{n\sin n}{2n^2 - 1}$

This post is by no means missing context. OP clearly stated at the beginning what he/she tried and where he/she got stuck. This post has also a detailed answer by a 50K+ user.

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    $\begingroup$ It might be worth noting that unfortunately that so-called detailed answer completely ignores the provided context, in that sense it's a rather poor answer, but arguably that's tangential. I'd tend to agree that this question does show reasonable context. If we take the context into account, likely it's a dupe. $\endgroup$ – quid Sep 23 '19 at 15:52
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    $\begingroup$ Instead of saying "what goes wrong with my thoughts" or "how can I go on with my original approach", OP explicitly stated that "Any help will be appreciated". And @Clement C. helped this user by directly giving an argument. Not a poor answer, and there have four people considering it good. This post is an extremely poor choice for deletion. $\endgroup$ – Jack Sep 23 '19 at 15:58
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    $\begingroup$ quid does make a fair point that an answer showing how OP could go on with the original approach may be more useful for the asker. One could add such an answer after this post is reopened. $\endgroup$ – Jack Sep 23 '19 at 16:06
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Undeleted, reopened

Please undelete and reopen this post:

How can one use the direct comparison test for $S=\sum_{k=1}^{\infty}{\frac{1}{\sqrt{k}+10k}}$?

Even in the very first version of the post, OP states his/her thoughts and difficulties he/she has. This post is NOT missing context. And it is NOT a duplicate of any other question.

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    $\begingroup$ Because we already have many questions where divergence of a series is deduced from a comparison with the harmonic series, this is bound to be a duplicate dozens of times over. Therefore the question should also be closed as too localized (not adding anything new to the site, and only helping the asker do their homework). Missing context is the best proxy we currently have, so if reopened I will vote to reclose and redelete. $\endgroup$ – Jyrki Lahtonen Sep 30 '19 at 4:53
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    $\begingroup$ @JyrkiLahtonen Maybe I should have trie to search a bit more, but by a quick search I only found this: Calculus II: Comparison Test for Divergence $\sum\limits_{n=1}^{\infty}\frac{1}{n+\sqrt{n}}$. (All I am saying that I am unable to confirm that there are dozens of posts which are close to duplicates. Although I agree that when I saw the question, I expected that there should be many similar questions.) $\endgroup$ – Martin Sleziak Sep 30 '19 at 8:09
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    $\begingroup$ @MartinSleziak It is one of the standard series to use a comparison test with :-) Anyway, when the question is a (near-) duplicate IMO showing effort/thoughts is NOT SUFFICIENT to stop closure/deletion. The questions should have wider interest, and the context should demonstrate that. $\endgroup$ – Jyrki Lahtonen Sep 30 '19 at 9:59
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    $\begingroup$ Mathematics Stack Exchange is a question and answer site for people studying math at ANY level and professionals in related fields. How many questions a student asks are not supposed to be "standard"?! And is any one of the four mentioned in the first comment not standard? $\endgroup$ – Jack Sep 30 '19 at 11:59
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    $\begingroup$ @Jack Your comment about MSE being for questions at any level is a red herring---the question was not closed because of its level. The question is a standard question which occurs (in one form or another) in many elementary texts on calculus. Moreover, it is a question which already has an answer on MSE. What new, valuable information does this particular question add to MSE which isn't already on the site? $\endgroup$ – Xander Henderson Sep 30 '19 at 20:18
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    $\begingroup$ How many questions a student asks are not supposed to be "standard"?! @Jack, not very many. The same applies to StackOverflow. Which is exactly why Jeff Atwood wrote that a new asker should spend 98% of their time on StackExchange searching. Getting more and more questions does not improve the site, and is not necessary for its success any longer. On occasion it is quite the opposite. $\endgroup$ – Jyrki Lahtonen Oct 1 '19 at 14:06
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Undeleted, deleted, undeleted, reopened, closed as duplicate, deleted, undeleted, deleted, undeleted

Please consider to undelete this question.

The OP brings some background about the problem and is just trying to find an easy solution. The one provided by me looks quite unknown (as it seems from the comments and the answers) thus I think it can be of future help if the question is undeleted.

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    $\begingroup$ It's good form to disclose that one has an answer on the post oneself. $\endgroup$ – quid Aug 15 '19 at 21:00
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    $\begingroup$ This question is a duplicate of many other questions on MSE. While I don't think that it is worth keeping (it brings nothing new to the table), if it is going to be kept, it should be closed as a duplicate. I left comments indicating a couple of potential dupe targets. $\endgroup$ – Xander Henderson Aug 17 '19 at 15:05
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    $\begingroup$ On the contrary I believe it brings something new to the table. My solution is different than any other from all the linked posts. $\endgroup$ – Zacky Aug 19 '19 at 20:24
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    $\begingroup$ @Zacky In that case your answer will stand its ground as an answer to the duplicate target as well, and you might consider relocating it there! Just by posting a new answer, the thread of your choice will be bumped to the front page, meaning that many viewers will see it and be able to vote it. <- This could be my template reply to all the users who are sad about a deletion of a duplicate. They can easily save their work this way! $\endgroup$ – Jyrki Lahtonen Aug 30 '19 at 18:19
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    $\begingroup$ "Partial fraction without defraction for an integral" What the heck does that mean? Is defraction really a word? Why has no one edited that title to something intelligible? $\endgroup$ – Gerry Myerson Aug 30 '19 at 23:00
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    $\begingroup$ @GerryMyerson I think that no one had edited the title into something intelligible because no one actually knows what the asker intended. That is part of the problem with the question (aside from the fact that it is a duplicate). $\endgroup$ – Xander Henderson Aug 31 '19 at 14:17
  • $\begingroup$ @Zacky Honestly, I think that Jyrki's advice is good advice. Indeed, I think that your answer would be a better fit for one of the other questions, if for no other reason than the current question specifically asks about partial fraction decomposition, while the others don't. Your answer would be a better fit for a different question. $\endgroup$ – Xander Henderson Aug 31 '19 at 14:18
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    $\begingroup$ @XanderHenderson something looks off. You said that it brings nothing new to the table and it's not worth to be kept, you even ranted on how my answer isn't a "pedagogical". Are you sure that you honestly believe that is a good advice? $\endgroup$ – Zacky Sep 1 '19 at 8:52
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    $\begingroup$ I also wrote in my pre-edit for OP: "Since this is just for your training, here's a more different approach for it. If it doesn't fit for you, I will delete it (it doesn't use any partial fractions)", when OP commented I think I received a positive feedback about it from OP.// Indeed it doesn't answer the exact question, but another answer (which got accepted by OP) shows a way with partial fractions. I only posted my answer after I saw it, and after the comments (from some high-rep users) who encouraged OP to not look for a better way? $\endgroup$ – Zacky Sep 1 '19 at 8:58
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Reopened, undeleted, closed, reopened

Please consider undeleting this question where I also provided an answer: Integrating $\int_0^1\frac{x\ln (1+x)}{1+x^2}dx$ with restricted techniques.

The OP from clearly provides background on how the integral appeared, although it was some kind of homework now it can be useful for others too.

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    $\begingroup$ It's good form to disclose that one has an answer on the post oneself. $\endgroup$ – quid Aug 15 '19 at 20:59
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    $\begingroup$ Hope I don't sound rude, but I don't understand what you wrote even with translate. Did you meant: It's a good thing to mention that I posted an answer to the linked question? $\endgroup$ – Zacky Aug 15 '19 at 21:07
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    $\begingroup$ Yes. You should mention this. It makes clear that you are not a neutral third party. You have a personal interest in the undeletion. This does not disqualify you from making the proposal. But it is good form to be upfront about it. $\endgroup$ – quid Aug 15 '19 at 21:12
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Deleted, undeleted, deleted.

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The close notice for How to prove that the permanent of a doubly stochastic matrix is positive? says, "Add details and clarify the problem being solved. This will help others answer the question."

The problem being solved is perfectly clear (to anyone who knows what a doubly stochastic matrix is, and what the permanent of a matrix is), and others have not needed any help answering the question, so the close notice strikes me as invalid. Please consider voting to reopen this question.

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    $\begingroup$ Possibly you did not get the memo: math.meta.stackexchange.com/questions/31147/… $\endgroup$ – quid Mar 5 at 15:31
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    $\begingroup$ @quid if a user is going to vote to close a question, and if none of the currently available close reasons applies, I reckon the user really ought to leave a comment explaining how the question doesn't measure up. What was done for this question, even if OP clicked to see the further information on "add details and clarify," gives OP no clue as to what to do. Heck, it gives me no idea as to what the close-voters actually had in mind. Let's reopen the question, and give the close-voters the chance to make their case. $\endgroup$ – Gerry Myerson Mar 5 at 21:30
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    $\begingroup$ Maybe that would have been better but there is only so much time and it fits well enough. As for you, it is hard to believe that you actually had not understood what the matter was (as opposed to disagreeing). Either way, I hope you keep the information obtained here in mind going forward. $\endgroup$ – quid Mar 6 at 4:36
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Please consider undeleting and reopening the post (question & answer both): Why integral of $dx$ is not $∫dxdx$.

This is a basic question came to OP's mind on the subject. There are a lot of people who have the same doubt about the fact. It will be very much helpful for those. Also my answer was accepted by OP and also being appreciated by the other members on this site. I think those question and answer are suitable for the site. I strongly recommend to undelete the post.

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Undeleted, Reopened

Please undelete and reopen Proof of $(1 + \delta)^{1/2 + \delta}(1- \delta)^{1/2 - \delta} > 1$ for $0 < \delta \leq 1/2$?. Let it be known that I answered the question. The question should now have sufficient context.

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Undeleted, deleted, undeleted, deleted, undeleted, reopened, closed, locked, unlocked, deleted, undeleted, locked (for historical significance).

I think this question should be reopened. Thank you!

The original poster looked for the possibility to use the substitution $t=\tan\frac{x}{2}$ and he got a number of solutions.

By the way, the first comment of Lord Shark the Unknown is nothing. At the least, it gives a very complicated solution which the topic starter tried to apply but without success.

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    $\begingroup$ Quote from the question: "I tried tangent half-angle substitution but it became too complicated." First comment on main: "Try half-angle substitution, but with a little more determination. – Lord Shark the Unknown Aug 9 at 9:54" Answer by the OP: [DNE]. $\endgroup$ – Did Sep 8 '18 at 22:17
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    $\begingroup$ "By the way, the first comment of Lord Shark the Unknown it's nothing" Pfff... $\endgroup$ – Did Sep 8 '18 at 22:44
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    $\begingroup$ And now there's a rollback war. $\endgroup$ – Gerry Myerson Oct 1 '18 at 12:35
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    $\begingroup$ This question feels like a joke that everyone gets but me. (1) Why is this question particularly bad? The OP said they tried the obvious thing but to no avail. Presumably they made a mistake, but so what? That doesn't invalidate the question. (2) Why is there such a fierce edit/close/delete war? The question is 2 months old - let it rest!!! $\endgroup$ – user1729 Oct 12 '18 at 17:11
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    $\begingroup$ Rollback to Revision 9 by Alex Francisco, rollback to revision 10 by Michael Rozenberg, rollback to 9 by Alex, back to 12 by Michael, to 9 by Alex, to 14 by Michael, to 9 by Alex, to 16 by Michael. Crazy! $\endgroup$ – Gerry Myerson Oct 15 '18 at 22:44
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    $\begingroup$ Please, Michael, please, please, please stop editing the tags on that question. Enough already! $\endgroup$ – Gerry Myerson Nov 6 '18 at 11:08
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    $\begingroup$ This is just sorta embarrassing to watch at this point. And it's already back to 2 undelete votes, so I expect another full cycle in the next day. $\endgroup$ – user296602 Nov 27 '18 at 18:28
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Undeleted, reopened

I humbly request undeletion and reopening of this question, if you see fit to do so, because

a) reasons for closing should be derived from the question itself and not from speculation about the skills of the asker.

b) the given reasons for closing are incorrect - while the question sails close to an open problem, it asks a different but related question - it asks if the open problem (which relates to a series of polynomials) can be extended to its limit point.

c) the time from entering the reopen queue to deletion was less than two hours, denying any who might have been interested in its reopening the opportunity to declare the same.

d) moreover, I think others will find the question and any prospective answers of interest and value.

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    $\begingroup$ The time from entering the reopen queue to deletion may have been short, but the question had been on hold for five days already. As for the close reasons, I'll note that 2 of the close voters did not use "not math". I think that personnally I would have probably voted to close as unclear what you're asking, because on a first reading I honestly had no idea what you were talking about. $\endgroup$ – Arnaud D. Dec 1 '18 at 12:43
  • $\begingroup$ @ArnaudD. Yes, but the reopen request followed an edit which increased focus on the actual question and removed the motivation for it, when I believe the question's motivation was almost certainly the cause for the down votes and closure in the first place, rather than the question itself which stands alone. $\endgroup$ – samerivertwice Dec 1 '18 at 12:47
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    $\begingroup$ By the way, I think the most recent edit on your question (post-deletion) already makes the question more interesting. It only consists of adding a Wikipedia link, but that link already explains some of the context better than your question (for example, I hadn't realized that you were talking about polynomial bijections $\mathbb{N}^k \to\mathbb{N}$, but maybe that's just me). $\endgroup$ – Arnaud D. Dec 1 '18 at 12:57
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    $\begingroup$ For what it's worth, the reopen review was completed before the deletion, as one can see here. $\endgroup$ – Arnaud D. Dec 1 '18 at 13:15
  • $\begingroup$ @ArnaudD. good point, although predominantly the same users. Is there a better review link that shows timings etc.? I thought I saw one before but can never re-find it. $\endgroup$ – samerivertwice Dec 1 '18 at 13:39
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    $\begingroup$ There's the timeline, but I don't know if it is as detailed as you'd want. (By the way, there's another question about how to find it). $\endgroup$ – Arnaud D. Dec 1 '18 at 13:44
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    $\begingroup$ Setting aside all other issues, there are some obvious (and, I think, easily correctible) errors in your question's key equation, stating the F-P conjecture: the right hand side has no $y$ in it, and the index of summation $n$ appears nowhere in the expression being summed. I'm pretty sure you want the summation to go from $k=1$ to $n$, not $n=1$ to $k$, with $f_n(x_1,\ldots,x_n)$ on the left hand side, not $f_k(y)$. $\endgroup$ – Barry Cipra Dec 1 '18 at 15:28
  • $\begingroup$ @BarryCipra thank-you. I think I intended $y$ to be the set $\{x_1,x_2,\ldots x_k\}$ and yes, I agree I appear to have exchanged an $n$ with a $k$ but alas no edits are allowed on a deleted post so I can't correct it :( $\endgroup$ – samerivertwice Dec 1 '18 at 18:17
  • $\begingroup$ @RobertFrost That's surprising. In any case, I've just done the correction. $\endgroup$ – Arnaud D. Dec 1 '18 at 18:57
  • $\begingroup$ @ArnaudD. Thanks and you even got the $f_n$ I thought might get missed. $\endgroup$ – samerivertwice Dec 1 '18 at 19:00
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    $\begingroup$ What do you mean by "timings"? The time of the review is there exactly. As usual, hover over the time stamp to get a resolution to the second. $\endgroup$ – quid Dec 1 '18 at 23:44
  • $\begingroup$ @quid thank-you. That probably covers it although I can only see a deleted post on desktop so I'll have to check later $\endgroup$ – samerivertwice Dec 2 '18 at 4:07
  • $\begingroup$ @ArnaudD. Thank-you for the edit. A reopen vote would be greatly appreciated (only if you think it appropriate). Else the deleters will reverse the undeleters! $\endgroup$ – samerivertwice Dec 3 '18 at 10:39
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Can this answer (on permutation groups) be undeleted:

https://math.stackexchange.com/a/3197094/10513

It was deleted for not providing an answer to the question. However, it gives a decent hint which leads to a solution. The hint is essentially "consider odd vs. even numbers". This was made formal in the accepted answer, which used mod 2.

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  • $\begingroup$ I think it's a bit of a stretch from that hint to the accepted answer. The hint ought to be a comment (in my opinion). $\endgroup$ – Gerry Myerson Apr 29 '19 at 12:50
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    $\begingroup$ @GerryMyerson Hints are a grey area. But I don't think deleting them because they are "a bit of a stretch" is the right thing to do. $\endgroup$ – user1729 Apr 29 '19 at 12:55
  • $\begingroup$ It would have been better, had a moderator converted the "answer" to a comment. You could try flagging the question for moderator attention, and making that suggestion. $\endgroup$ – Gerry Myerson Apr 29 '19 at 12:59
  • $\begingroup$ I flagged it beforehand, and was told to post my request here (admittedly i didn't suggest making it into a comment) $\endgroup$ – user1729 Apr 29 '19 at 13:19
  • $\begingroup$ OK, now you know what to do. $\endgroup$ – Gerry Myerson Apr 29 '19 at 13:22
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Undeleted, Merged and then deleted

Please reopen https://math.stackexchange.com/q/3208119/290189 since OP has self-deleted his/her own post after getting an answer. The community should have been given enough time to vote.

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    $\begingroup$ You can't reopen a question that isn't closed, GNU. Now it has been closed, as a duplicate, but I don't see how $x^y+y^x=(xy)^2-19$ is a duplicate of $x^y-y^x=xy^2-19$. So, I'm flagging for moderator attention. $\endgroup$ – Gerry Myerson Apr 30 '19 at 13:47
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    $\begingroup$ @GerryMyerson By the moment I posted this answer, I did voted to close this question as missing context. I did notice the difference between the two. Thanks for reminding me. $\endgroup$ – GNUSupporter 8964民主女神 地下教會 Apr 30 '19 at 13:52
  • $\begingroup$ @GerryMyerson a problem is that the original version of the question was incorrect. A third party edited it to this version, while main OP had re-asked a different version. $\endgroup$ – quid Apr 30 '19 at 15:32
  • $\begingroup$ I deleted it after the merge now. This did not work out ideally, but it's near impossible to fix. If somebody wants that version of the question on the site I recommend to ask it anew. $\endgroup$ – quid May 2 '19 at 18:17
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Please unmark the duplicate of this question. I have clearly said that my question has nothing to do with duplicate one. I am asking totally different thing and the duplicate question's answer does not help me at all. It is also an unique question which follows m previous question.

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    $\begingroup$ I don't believe the relationship between the Questions is quite so simple as one "has nothing to do with" the other. Both involve intersections between two spherical caps, although you might not have visualized your problem in this way. There might still be a further problem in stating the area in terms of how you parameterized the spherical region, I grant, but it would be worth understanding the solution of the proposed duplicate to try restating the area given there in your terms. $\endgroup$ – hardmath Jun 6 '19 at 17:11
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Undeleted, closed, deleted, undeleted, redeleted

I nominate Solve the recurrence $\sqrt {x_n} − 5 \sqrt {x _{n−1} }+ 6\sqrt {x_{n−2}} = 0$ for undeletion because OP has self-deleted his post shortly after receiving an answer. That's rude to the answerer.

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    $\begingroup$ I agree that the question should not have been deleted by the original asker, and would have voted to undelete had I seen this before the question was undeleted. That being said, it is a pretty low quality question, and would like to now nominate it for closure and (if it is not improved) eventual deletion. $\endgroup$ – Xander Henderson Nov 12 '18 at 23:26
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    $\begingroup$ @XanderHenderson Agreed and updated the status of my answer. $\endgroup$ – GNUSupporter 8964民主女神 地下教會 Nov 13 '18 at 1:27
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    $\begingroup$ It is inconceivable this question is undeleted. Even the answer is pretty low-quality. $\endgroup$ – YuiTo Cheng Jun 26 '19 at 4:53
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    $\begingroup$ I find that revision history really surprising : one (now removed) user voted to undelete and then close the question, OP initially self-deleted the question but 5 days later voted to undelete after the second deletion, and now you've voted to undelete and then redelete the day after. Pardon my French, but WTF? $\endgroup$ – Arnaud D. Jun 27 '19 at 9:31
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Undeleted, reopened

Please consider undeleting and reopening this post:

If $\sin(\pi x)=a_0+\sum\limits_{n=1}^{\infty}a_n \cos(n\pi x)$ for $0<x<1$ then what is the value of $(a_0+a_1)\pi$?,

where OP gave his/her thoughts:

Honestly I have no idea about how to approach this problem. I tried the expansion of $\sin x$ on the left hand side and then expand $\cos x$ on the right but end up with a mess. Then again $\cos x=(\sin x)'$, but by converting each term on the right hand side into derivative of sine does not give any sensible identity to draw out something. Really out of ideas on this one. Is there any way to do this? Please give me some hints. Thanks for your time.

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Undeleted, deleted, undeleted, reopened, closed as duplicate (mod)

Please consider undeleting this post: 2011 IMC Section A Problem 3

This is not an ongoing contest problem. The answer is known online. @Aqua's answer (with 14 net upvotes) is a useful one.

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    $\begingroup$ Poorly explained answer. Had downvoted it back then. $\endgroup$ – quid Jul 6 '19 at 19:53
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    $\begingroup$ That was a really bad excuse for deletion. $\endgroup$ – Jack Jul 6 '19 at 21:09
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    $\begingroup$ Since I did not delete it, I don't need any excuse (and I doubt the delete-voters where aware of my voting back then). Please do not write comments that might suggest otherwise as not everybody can check it easily. $\endgroup$ – quid Jul 6 '19 at 22:20
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Undeleted, deleted, undeleted, deleted, undeleted, reopened

Please consider casting the final undelete vote and reopening this post:
Evaluate $1+2-3-4+5+6-7-8+\cdots+50.$

This question is well written and with effort. It has very well written answers, one of which gives a generalization of the problem.

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Please undelete this post: $n^4 + 4^n$ is a not a prime

OP clearly states context for his/her question:

This question appeared in the undergrad entrance exam of the Indian Statistical institute.

When $n$ is even the proof is simple. For $𝑛=2𝑚+1$ I am utterly stuck.

True that this is a duplicate of some previous posts, but duplicate can be useful. Moreover, it is not necessarily easy at all to identify this question as a duplicate:

  • The useful searching engine Approach0 is NOT well known, neither it is mentioned on the page of help center of MSE.
  • A quick search of the expression $n^4+4^n$ on the MSE searching box returns something irrelevant: enter image description here
  • If one posts this as a new question: the "similar questions" box does not show any one of the duplicate:

    enter image description here

"Questions that are extremely off topic, or of very low quality, may be removed at the discretion of the community and moderators." This 5 upvoted one with three answers is not.

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    $\begingroup$ At some point, every math student really ought to be shown the factorization of $x^4+4$. $\endgroup$ – Gerry Myerson Sep 26 '19 at 7:47
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    $\begingroup$ This is an umpteenth reincarnation. Even in 2013 when André Nicolas answered it we had already covered it many times. Also, while duplicates were seen as somewhat useful back in 2013 (when Jeff Atwood's strangedupe comment was often cited), the site has moved on since. The most recent word from above (don't remember for sure whether it is from JA or the current CEO Joel Spolsky) reads: Over time duplicates become vast landmine fields. $\endgroup$ – Jyrki Lahtonen Sep 26 '19 at 16:24
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    $\begingroup$ The sentence ("... landmine fields") Jyrki mentioned is from this article by Jeff Atwood. That one is specifically talk about StackOverflow, which is sort of the counterpart of MathOverflow. (cont.) $\endgroup$ – Jack Sep 26 '19 at 17:18
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    $\begingroup$ Jeff explicitly writes in that article the following : But I will point out that there is plenty of precedent on the Stack Exchange network for splitting sites into "expert" and "beginner" areas with slightly different rulesets. We've seen this for Math vs. MathOverflow, English vs. English Learners, Unix vs. Ubuntu... perhaps it's time for a more beginner focused Stack Overflow where duplicates are less frowned upon, and conversational rules are a bit more lenient? $\endgroup$ – Jack Sep 26 '19 at 17:18
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    $\begingroup$ I am not following SO much at all, but what little I have seen of it it is nowhere near the level of sophistication of MathOverflow (admittedly my observations are likely biased). But I would support walling off lower level math (like freshman and below) to a Math Learner site. $\endgroup$ – Jyrki Lahtonen Sep 26 '19 at 21:49
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    $\begingroup$ I don't get what's the contradiction even. Certainly duplicates will happen, some will be caught other won't be caught. Of those that do get caught we'll keep some that seems relevant as "sign posts" and we remove the rest without much ado (either way). Why would we want to keep them? $\endgroup$ – quid Sep 26 '19 at 22:26
  • $\begingroup$ On search, searching n^4 + 4^n for works like a charm Why would one use strange spacing, or in any case just check one version, and include the completely irrelevant dollars? $\endgroup$ – quid Sep 26 '19 at 22:34
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    $\begingroup$ @quid: your linked search of n^4 + 4^n , which does return a duplicate result, uses "strange spacing". If one searches n^4+4^n without the spacing before and after the plus sign, one would not see the charm. $\endgroup$ – Jack Sep 27 '19 at 0:04
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    $\begingroup$ The answers are all dupes of answers given many times in the past. This FAQ occurs many times every year. There is no need to keep adding duplicate answers every time it reoccurs. Doing so makes it difficult if not impossible for users to locate the most enlightening answers. $\endgroup$ – Gone Sep 27 '19 at 0:45
  • $\begingroup$ @Jack I forgot to add that Theoretical Computer Science is the StackOverflow analogue of MathOverflow. Don't get mislead by the inclusion of the word Overflow. If SO walls off a learners' site, we should do the same. $\endgroup$ – Jyrki Lahtonen Sep 27 '19 at 3:42
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    $\begingroup$ And also, the voting already gives low level math an unhealthy advantage in the gamification aspects of the site. Simply because users are obviously not going to (and should not) vote on material over their head. You wanting to be more lenient about low level duplicates would exacerbate an already serious problem. $\endgroup$ – Jyrki Lahtonen Sep 27 '19 at 3:45
  • $\begingroup$ Well, let's forget about which spacing is strange or not. One might learn the lesson that one should search for both variations. I'd still maintain that the spacing there is the more natural way to type it. But I am glad that you seem to agree now that the dollars are useless. // And even if it was completely unnatural it is the very spacing that was used in that post that you want undel too. Thus at least that user likely would have used the same spacing if they had searched and keeping this as a dupe would not even help future searches (except maybe via answer, but then merge could do). $\endgroup$ – quid Sep 27 '19 at 8:44
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Reopened

I propose reopening Has Gödel’s second incompleteness theorem been formalized?

The question simply asks whether there is a computer-verified version of the proof of the theorem. This strikes me as a perfectly reasonable mathematical question, and I can't see why anyone would vote to close it, much less delete it (there are currently two votes to delete).

Full disclosure: I posted an answer (the only answer) to the question, which OP has accepted. The answer attracted a downvote – I'm guessing that was done to make it easier to close and delete the question, since I can't see what objection anyone could have to the answer.

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REOPENED, Closed

I request to reopen Why does $f(z) = z^n$ have no antiderivative only for $n=-1$? It was closed as a duplicate of Antiderivative 1/z on C.

I disagree that it is a duplicate. The first question was a soft question and the second was a hard question. OP wasn't literally asking for a proof that $1/z$ doesn't have an antiderivative. He was asking what makes $n=-1$ "special". This is a different question. One question is soft and the other is hard.

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Undeleted, Still Closed

I want this question to be to undeleted because I wrote a long answer for it and it was a correct and well-explained question. The question now has more context.

Edit: Can someone suggest for edits that should be made to reopen this post also? This problem is quite similar to Siegel's lemma and I don't think MSE has a post regarding such problems. I think it should be reopened also. Please consider this to be reopened.

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    $\begingroup$ Thank you for being upfront about your reasons for wanting this question undeleted / reopened. Unfortunately, I do not feel that I can vote to undelete the question–the question is a "problem statement question", and does not meet the quality standards of Math SE. The asker has not provided any context, such as a source for the problem or any thoughts about how to approach the problem. I am sorry that you invested your time into answering this question. Perhaps, in the future, you will be a little choosier about which questions you choose to answer. $\endgroup$ – Xander Henderson yesterday
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    $\begingroup$ @Xander, we do sometimes keep low quality questions that provoke high quality answers. I can't see question or answer so I can't say whether that applies here. $\endgroup$ – Gerry Myerson yesterday
  • $\begingroup$ @GerryMyerson What do you mean "we"? I was expressing my opinion (note the large number of first person singular pronouns in the text), and hoping to convince others to follow my lead. You appear to have a contrary opinion. Wuderbar. But please don't phrase your opinion in such a way that you give the appearance of speaking for the rest of the community. Again, what do you mean "we"? $\endgroup$ – Xander Henderson yesterday
  • $\begingroup$ BTW you linked to your answer rather than to the question. The wording suggests that you probably wanted to link to the question. $\endgroup$ – Martin Sleziak yesterday
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    $\begingroup$ @MartinSleziak I just requested to undelete this because I have invested time to think about the solution and I solved this using pigeon hole principle. It's an undeniable fact that this a very good and seemingly unexpected application of pigeon hole principle. I think it should be undeleted. I have nothing more to say. $\endgroup$ – Shubhrajit Bhattacharya yesterday
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    $\begingroup$ I thought I made it clear, Xander, that I was not expressing any opinion on the merits or otherwise of the deleted question or of Shubhrajit's answer. On the other matter you raise, I think it is a fact, and not an opinion, that occasionally we – the m.se community – find an answer of such high quality that we choose to keep it on site despite the failings of the question that prompted it. $\endgroup$ – Gerry Myerson 23 hours ago
  • $\begingroup$ @GerryMyerson My understanding is that the community usually have huge argument about what to do with low quality question with great answers. In some heated case the mods even had to lock the question to stop the delete/un-delete war. $\endgroup$ – Arctic Char 12 hours ago
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    $\begingroup$ @Arc OK, but I see nothing in what you've written that contradicts what I wrote. $\endgroup$ – Gerry Myerson 6 hours ago
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Reopened


Please consider reopening Indian Mathematicians. The question was closed as off-topic, apparently because of the existence of the History of Science and Mathematics site.

As I pointed out in a comment, first, questions about history of mathematics are on-topic here. The tag has almost two thousand questions and a decent amount of recent activity.

Second, the quality of many interactions at the HSM site is sadly rather poor (I was very enthusiastic about it when it started, so this poor quality quite bothers me), and it may end up being a disservice to send this question there.

(I would actually love to see the quality at that site to raise uniformly, and hope it happens relatively soon. But that is another matter.)

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  • $\begingroup$ I think at least the formulation of the question should be clarified a bit before it is reopened. The first answerer already misunderstood it. $\endgroup$ – quid Aug 30 '18 at 21:11
  • $\begingroup$ Fair enough. I'll do a slight edit. $\endgroup$ – Andrés E. Caicedo Aug 30 '18 at 21:16
  • $\begingroup$ I had decided to do an edit myself. Sorry for the confusion. I'll check back in one or two hours. If it's still closed then I'll give the final vote(s). This allows also for further editing. $\endgroup$ – quid Aug 30 '18 at 21:19
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    $\begingroup$ No problem. I gave the edit a second pass. (And thank you.) $\endgroup$ – Andrés E. Caicedo Aug 30 '18 at 21:22
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Undeleted, deleted, undeleted, reopened

Please undelete On a colon ideal in the polynomial ring $\mathbb R[x,y]$ because this question has got a score of 2 after OP included his/her own thoughts on the problem. This algebraic geometry question might add value to the main site.

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    $\begingroup$ Selfdeleted without answer. No need to undelete. $\endgroup$ – quid Feb 25 '19 at 9:16

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